How It Works Book of the Human Body 6th Edition

8/18/2019 How It Works Book of the Human Body 6th Edition

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Structureof the

ribcage

How domuscles

work?

Behindthe kidney

walls

N E W

HUMAN

BODY

THE

ACKED FULL OF FASCINATING FACTS, IMAGES & ILLUSTRATIONS

BOOK OF

Everything you need to know about the human body

THE BODY AT WORK CURIOUS QUESTIOHUMAN ANATOMY

down ofmmunestem

How manybones in thehuman foot?

Complexbrain

functions

Dissecting thestomach

didndsve?

AMAZING FACTS

500 OVER

Inside human h

What doesthe spinalcord do?

ise?

How yourblood works

Tour thelymphatic

system

Bonefracturehealingprocess

explained

How areteeth

formed?


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The human body is truly an amazing thing. Capable of awe-inspiring feats ofspeed and agil ity, while being mind-blowing in complexity, our bodies areunmatched by any other species on Ear th. In this new edition of the Bookof the Human Body, we explore our amazing anatomy in ne detail before

delving into the intricacies of the complex processes, functions and systemsthat keep us going. For instance, did you know you really have 16 senses?

We also explain the weirdest and most wonderful bodi ly phenomena, fromblushing to hiccuping, cramps to blisters. We will tour the human bodyfrom head to toe, using anatomical illustrations, amazing photography

and authoritative explanations to teach you more. This book wil l help youunderstand the wonder that is the human body and in no time you will beg in

to see yourself in a whole new light!

Welcome toBOOK OF

HUMAN

BODY

THE


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Imagine Publishing LtdRichmond House33 Richmond Hill

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How It Works Book of the Human Body Sixth Edition © 2016 Imagine Publishing Ltd

bookazine series

Part of the

BOOK OF

HUMANBODY

THE


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010 50 amazing body facts

018 Human cells

020 Inside a nucleus

021 What are stem cells?

022 Brain power

026 Vision and eyesight

028 How ears work

030 The tonsils031 Vocal cords

032 All about teeth

034 Anatomy of the neck

036 The human skeleton

038 The spine

040 How the body moves

042 How muscles work 044 Skin colour / Skin grafts

045 Under the skin

046 Heart attacks

047 Heart bypasses

048 The human kidneys

050 Kidney transplants

052 Vestigial organs

053 How the spleen works

054 How the liver works

056 The small intestine

058 The human ribcage

060 How the pancreas works

062 How your bladder works

064 The urinary system

066 Inside the human stomach

068 The human hand

070 Finger nails / Achilles’ tendon

071 Inside the knee

072 How your feet work

006

Human anatomy

CONTENTS

The body at work076 The science of sleep

084 The blood-brain barrier

085 Pituitary gland up close

086 Human digestion explained

088 Altitude sickness / Synapses

089 Adrenaline

090 Human respiration

092 Dehydration / Sweating

093 Scar types

094 The immune system

026

092The power of your brain

How do webreathe?

Urinarysystemexplained

064


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007

170Why is it calleda funny bone?

Curious questions142 Left or right brained?

144 Brain freeze

145 Runny nose / Comas 146 Sore throat / Ears pop /

Freckles

147 Memory / Toothpaste /Epidurals

148 Blush / Caffeine / Fainting

149 What is Tinnitus? / When doesthe brain stop growing?

150 72-hour deodorant /Modern llings

151 What powers cells?152 Can we see thoughts?

154 How anaesthesia works

155 Stomach ulcers / Mouth ulcers

156 Enzymes / Love

157 Correcting heart rhythms /Salt / Adam’s apple

158 Seasickness / Rumblingstomachs

159 Cravings160 Feet smell / Knee-jerk

reaction

161 Blisters / Cramp

162 Brain control / Laughing

163 Dandruff / Eye adjustment /Distance the eye can see

164 Allergies / Eczema

165 Growing pains / Squinting

166 What are twins?

168 Alveoli

169 Migraines / Eyedrops

170 Paper cuts / Pins andneedles / Funny bones

171 Aching muscles / Fat hormone

172 Raw meat / Inoculations /Upper arm and leg

173 What causes insomnia?

174 Hair growth / Blonde hair

appearance175 Why do we get angry?

046How do heartattacks happen?

100Healing bonefractures

98 Bone fracture healing 99 Making protein

100 The cell cycle

102 Human pregnancy

104 Embryo development

106 How we taste / Taste buds

107 What is saliva?

108 Neurotransmitters and your feelings

109 Short term memory

110 White blood cells

112 The science of genetics 117 What is anxiety? 118 Circulatory system

120 How your blood works124 Blood vessels /

Hyperventilation

125 Tracheotomy surgery

126 Hormones

128 Exploring the sensory system

132 Chickenpox

133 Why we cry

134 The other senses


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The humanhand

040 How the body moves The types of joints explained

042 How muscles work Muscle power revealed

044 Skin colour / Skin grafts Skin facts explained

045 Under the skin Anatomy of our largest organ

046 Heart attacks Why do they happen?

047 Heart bypasses How are blockages bypassed?

048 Human kidneys How do your kidneys function? 050 Kidney transplants The body’s natural lters

031 Vocal cords See how they help us talk

032 All about teeth Dental anatomy and more

034 Anatomy of the neck Impressive anatomical design

036 The human skeleton A bounty of boney facts

038 The human spine 33 vertebrae explained

HUMANANATOMY

008

Inside the eye026

021Stem cells068

024 How we think

© S

P L

010 50 amazing body facts From head to toe

018 Human cells How are they structured?

020 Inside a nucleus Dissecting a cell’s control centre

021 What are stem cells? Building block bring new life

022 Brain power About our most complex organ

026 The science of vision Inside the eye

028 How ears work Sound and balance explained

030 The tonsils What are these eshy lumps?

01050 fantasticfacts aboutthe body


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009

052 Vestigial organs Are they really useless?

053 How the spleen works Learn how it staves off infections

054 How the liver works The ultimate multitasker 056 The small intestine How does this organ work?

058 The human ribcage The protective function of the ribs

060 How the pancreas works The body’s digestive workhorse

062 How your bladder works Waste removal facts

064 The urinary system How we process waste

066 Inside the human stomach How does this organ digest food?

068 The human hand Our most versatile body part

070 Finger nails / Achilles’ tendon A look at ngernails and more

071 Inside the knee See how it allows us to walk 072 How your feet work Feet facts and stats

024 How do

we smell?

Cell structurerevealed

018

058Thehumanribcage

Inside the heart046

022Understandthe nerves

072How do ourfeet work?


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HUMAN ANATOMYHUMAN ANATOMY

50There are lots of medicalquestions everybody wantsto ask but we just neverget the chance… until now!

Amazing factsabout the

humanbody

10

The human body is the most complexorganism we know and if humanstried to build one articially, we’d

fail abysma lly. There’s more we don’tknow about the body than we do know.This includes many of the quirks andseemingly useless traits that ourspecies carry. However, not all ofthese traits are as bizarre as theymay seem, and many have anevolutionary tale behind them.

Asking these questions is onlynatural but most of us are tooembarrassed or never get theopportunity – so here’s achance to clear up all thoseniggling queries. We’ll take ahead-to-toe tour of thequirks of human biology,looking at everythingfrom tongue rolling and why we are ticklishthrough to pulledmusclesand why we dream.


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Useless body parts include the appendix, the coccyx and wisdom teethDID YOU KNOW?

What are thoughts? This question willkeep scientists, doctors andphilosophers busy for decades tocome. It all depends how you want todene the term ‘thoughts’. Scientistsmay talk about synapse formation,pattern recognition and cerebralactivation in response to a stimulus(such as seeing an apple andrecognising it as such). Philosophers,and also many scientists, will arguethat a network of neurons cannotpossibly explain the many thousandsof thoughts and emotions that wemust deal with. A sports doctor mightstate that when you choose to run, youactivate a series of well-troddenpathways that lead from your brain to your muscles in less than a second.There are some specics we do knowthough – such as which areas of yourbrain are responsible for various typesof thoughts and decisions.

1 How dowe think?

Although we’re often taught in school thattongue rolling is due to genes, the truth is

likely to be more complex. There is likelyto be an overlap of genetic factors andenvironmental inuence. Studies onfamilies and twins have shown that itcannot be a case of simple geneticinheritance. Ask around – the fact thatsome people can learn to do it suggeststhat in at least some people it’senvironmental (ie a learned behaviour)rather than genetic (inborn).

Only a small amount– hence why babiesappear so beautiful, astheir eyes are slightlyout of proportion andso appear bigger.

5 Why cansome peopleroll theirtongues butothers can’t?

3 Do eyeballsgrow like therest of the body?

Frontal lobeThe frontal lobe is where yourpersonality is, and where yourthoughts and emotions form.Removing this or damaging it canalter your persona.

Broca’sareaBroca’s area iswhere you formcomplex wordsand speechpatterns.

Pre-motor cortexThe pre-motor cortex is wheresome of your movements areco-ordinated.

Parietal lobeThe parietal lobe is responsible for

your complex sensory system.

Occipital lobeThe occipital lobe is all

the way at the back, butit interprets the light

signals in your eyes intoshapes and patterns.

Wernicke’s areaWernicke’s area is where you interpretthe language you hear, and then youwill form a response via Broca’s area.

Primary auditorycomplex

The primary auditorycomplex is right next to

the ear and is where youinterpret sound waves

into meaningfuinformation

Temporal lobeThe temporal lobe decides what todo with sound information and alsocombines it with visual data.

Primary motor cortexThe primary motor cortex and the primarysomatosensory cortex are the areas which

receive sensory innervations and thenco-ordinate your whole range of movements.

When you feel yourown pulse, you’refeeling the directtransmission of your heartbeatdown an artery. You can feel a pulse where you cancompress an arteryagainst a bone, egthe radial artery atthe wrist. Thecarotid artery can

be felt against the vertebral body, butbeware: a) presstoo hard and youcan faint, b) pressboth at the sametime and you’ll cutoff the blood to your brain and, asa protectivemechanism, you’lldenitely faint!

6 What isa pulse?

Sleep is a gift from nature, which ismore complex than you think. Thereare ve stages of sleep which representthe increasing depths of sleep – when you’re suddenly wide awake and youreyes spring open, it’s often a naturalawakening and you’re coming out ofrapid eye movement (REM) sleep; youmay well remember your dreams. If you’re coming out of a different phase,eg when your alarm clock goes off, it will take longer and you might not want to open your eyes straight away!

2 In themornings,do we wake upor open oureyes rst?

This is a behavioural response –some people play with their hair when they’re nervous or bored. Forthe vast majority of people suchtraits are perfectly normal. If theybegin to interfere with your life,behavioural psychologists can help– but it’s extremely rare that you’llend up there.

4 Why do we ddlesubconsciously?I’m constantlyplaying with my hair

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o r a

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HUMAN ANATOMY

12

The human eld of vision is just about 180degrees. The central portion of this(approximately 120 degrees) is binocular or

stereoscopic – ie both eyes contribute,allowing depth perception so that we cansee in 3D. The peripheral edges aremonocular, meaning that there is nooverlap from the other eye so we see in 2D.

The tonsils are collectionsof lymphatic tissues whichare thought to help ght offpathogens from the upperrespiratory tract. However,they themselves cansometimes becomeinfected – leading totonsillitis. The ones youcan see at the back of yourthroat are just part of thering of tonsils. You won’tmiss them if they’re takenout for recurrent infectionsas the rest of your immunesystem will compensate.

It’s different for everybody – yourage, nutrition, health status, genesand gender all play a role. In termsof length, anywhere between0.5-1 inch (1.2-2.5cm) a monthmight be considered average,but don’t be surprised if you’reoutside this range.

A burp is a naturalrelease of gas fromthe stomach. This gashas either beenswallowed or is theresult of something you’ve ingested – suchas a zzy drink. Thesound comes from the vibration of theoesophagealsphincter at theoesophago-gastric junction, which is thenarrowest part of thegastrointestinal tract.

7 What’s myeld of visionin degrees?

13 How many

inches ofhair does theaverage persongrow from theirhead each year?

12 Why dowe burp?

You’re actually hitting the ulnar nerve as it wraps around thebony prominence of the ‘humerus’ bone, leading to a ‘funny’sensation. Although not so funny as the brain interprets thissudden trauma as pain to your forearm and ngers!

10 Why does it feel so weird whenyou hit your funny bone?

3D eldThe central 120-degreeportion is the 3D part ofour vision as both eyescontribute – this is the partwe use the most.

2D eldThe areas from 120 to 180degrees are seen as 2D asonly one eye contributes, butwe don’t really notice.

Your total ‘circulating volume’ is about ve litres. Eachred blood cell within this has to go from your heart,down the motorway-like arteries, through the

back-road capillary system, and then back through therush-hour veins to get back to your heart. The processtypically takes about a minute. When you’re in a rushand your heart rate shoots up, the time reduces as theblood diverts from the less-important structures (eglarge bowel) to the more essential (eg muscles).

11 How fast doesblood travel roundthe human body?

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1. The mostimportant organThe brain has its ownspecial blood supplyarranged in a circle.

4. The inferiorvena cavaThis massive vein sitsbehind the aorta but isno poor relation –without it, bloodwouldn’t get backto your heart.

5. Thefurthest point

These arteries andveins are the furthest

away from yourheart, and blood flow

here is slow. As yougrow older, these

vessels are often thefirst to get blocked by

fatty plaques.

2. Under pressureBlood is moving fastest

and under the highestpressure as it leaves the

heart and enters theelastic aorta.

3. The kidneysThese demand a massive25 per cent of the bloodfrom each heart beat!

© S P L

Lips are predominantly used as a tactile sensory organ,typically for eating, but also for pleasure when kissing. Theyare also used to help ne-tune our voices when we speak.

9 What arelips for?

© M a t t W i l l m a n

ULNAR NERVE

8 What isthe pointof tonsils?


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1

Most of it is down to the genes that resultfrom when your parents come together tomake you. Some hair colours win out(typically the dark ones) whereas some (egblonde) are less strong in the genetic race.

17 Why do we allhave differentcoloured hair?

Your ngerprints are ne ridges ofskin in the tips of your ngers andtoes. They are useful for improvingthe detection of small vibrationsand to add friction for better grip.No two ngerprints are the same– either on your hands or betweentwo people – and that’s down to your unique set of genes.

Hair follicles in different parts of yourbody are programmed by your genes todo different things, eg the follicles on your arm produce hair much slowerthan those on your head. Men can gobald due to a combination of genes andhormonal changes, which may nothappen in other areas (eg nasal hair).It’s different for everybody!

14 Why areeveryone’sngerprintsdifferent?

16 Why, as weget older,does hair growthbecome so erratic?

Researchers have spent their whole lives trying toanswer this one. Your personality forms in the frontlobes of your brain, and there are clear personalitytypes. Most of it is your environment – that is, yourupbringing, education, surroundings. However someof it is genetic, although it’s unclear how much. Thestrongest research in this comes from studying twins– what inuences one set of twins to grow up and bebest friends, yet in another pair, one might become aprofessor and the other a murderer.

19 What gives memy personality?

20 WHY DO MENHAVE NIPPLES?Men and women are built fromthe same template, and theseare just a remnant of a man’searly development.

21 WHAT’S THEPOINT OFEYEBROWS? Biologically, eyebrows canhelp to keep sweat andrainwater from falling into your eyes. More importantly inhumans, they are key aids tonon-verbal communication.

22 WHAT IS ABELLY BUTTON?The umbilicus is where ababy’s blood flows through toget to the placenta to exchangeoxygen and nutrients with themother’s blood. Once out, theumbilical cord is clampedseveral centimetres away fromthe baby and left to fall off. Noone quite knows why you’ll getan ‘innie’ or an ‘outie’ – it’sprobably all just luck.

23 WHY DOFINGERNAILSGROW FASTER THANTOENAILS?

The longer the bone at the endof a digit, the faster the growthrate of the nail. However thereare many other influences too– nutrition, sun exposure,activity, blood supply – andthat’s just to name a few.

24 WHY DOES MYARM TINGLEAND FEEL HEAVY IF I

FALL ASLEEP ON IT?This happens because you’recompressing a nerve as you’relying on your arm. There areseveral nerves supplying theskin of your arm and threesupplying your hand (theradial, median and ulnarnerves), so depending on which part of your arm you lieon, you might tingle in yourforearm, hand or fingers.

Dreams have fascinated humansfor thousands of years. Somepeople think they are harmless while others think they are vital toour emotional wellbeing. Mostpeople have four to eight dreamsper night which are inuenced bystress, anxiety and desires, butthey remember very few of them.There is research to prove that if you awake from the rapid eyemovement (REM) part of your sleepcycle, you’re likely to remember your dreams more clearly.

15 Why dowe onlyremembersome dreams?

Your eyes remain shut as adefence mechanism to preventthe spray and nasal bacteriaentering and infecting youreyes. The urban myth that your eyes will pop out if youkeep them open is unlikelyto happen – but keepingthem shut will providesome protection againstnasty bugs and viruses.

18 Is it possible tokeep your eyesopen when you sneeze?

© T

r i s t a n

b

The average person breaks wind between 8-16 times per dayDID YOU KNOW?


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HUMAN ANATOMY

14

Your blood type is determined by protein markers known as antigens on the surface of yourred blood cells. You can have A antigens, B antigens, or none – in which case you’re blood typeO. However, if you don’t have the antigen, your antibodies will attack foreign blood. If you’re

type A and you’re given B, your antibodies attack the B antigens. However, if you’re blood type AB, you can safely receive any type. Those who are blood group O have no antigens so can giveblood to anyone, but they have antibodies to A and B so can only receive O back!

25 What makes some bloodgroups incompatible whileothers are universal?

26 What is a pulledmuscle?

A You have A antigens and Bantibodies. You can receive bloodgroups A and O, but can’t receive B. You can donate to A and AB.

B You have B antigens and Aantibodies. You can receive bloodgroups B and O, but can’t receive A. You can donate to B and AB.

AB You have A and B antigens and noantibodies. You can receive bloodgroups A, B, AB and O (universalrecipient), and can donate to AB.

O You have no antigens but have A and Bantibodies. You can receive blood groupO, but can’t receive A, B or AB and candonate to all: A, B, AB and O.

The heart is the mostefcient – it extracts80 per cent of theoxygen from blood.But the liver gets themost blood – 40 percent of the cardiacoutput compared tothe kidneys, whichget 25 per cent, andheart, which onlyreceives 5 per cent.

27 Whichorganuses up themost oxygen?

The appendix is useful in cows fordigesting grass and koala bears fordigesting eucalyptus – koalas can havea 4m (13ft)-long appendix! In humans,however, the appendix has no usefulfunction and is a remnant of ourdevelopment. It typically measures5-10cm (1.9-3.9in), but if it gets blocked itcan get inamed. If it isn’t quicklyremoved, the appendix can burst andlead to widespread infection which canbe lethal.

28 What is theappendix? I’veheard it has no usebut can kill you…

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The hamstringsThese are a group ofthree main muscles

which flex the knee.

StrainA pulled muscle, or

strain, is a tear in a grouof muscle fibres as aresult of overstretching

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This yellow discolouration of the skinor the whites of the eyes is called jaundice. It’s due to a buildup ofbilirubin in your body, when normallythis is excreted in the urine (hence why urine has a yellow tint). Diseasessuch as hepatitis and gallstones canlead to a buildup of bilirubin due toaltered physiological processes,although there are many other causes.

29 Why doespeople’sskin turn yellowif they contractliver disease?

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Though warming up can help prevensprains, they can happen to anyone

from walkers to marathon runnersPulled muscles are treated with RICE

rest, ice, compression and elevation

30 Whatis thegag reex?

1. Foreign bodiesThis is a protective mechanism to preventfood or foreign bodies entering the back ofthe throat at times other than swallowing.

2. Soft palateThe soft palate (the fleshy part of themouth roof) is stimulated, sending signalsdown the glossopharyngeal nerve.

3. Vagus nerveThe vagus nerve is stimulated,leading to forceful contractionof the stomach and diaphragmto expel the object forwards.

4. The gagThis forceful expulsionleads to ‘gagging’, whichcan develop into retchingand vomiting.


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1

Light touches, by feathers, spiders, insects or otherhumans, can stimulate ne nerve-endings in the skin which send impulses to the somatosensory cortex inthe brain. Certain areas are more ticklish – such as thefeet – which may indicate that it is a defence

mechanism against unexpected predators. It is theunexpected nature of this stimulus that means you canbe tickled. Although you can give yourself goosebumpsthrough light tickling, you can’t make yourself laugh.

Your eyelashes are formed from hair follicles, just like those on yourhead, arms and body. Each follicle is genetically programmed tofunction differently. Your eyelashes are programmed to grow to acertain length and even re-grow if they fall out, but they won’t growbeyond a certain length, which is handy for seeing!

The immune response leads to inammation and the release ofinammatory factors into your blood stream. These lead to anincreased heart rate and blood ow, which increases your core bodytemperature – as if your body is doing exercise. This can lead toincreased heat production and thus dehydration; for this reason, it’simportant to drink plenty of clear uids when you’re feeling unwell.

31 Why are weticklish?

32 Why don’t eyelasheskeep growing?

34 Couldwesurvive onvitaminsalone?

35 Why do we get ahigh temperaturewhen we’re ill?

36 WHY DOSOME PEOPLEHAVE FRECKLES? Freckles are concentrationsof the dark skin pigmentmelanin in the skin. Theytypically occur on the faceand shoulders, and are more

common in light-skinnedpeople. They are also a well-recognised genetic traitand become more dominantduring sun-exposure.

37 WHAT ISA WART? Warts are small, rough,round growths of the skincaused by the humanpapilloma virus. There aremany different types whichcan occur in different partsof the body, and they can becontagious. They commonlyoccur on the hands, but canalso come up anywhere fromthe genitals to the feet!

38 WHY DO ITWITCH INMY SLEEP? This is common and knownin the medical world as amyoclonic twitch. Althoughsome researchers say thesetwitches are associated withstress or caffeine use, theyare likely to be a natural partof the sleep process. If ithappens to you, it’s perfectlynormal.

No, you need a dietbalanced incarbohydrate,protein, fat, vitamins andminerals to survive. You can’t cut one ofthese and expect tostay healthy.However, it’s theproportions of these which keep ushealthy and t. Youcan get these fromthe ve major foodgroups. Food chartscan help with this

balancing act.

33 Whatmakes usleft-handed?One side of the brain istypically dominant over theother. Since each hemisphereof the brain controls theopposite side (ie the leftcontrols the right side of yourbody), right-handed peoplehave stronger left brain

hemispheres. Occasionally you’ll nd an ambidextrousperson, where hemispheresare co-dominant, and thesepeople are equally capable with both right and left hands!

© L o y n a

© s h l o m i t g

© J e i n n y S o l i s

Your brain interprets pain from the rest of the body, but doesn’t have any pain receptors itself DID YOU KNOW?


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HUMAN ANATOMY

16

The heart keeps itself beating. Thesinoatrial node (SAN) is in the wall of theright atrium of the heart, and is where theheartbeat starts. These beats occur due tochanges in electrical currents as calcium,sodium and potassium move acrossmembranes. The heart can beat at a rate of60 beats per minute constantly if left alone.However – we often need it to go faster. Thesympathetic nervous system sends rapidsignals from the brain to stimulate theheart to beat faster when we need it to – in‘ght or ight’ scenarios. If the SAN fails, apacemaker can send articial electricalsignals to keep the heart going.

Blood doesn’t circulate around your body asefciently when you’re asleep so excess water canpool under the eyes, making them puffy. Fatigue,nutrition, age and genes also cause bags.

A bruise forms when capillaries under the skin leak and allowblood to settle in the surrounding tissues. The haemoglobin ined blood cells is broken down, and these by-products give a

dark yellow, brown or purple discolouration depending on thevolume of blood and colour of the overlying skin. Despitepopular belief, you cannot age a bruise – different people’sbruises change colour at different rates.

Onions make your eyes water due to their expulsion ofan irritant gas once cut. This occurs as when an onionis cut with a knife, many of its internal cells are brokendown, allowing enzymes to break down amino acidsulphoxides and generate sulphenic acids. Thesesulphenic acids are then rearranged by anotherenzyme and, as a di rect consequence, syn-propanethial-S-oxide gas is produced, which is volatile.This volatile gas then diffuses in the air surroundingthe onion, eventually reaching the eyes of the cutter, where it proceeds to activate sensory neurons andcreate a stinging sensation. As such, the eyes thenfollow protocol and generate tears from their tearglands in order to dilute and remove the irritant.Interestingly, the volatile gas generated by cuttingonions can be largely mitigated by submerging theonion in water prior to or midway through cutting, with the liquid absorbing much of the irritant.

39 What triggersthe heart andkeeps it beating?

43 When we’retired, why dowe get bags underour eyes?

40 Why do bruises gopurple or yellow? 41 Whydoescuttingonions makeus cry?

DenitionsSystole = contractionDiastole = relaxation3. Ventricular diastole

The heart is now relaxed and can

refill, ready for the next beat.

1. Atrial systoleThe atria are thelow-pressure upper

chambers, and are thefirst to contract, emptyingblood into the ventricles.

2. Ventricular systoleThe ventricles contract next,

and they send high-pressureblood out into the aorta tosupply the body.

3 x © S P L

‘Simple’ male pattern baldness is dueto a combination of genetic factorsand hormones. The most implicatedhormone is testosterone, which menhave high levels of but women havelow levels of, so they win (or lose?) inthis particular hormone contest!

44 Why domoremen go baldthan women?

42 What isthe littletriangle shapeon the side ofthe ear?This is the tragus. It servesno major function that weknow of, but it may help toreect sounds into the earto improve hearing.

3. DiscolourationHaemoglobin is thenbroken down into itssmaller components, whichare what give the darkdiscolouration of a bruise.

Blood leaksto the skinood settles into thesues surrounding thessel. The pressurem the bruise thenps stem the bleeding.

Damage to theood vesselster trauma such as a fall,small capillaries are

n and burst.

© L a l i M a s r i e r a

© David Benbennick


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1

Genes work in pairs. Some genes are‘recessive’ and if paired with a‘dominant’ half, they won’t shinethrough. However, if two recessivegenes combine (one from yourmother and one from your father),the recessive trait will show through.

Blinking helps keep your eyes clean and moist. Blinkingspreads secretions from the tear glands (lacrimal uids)over the surface of the eyeball, keeping it moist and alsosweeping away small particles such as dust.

The gluteus maximus is the largest muscle and forms the bulk of your buttock. The heart (cardiacmuscle) is the hardest-working muscle, as it is constantly beating and clearly can never take a break!However the strongest muscle based on weight is the masseter. This is the muscle that clenches the jaw shut – put a nger over the lowest, outer part of your jaw and clench your teeth and you’ll feel it.

48 Why do somehereditaryconditions skip ageneration?

45 Why do

we blink?

50 Which muscle produces themost powerful contractionrelative to its size?

1. Taking the rst stepMuscle contraction starts with an impulse received from thenerves supplying the muscle – an action potential. Thisaction potential causes calcium ions to flood across theprotein muscle fibres. The muscle fibres are formed from twokey proteins: actin and myosin.

2. PreparationThe calcium binds to troponin which is a receptor onthe actin protein. This binding changes the shape oftropomyosin, another protein which is bound to actin.These shape changes lead to the opening of a series ofbinding sites on the actin protein.

3. BindingNow the binding sites are free on actin, the myosin headsforge strong bonds in these points. This leads to thecontraction of the newly formed protein complex; when allof the proteins contract, the muscle bulk contracts.

4. Unbinding When the energy runs out, the proteins lose theirstrong bonds and disengage, and from there theyreturn to their original resting state.

Itching is caused by the release of atransmitter called histamine frommast cells which circulate in your body.These cells are often released in

response to a stimulus, such as a beesting or an allergic reaction. They leadto inammation and swelling, andsend impulses to the brain via nerves which causes the desire to itch.

47 Why do weget itchy?

This is ‘phantom limb pain’ and can range from a mildannoyance to a debilitating pain. The brain cansometimes struggle to adjust to the loss of a limb, andit can still ‘interpret’ the limb as being there. Since thenerves have been cut, it interprets these new signalsas pain. There isn’t a surgical cure as yet, though timeand special medications can help lessen the pain.

49 Why do amputeessometimes stillfeel pain in theiramputated limbs?

Most people’s feet are different sizes – in fact the twohalves of most people’s bodies are different! We all startfrom one cell, but as the cells multiply, genes give them varying characteristics.

46 How come mostpeople have one footlarger than the other?

Myosin head Actin lament Actin lamentis pulled

Cross bridgedetaches

Energised myosinhead

There are many home remedies for baggy eyes, including tea bags, potatoes and cold spoonsDID YOU KNOW?


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HUMAN ANATOMY

C ells are life and cells are alive. You are here because every cellinside your body has a specicfunction and a very specialised job todo. There are many different types ofcell, each one working to keep thebody’s various systems operating. Asingle cell is the smallest unit of livingmaterial in the body capable of life.When grouped together in layers orclusters, however, cells with similar jobs to do form tissue, such as skin ormuscle. To keep these cells working,there are thousands of chemicalreactions going on all the time.

All animal cells contain a nucleus, which acts like a control hub telling thecell what to do and contains the cell’sgenetic information (DNA). Most of thematerial within a cell is a watery, jelly-like substance called cytoplasm(cyto means cell), which circulatesaround the cell and is held in by a thinexternal membrane, which consists oftwo layers. Within the cytoplasm is a variety of structures called organelles, which all have different tasks, such asmanufacturing proteins – the cell’s keychemicals. One vital example of anorganelle is a ribosome; these numerousstructures can be found either oatingaround in the cytoplasm or attached tointernal membranes. Ribosomes arecrucial in the production of proteinsfrom amino acids.

In turn, proteins are essential tobuilding your cells and carrying out thebiochemical reactions the body needs inorder to grow and develop and also torepair itself and heal.

Cell structureexplainedThere are around 75 trillion cellsin the human body, but what arethey and how do they work?

Cell membraneSurrounding and supporting

each cell is a plasma membranethat controls everything that

enters and exits.

NucleusThe nucleus is the cell’s ‘brain’or control centre. Inside thenucleus is DNA information,which explains how to makethe essential proteins neededto run the cell.

MitochondriaThese organelles supply cells with the energynecessary for them to carry out their functions.The amount of energy used by a cell is measuredin molecules of adenosine triphosphate (ATP).Mitochondria use the products of glucosemetabolism as fuel to produce the ATP.

Golgi bodyAnother organelle, the Golgi body is onethat processes and packages proteins,including hormones and enzymes, fortransportation either in and around thecell or out towards the membrane forsecretion outside the cell where it canenter the bloodstream.

RibosomesThese tiny structures make proteins andcan be found either floating in thecytoplasm or attached like studs to the

endoplasmic reticulum, which is a conveyorbelt-like membrane that transports proteinsaround the cell.

Endoplasmic reticulumThe groups of folded membranes (canals)connecting the nucleus to the cytoplasm arecalled the endoplasmic reticulum (ER). Ifstudded with ribosomes the ER is referred toas ‘rough’ ER; if not it is known as ‘smooth’ER. Both help transport materials around thecell but also have differing functions.

Rough endoplasmicreticulum (studdedwith ribosomes)

Smoothendoplasmicreticulum


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Bacteria are the simplest living cells and the most widespread life form on EarthDID YOU KNOW?

CytoplasmThis is the jelly-like

substance – made of

water, amino acids andenzymes – found inside

the cell membrane.Within the cytoplasm are

organelles such as thenucleus, mitochondria

and ribosomes, eachof which performs a

specific role, causingchemical reactions in

the cytoplasm.

LysosomesThis digestive enzyme breaks down

unwanted substances and worn-outorganelles that could harm the cell by

digesting the product and thenejecting it outside the cell.

Pore

Cell anatomy

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NERVE CELLSThe cells that make up the nervoussystem and the brain are nerve cellsor neurons. Electrical messagespass between nerve cells alonglong filaments called axons. Tocross the gaps between nervecells (the synapse) that electricalsignal is converted into a chemicalsignal. These cells enable us to feelsensations, such as pain, and theyalso enable us to move.

BONE CELLSThe cells that make up bone matrix – the hardstructure that makes bones strong – consist of threemain types. Your bone mass is constantly changingand reforming and each of the three bone cells playsits part in this process. First the osteoblasts, whichcome from bone marrow, build up bone mass andstructure. These cells then become buried in the

matrix at which point they becomeknown as osteocytes. Osteocytes

make up around 90 per cent ofthe cells in your skeleton andare responsible formaintaining the bonematerial. Finally, while theosteoblasts add to bone mass,osteoclasts are the cells

capable of dissolving bone andchanging its mass.

PHOTORECEPTOR CELLSThe cones and rods on the retina at the back of theeye are known as photoreceptorcells. These contain light-sensitive pigments thatconvert the image thatenters the eye into nervesignals, which the braininterprets as pictures. Therods enable you to perceivelight, dark and movement, while the cones bring colourto your world.

LIVER CELLSThe cells in your liver are responsible for regulating the

composition of your blood. These cellsfilter out toxins as well as controlling

fat, sugar and amino acid levels. Around 80 per cent of the liver’s

mass consists of hepatocytes,

which are the liver’sspecialised cells that areinvolved with the production of

proteins and bile.

MUSCLE CELLSThere are three types of muscle

cell – skeletal, cardiac and smooth – andeach differs depending on the functionit performs and its location in thebody. Skeletal muscles contain longfibres that attach to bone. Whentriggered by a nerve signal, themuscle contracts and pulls the bone with it, making you move. We cancontrol skeletal muscles because

they are voluntary. Cardiac muscles, meanwhile,are involuntary, which is fortunate because

they are used to keep your heart beating.Found in the walls of the heart, these musclescreate their own stimuli to contract withoutinput from the brain. Smooth muscles, which are pretty slow and also involuntary,make up the linings of hollow structures

such as blood vessels and your digestivetract. Their wave-like contraction aids the

transport of blood around the body and thedigestion of food.

FAT CELLSThese cells – also known asadipocytes or lipocytes – makeup your adipose tissue, orbody fat, which cancushion, insulate andprotect the body. Thistissue is found beneath your skin and alsosurrounding your otherorgans. The size of a fatcell can increase ordecrease depending on theamount of energy it stores. If wegain weight the cells fill with more watery fat, and eventually the number of fat cells willbegin to increase. There are two types of adiposetissue: white and brown. The white adipose tissuestores energy and insulates the body by maintainingbody heat. The brown adipose tissue, on the otherhand, can actually create heat and isn’t burned forenergy – this is why animals are able to hibernate formonths on end without food.

EPITHELIAL CELLSEpithelial cells make up the epithelial tissue thatlines and protects your organs andconstitute the primarymaterial of your skin.These tissues form a

barrier between theprecious organs and

unwanted pathogens orother fluids. As well ascovering your skin, you’llfind epithelial cells inside your nose, around your lungsand in your mouth.

RED BLOOD CELLSUnlike all the other cells in yourbody, your red blood cells (alsoknown as erythrocytes) donot contain a nucleus. Youare topped up witharound 25 trillion redblood cells – that’s a thirdof all your cells, making

them the mostcommon cell in

your body. Formedin the bone marrow,these cells areimportant because they carry oxygen to allthe tissues in your body. Oxygen is carriedin haemoglobin, a pigmented protein that

gives blood cells their red colour.

Types of human cellSo far around 200 different varieties of cell have been

identied, and they all have a very specic function toperform. Discover the main types and what they do…

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Prokaryotic cells are much more basic thantheir eukaryotic counterparts. Up to 100 timessmaller and mainly comprising species of

bacteria, prokaryotic cells have fewerfunctions than other cells, so they do notrequire a nucleus to act as the control centre forthe organism.

Instead, these cells have their DNA movingaround the cell rather than being housed in anucleus. They have no chloroplasts, nomembrane-bound organelles and they don’tundertake cell division in the form of mitosis ormeiosis like eukaryotic cells do.

Prokaryotic cells divide asexually with DNAmolecules replicating themselves in a processknown as binary ssion.

How do cellssurvive withouta nucleus?

Take a peek at what’s happening insidethe ‘brain’ of a eukaryotic cell

Central command

Explore the larger body that anucleus rules over and meetits ‘cellmates’

Nucleus in context

S urrounded by cytoplasm, the nucleuscontains a cell’s DNA and controls allof its functions and processes such asmovement and reproduction.

There are two main types of cell:eukaryotic and prokaryotic. Eukaryotic cellscontain a nucleus while prokaryotic do not.Some eukaryotic cells have more than onenucleus – called multinucleate cells –occurring when fusion or division createstwo or more nuclei.

At the heart of a nucleus you’ll nd thenucleolus; this part icular area is essential in

the formation of ribosomes. Ribosomes are

responsible for making proteins out of aminoacids which take care of growth and repair.

Being so important, the nucleus is the mostprotected part of the cell. In animal cells it isalways located near its centre and away fromthe membrane to ensure it has the maximumcushioning. As well as the jelly-likecytoplasm around it, the nucleus itself islled with nucleoplasm, a viscous liquid which maintains its structural integrity.

Conversely, in plant cells, the nucleus ismore sporadically placed. This is due to thelarger vacuole in a plant cell and the added

protection that is granted by a cell wall.

Dissecting the control centre of a cell

Inside a nucleus

1 Nuclear poreThese channels control the movement of molecules

between the nucleus and cytoplasm.

3 NucleolusMade up of protein and RNA, this is the heart of the

nucleus which manufactures ribosomes.

2 Nuclear envelopeActs as a wall to protect the DNA within the nucleus

and regulates cytoplasm access.

4 NucleoplasmThis semi-liquid, semi-jelly material surrounds the

nucleolus and keeps the organelle’s structure.

5 ChromatinProduces chromosomes and aids cell division bycondensing DNA molecules.

RibosomesMade up of two separateentities, ribosomes makeproteins to be used bothinside and outside the cell.

Nucleus

Golgi apparatusNamed after the Italianbiologist Camillo Golgi,they create lysosomesand also organise theproteins for secretion.

MitochondrionDouble membraned,this produces energy forthe cell by breakingdown nutrients viacellular respiration.

1

2

3

4

5

LysosomeSmall and spherical,this organelle containsdigestive enzymes thatattack invading bacteria.


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S tem cells are incrediblyspecial because they havethe potential to becomeany kind of cell in the body, fromred blood cells to brain cells. Theyare essential to life and growth, asthey repair tissues and replacedead cells. Skin, for example, isconstantly replenished by skinstem cells.

Stem cells begin their life cycleas generic, featureless cells thatdon’t contain tissue-specicstructures, such as the ability tocarry oxygen. Stem cells becomespecialised through a processcalled differentiation. This istriggered by signals inside andoutside the cell. Internal signalscome from strands of DNA thatcarry information for all cellularstructures, while external signalsinclude chemicals from nearbycells. Stem cells can replicatemany times – known as

proliferation – while others suchas nerve cells don’t divide at all.

There are two stem cell types,as Professor Paul Fairchild,co-director of the Oxford Stem CellInstitute at Oxford Martin Schoolexplains: “Adult stem cells aremultipotent, which means theyare able to produce numerouscells that are loosely related, suchas stem cells in the bone marrowcan generate cells that make upthe blood,” he says. “In contrast,pluripotent stem cells, found within developing embryos, areable to make any one of theestimated 210 cell types that makeup the human body.”

This fascinating ability totransform and divide has madestem cells a rich source formedical research. Once their truepotential has been harnessed,they could be used to treat a hugerange of diseases and disabilities.

What are stem cells?Understand how these building blocks bring new life

Cloning cellsScientists can reprogram cells toforget their current role andbecome pluripotent cellsindistinguishable from earlyembryonic stem cells. Inducedpluripotent stem cells (IPSCs) can beused to take on the characteristics ofnearby cells.

IPSCs are more reliable than stemcells grown from a donated embryobecause the body is more likely toaccept self-generated cells. IPSCs cantreat degenerative conditions such asParkinson’s disease and baldness, which are caused by cells dying without being replaced. The IPSCs llthose gaps in order to restore thebody’s systems.

Professor Fairchild explains, “byderiving these cells from individuals with rare conditions, we are able tomodel the condition in the laboratoryand investigate the effects of newdrugs on that disease.“

A stem cell surrounded byred blood cells. Soon it

could become one of them

Stem cells have the ability to self-renew DID YOU KNOW?


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HUMAN ANATOMY

I t’s a computer, a thinking machine, a pink organ, and a vast collection ofneurons – but how does it work? The human brain is amazingly complex– in fact, more complex than anything in the known universe. The brain

effortlessly consumes power, stores memories, processes thoughts, andreacts to danger.

In some ways, the human brain is like a car engine. The fuel – which couldbe the sandwich you had for lunch or a sugar doughnut for breakfast –causes neurons to re in a logical sequence and to bond with otherneurons. This combination of neurons occurs incredibly fast, but thechain reaction might help you compose a symphony or recall entirepassages of a book, help you pedal a bike or write an email to a friend.

Scientists are just beginning to understand how these brain neurons work – they have not gured out how they trigger a reaction when youtouch a hot stove, for example, or why you can re-generate brain cells when you work out at the gym.

The connections inside a brain are very similar to the internet – theconnections are constantly exchanging information. Yet, even the internet israther simplistic when compared to neurons. There are ten to 100 neurons, andeach one makes thousands of connections. This is how the brain processesinformation, or determines how to move an arm and grip a surface. Thesecalculations, perceptions, memories, and reactions occur almostinstantaneously, and not just a few times per minute, but millions. Accordingto Jim Olds, research director with George Mason University, if the internet were as complex as our solar system, then the brain would be as complex asour galaxy. In other words, we have a lot to learn. Science has not given uptrying, and has made recent discoveries about how we adapt, learn newinformation, and can actually increase brain capability.

In the most basic sense, our brain is the centre of all input and outputs in thehuman body. Dr Paula Tallal, a co-director of neuroscience at RutgersUniversity, says the brain is constantly processing sensory information – evenfrom infancy. “It’s easiest to think of the brain in terms of inputs and outputs,”says Tallal. “Inputs are sensory information, outputs are how our brainorganises that information and controls our motor systems.”

Tallal says one of the primary functions of the brain is in learning to predict what comes next. In her research for Scientic Learning, she has found that young children enjoy having the same book read to them again and againbecause that is how the brain registers acoustic cues that form into phonemes(sounds) to become spoken words.

“We learn to put things together so that they become smooth sequences,”she says. These smooth sequences are observable in the brain, interpreting

The human brain is the most

mysterious – and complex –entity in the known universe

HypothalamusControls metabolic functions such asbody temperature, digestion,breathing, blood pressure, thirst,hunger, sexual drive, pain relays, andalso regulates some hormones.

Parts ofthe brain

So what are the parts of the brain? According

to Olds, there are almost too many to count– perhaps a hundred or more, depending on

who you ask. However, there are some keyareas that control certain functions and store

thoughts and memories.

Yourbrain

Basal ganglia (unseen)Regulates involuntary movementssuch as posture and gait when wewalk, and also regulates tremors andother irregularities. This is thesection of the brain whereParkinson’s Disease can develop.


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CerebellumConsists of two cerebralhemispheres that controls motoractivity, the planning ofmovements, co-ordination, andother body functions. This sectionof the brain weighs about 200grams (compared to 1,300 gramsfor the main cortex).

“In a sense, the main function ofthe brain is in ordering information

– interpreting the outside world andmaking sense of it”

Limbic systemThe part of the brain thatcontrols intuitive thinking,emotional response,sense of smell and taste.

the outside world and making sense of it. The brainis actually a series of interconnected‘superhighways’ or pathways that move ‘data’ fromone part of the body to another.

Tallal says another way to think about the brainis by lower and upper areas. The spinal cord movesinformation up to the brain stem, then up into thecerebral cortex which controls thoughts andmemories. Interestingly, the brain really does worklike a powerful computer in determining not onlymovements but registering memories that can bequickly recalled.

According to Dr Robert Melillo, a neurologistand the founder of the Brain Balance Centers( www.brainbalancecenters.com ), the brainactually predetermines actions and calculates theresults about a half-second before performing

them (or even faster in some cases). This meansthat when you reach out to open a door, yourbrain has already predetermined how to move your elbow and clasp your hand around the door

handle – maybe even simulated this movementmore than once, before you even actually performthe action.

Another interesting aspect to the brain is thatthere are some voluntary movements and someinvoluntary. Some sections of the brain mightcontrol a voluntary movement – such as patting your knee to a beat. Another section controlsinvoluntary movements, such as the gait of your walk – which is passed down from your parents.Reexes, long-term memories, the pain reex –these are all aspects that are controlled by sectionsin the brain.

Functions of thecerebral cortex

Prefrontal cortexExecutive functions such as complexplanning, memorising, social and verbalskills, and anything that requiresadvanced thinking and interactions. Inadults, helps us determine whether an

action makes sense or is dangerous.

Parietal lobeWhere the brain sensestouch and anything that

interacts with the surfaceof the skin, makes usaware of the feelings

of our body andwhere we are

in space

Frontal lobePrimarily controls sensessuch as taste, hearing, andsmell. Association areasmight help us determinelanguage and the tone ofsomeone’s voice.

Temporal lobeWhat distinguishes the human

brain – the ability to processand interpret what other parts

of the brain are hearing,sensing, or tasting and

determine a response.

The cerebral cortex is the wrinklingpart of our brain that shows up when you see pictures of the brain

Complexmovements

Problemsolving

Skeletal movement

Analysis ofsounds

Cerebral cortexThe ‘grey matter’ of the brain controls

cognition, motor activity, sensation, andother higher level functions. Includes

the association areas which helpprocess information. Theseassociation areas are what

distinguishes the humanbrain from other brains.

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Touch and skin

sensations

Language

Receivessignals

from eyes

Analysis ofsignal from eyes

Speech

Hearing

The average human brain is 140mm wide x 167mm long x 93mm highDID YOU KNOW?


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24

NeuronsexplainedNeurons re like electrical circuitsNeurons are a kind of cell in the brain (humans have many cells inthe body, including fat cells, kidney cells, and gland cells). Aneuron is essentially like a hub that works with nearby neurons togenerate an electrical and chemical charge. Dr Likosky of theSwedish Medical Institute says another way of thinking aboutneurons is that they are like a basketball and the connections(called axons) are like electrical wires that connect to otherneurons. This creates a kind of circuit in the human body. Tallalexplained that input from the ve senses in the body causeneurons to re.

“The more often a collection of neurons are stimulated togetherin time, the more likely they are to bind together and the easierand easier it becomes for that pattern of neurons to re insynchrony as well as sequentially,” says Tallal.

NeuronA neuron is a nerve cell in

the brain that can beactivated (usually by

glucose) to connect withother neurons and form a

bond that triggers anaction in the brain.

NeurotransmitterA neurotransmitter is the

electro-chemical circuitthat carries the signal from

one neuron to anotheralong the axon.

A thin synapseA thin synapse

(measuring just a fewnanometres) betweenthe neurotransmitter,

carried along the axon inthe brain, forms the

electro-chemicalconnection.

In pictures that we are all accustomed to seeing, the humanbrain often looks pink and spongy, with a sheen of slime. According to Dr William Likosky, a neurologist at the SwedishMedical Institute ( www.swedish.org ), the brain is actuallyquite different from what most people would immediatelythink it is.

Likosky described the brain as being not unlike feta cheesein appearance – a fragile organ that weighs about 1,500 gramsand sags almost like a bag lled with water.

In the skull, the brain is highly protected and has hardtissue, but most of the fatty tissue in the brain – which helpspass chemicals and other substances through membranes –is considerably more delicate.

What is mybrain like?If you could hold it in your hand…

Brain mapsTrackVis generates unique maps of the brain

TrackVis is a free program used by neurologists to see a map of the brain thatshows the bre connections. On every brain, these neural pathways help

connect one part of the brain to another so that a feeling you experience in onepart of the brain can be transmitted and processed by another part of the brain

(one that may decide the touch is harmful or pleasant). TrackVis uses fMRIreadings on actual patients to generate the colourful and eye-catching images.

To construct the maps, the program can take several hours to determine exactlyhow the bres are positioning in the brain.

The computers used togenerate the TrackVismaps might use up to1,000 graphics processorsthat work in tandem toprocess the data.

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“The brain - a fragileorgan that weighsabout 1,500 grams”


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How donerveswork?Nerves carry signals throughout thebody – a chemical superhighway Nerves are the transmission cables that carry brain waves in thehuman body, says Sol Diamond, an assistant professor at the ThayerSchool of Engineering at Dartmouth. According to Diamond, nervescommunicate these signals from one point to another, whether from your toenail up to your brain or from the side of your head.

Nerve transmissionsSome nerve transmissions travel greatdistances through the human body,others travel short distances – both usea de-polarisation to create the circuit.De-polarisation is like a wound-upspring that releases stored energy onceit is triggered.

Myelinated andun-mylinatedSome nerves are myelinated(or insulated) with fatty tissuethat appears white and forms aslower connection over alonger distance. Others areun-myelinated and areun-insulated. These nervestravel shorter distances.

What does thespinal cord do?The spinal cord actuallyis part of the brain andplays a major roleScientists have known for the

past 100 years or so that thespinal cord is actually part ofthe brain. According toMelillo, while the brain hasgrey matter on the outside(protected by the skull) andprotected white matter onthe inside, the spinal cord isthe reverse: the grey matter isinside the spinal cord and the white matter is outside.

Grey matter cellsGrey matter cells in the spinal cordcannot regenerate, which is why

people with a serious spinal cord injurycannot recover over a period of time.White matter cells can re-generate.

White matter cellsWhite matter cells in the spinal cordcarry the electro-chemical pulses up tothe brain. For example, when you arekicked in the shin, you feel the pain inthe shin and your brain then tells youto move your hand to cover that area.

NeuroplasticityIn the spinal cord and in the brain, cells

can rejuvenate over time when youexercise and become strengthened. This

process is called neuroplasticity.

NeurogenesisAccording to Tallal, by repeating brain

activities such as memorisation andpattern recognition, you can grow new

brain cells in the spinal cord and brain.

Neuronal bre

tracts

Spinal nerveNerve root

Spinal cord coreIn the core of the spinal cord, grey matter

– like the kind in the outer layer of thebrain – is for processing nerve cells such

as touch, pain and movement.

Nerve triggersWhen many neurons are activated togetherat the same time, the nerve is excited – this

is when we might feel the sensation oftouch or a distinct smell.

The adult human brain weighs about 1.4kg (or three pounds)DID YOU KNOW?


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HUMAN ANATOMY

T he structure of t he human eye is socomplex that it’s hard to believe thatit’s not the product of intelligent

design. But by looking at the eyes of otheranimals, scientists have shown that itevolved very gradually from a simplelight-dark sensor over the course of around100 million years.

It functions in a very similar way to acamera, with an opening through which thelight enters, a lens for focusing and a li ght-sensitive membrane at the back.

The amount of light that enters the eye iscontrolled by the circular and radial muscles

in the iris, which contract and rela x toalter the size of the pupil. The light rstpasses through a tough protective sheet

called the cornea, and then moves into thelens. This adjustable structure bends thelight, focusing it down to a point on theretina, at the back of the eye.

The retina is covered in mill ions oflight-sensitive receptors known as rodsand cones. Each receptor containspigment molecules, which change shape when they are h it by light, tr igger ing anelectrical message that travels to thebrain via the optic nerve.

Inside thehuman eyeUncovering one of the most complexconstructs in the natural world

Seeing in three dimensionsOur eyes are only able to produce two-dimensional images, but with some cleverprocessing, the brain is able to build these at pictures into a three-d imensional view. Our eyes are positioned about ve centimetres (two inches) apart, so each seesthe world from a slightly different angle. The brain compares the two pictures,using the differences to create the illusion of depth.

Each eye sees a slightly different image, allowing the brain to perceive depth

Individual imageDue to the positioning of our eyes,when objects are closer than about5.5m (18ft) away, each eye sees aslightly different angle.

Combined imageThe incoming signals from botheyes are compared in the brain, andthe subtle differences are used tocreate a three-dimensional image.

Try it for yourselfBy holding your hand in front ofyour face and closing one eye at atime, it is easy to see the different2D views perceived by each eye.

IrisThis circular musclecontrols the size ofthe pupil, allowing itto be closed down inbright light, or openedwide in the dark.

RetinaThe retina is covered inreceptors that detect light.It is highly pigmented,preventing the light fromscattering and ensuring acrisp image.

Optic nerveSignals from the retinatravel to the brain via the

optic nerve, a bundle ofbres that exitsthrough the backof the eye.

Blind spotAt the position where theoptic nerve leaves the eye,there is no space for lightreceptors, leaving a naturalblind spot in our vision.

FoveaThis pit at the centre of theback of the eye is rich in lightreceptors and is responsiblefor sharp central vision.


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285 million people in the world are estimated to be visually impaired and 39 million of them are blindDID YOU KNOW?

Protection

The most common problems withour eyesight

PupilThe pupil is a hole thatallows light to reachthe back of the eye.

LensThe lens is responsible forfocusing the light, and canchange shape toaccommodate objectsnear and far from the eye.

Ciliary bodyThis tissue surrounds thelens and contains themuscles responsible forchanging its shape.

CorneaThe pupil and iris arecovered in a tough,transparentmembrane, whichprovides protectionand contributes tofocusing the light.

EyelashesEyelashes not only catchdust before it enters theeye, they are also sensitive,like whiskers, and theslightest unexpected touchtriggers a protective blink.

Lachrymal glandTears are produced hereand wash across to theinner corner of the eye,helping to clean andnourish the surface.

ScleraA tough white membraneknown as the sclera helpsto maintain the eye’sspherical shape.

Vision problems

Farsightedness (hyperopia)If the eye is too short, the cornea is too at, or if

the lens sits too far back, incoming light isfocused behind the retina, making nearbyobjects appear blurry, particularly in the dark.

Nearsightedness (myopia)If the eye is too long, or the cornea and lens aretoo curved, the light is focused before it hits theback of the eye, and then start s to defocusagain as it reaches the retina, making distantobjects difcult to see.

Colour-blindnessThis rare condition is often linked to a gene onthe X-chromosome and occurs more commonlyin men than in women. A defect in the conecells of the eye reduces the number of coloursthat can be detected.

The eyes are shielded by several layers of protection.They are almost completely encased in bone at the backand insulated from shock by layers of muscle andconnective tissue. The front is kept moist with tears andare constantly wiped by the blinking of the eyelids, whilethe hairs of the eyebrows and eyelashes catch any debristhat might fall in.

EyebrowsThe arch of the eyebrows helps

to keep sweat and rain awayfrom the eyes, channelling it

down the sides of the face.


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HUMAN ANATOMY

T he thing to remember when learningabout the human ear is that sound is al labout movement. When someone speaksor makes any kind of movement, the air aroundthem is disturbed, creating a sound wave ofalternating high and low frequency. These wavesare detected by the ear and interpreted by thebrain as words, tunes or sounds.

Consisting of air-lled cavities, labyrinthineuid-lled channels and highly sensitive cells,the ear has external, middle and internal parts.The outer ear consists of a skin-covered exiblecartilage ap called the ‘auricle’, or ‘pinna’. Thisfeature is shaped to gather sound waves andamplify them before they enter the ear forprocessing and transmission to the brain. Therst thing a sound wave entering the earencounters is the sheet of tightly pulled tissueseparating the outer and middle ear. This tissue isthe eardrum, or tympanic membrane, and it vibrates as sound waves hit it.

Beyond the eardrum, in the air-lled cavity ofthe middle ear, are three tiny bones called the‘ossicles’. These are the smallest bones in yourbody. Sound vibrations hitting the eardrum passto the rst ossicle, the malleus (hammer). Next the waves proceed along the incus (anvil) and then onto the (stapes) stirrup. The stirrup presses againsta thin layer of tissue called the ‘oval window’, andthis membrane enables sound waves to enter t heuid-lled inner ear.

The inner ear is home to the cochlea, whichconsists of watery ducts that channel the vibrations, as ripples, along the cochlea’sspiralling tubes. Running through the middle ofthe cochlea is the organ of Corti, which is lined with minute sensory hai r cells that pick up on the vibrations and generate nerve impulses that aresent to the brain as electrical signals. The braincan interpret these signals as sounds.

HowearsworkThe human ear performs arange of functions, sendingmessages to the brain when asound is made while alsoproviding your body with asense of balance

Structureof the ear

Auricle (pinna)This is the visible part

of the outer ear thatcollects sound wavevibrations and directsthem into the ear.

External acousticmeatus (outerear canal)This is the wax-lined tubethat channels soundvibrations from the outerpinna through the skull tothe eardrum.

Tympanic membrane(eardrum)

The slightly concave thin layer of skinstretching across the ear canal and

separating the outer and middle ear.Vibrations that hit the eardrum are

transmitted as movement to thethree ossicle bones.

Malleus(hammer)

One of the three ossicles,this hammer-shapedbone connects to the

eardrum and moves withevery vibration bouncing

off the drum.

Scala vestibuli(vestibular canal)Incoming vibrations travelalong the outer vestibularcanal of the cochlea.

Cochlear ductThe cochlear duct separates thetympanic and vestibular canals.The organ of Corti is found here.


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Where you can ndthe three pairs oftonsils in your head

Tonsillocations

T onsils are the small masses of esh foundin pairs at the back of the throats of manymammals. In humans the word is actuallyused to describe three sets of this spongylymphatic tissue: the lingual tonsils, thepharyngeal tonsils and the more commonlyrecognised palatine tonsils.

The palatine tonsils are the oval bits that hangdown from either side at the back of your throat – you can see them if you look in the mirror. Although the fu ll purpose of the palatine tonsilsisn’t yet understood, because they produceantibodies and because of their prominentposition in the throat, they’re thought to be therst line of defence against potential infection inboth the respiratory and digestive tracts.

The pharyngeal tonsils are also known as theadenoids. These are found tucked away in thenasal pharynx and serve a similar purpose to thepalatine tonsils but shrink in adulthood.

The lingual tonsils are found at the back of thetongue towards the root and, if you poke yourtongue right out, you should spot them. These aredrained very efciently by mucous glands so they very rarely get infected.

What purpose do these fleshy lumpsin the back of our throats serve?

What aretonsils for?

Tonsillitis is caused by certain bacteria (eggroup A beta-haemolytic streptococci), andsometimes viral infections, that result in asore and swollen throat, a fever, white spots atthe back of the throat and difculty

swallowing. Usually rest and antibiotics willsee it off, but occasionally the infection cancause serious problems or reoccur veryfrequently. In these cases, a tonsillectomy maybe considered,where the tonsils are removed.

The adenoids are less commonly infectedbut, when they are, they become inamed,obstruct breathing through the nose andinterfere with drainage from the sinuses, which can lead to further infections. In younger people, constant breathing throughthe mouth can stress the facial bones andcause deformities as they grow, which is whychildren will sometimes have their adenoidglands removed.

Tonsillitis in focusLots of bed rest, uidsand pain relief like

paracetamol are allrecommended fortreating tonsillitis

Palatine tonsilsThese are the best-known pairof tonsils, as they’re clearlyvisible at the back of your throat.

Lingual tonsilsThe lingual tonsils are found atthe rear of your tongue – one ateither side in your lower jaw.

Pharyngeal tonsilsThese are otherwise known asthe adenoids and are locatedat the back of the sinuses.


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The vocal cords remain open when you breathe, but close completely when you hold your breathDID YOU KNOW?

How dohumansspeak?

V ocal cords, also known as vocalfolds, are situated in the larynx, which is placed at the top of thetrachea. They are layers of mucousmembranes that stretch across thelarynx and control how air is expelledfrom the lungs in order to make certainsounds. The primary usage of vocalcords within humans is tocommunicate and it is hypothesisedthat human vocal cords actuallydeveloped to the extent we see now tofacilitate advanced levels ofcommunication in response to theformation of social groupings during

phases of primate, and specicallyhuman, evolution.

As air is expelled f rom the lungs, the vocal folds vibrate and collide toproduce a range of sounds. The type ofsound emitted is effected by exactlyhow the folds collide, move and stretchas air passes over them. An individual‘fundamental frequency’ isdetermined by the length, size andtension of their vocal cords. Movementof the vocal folds is controlled by the vagus nerve, and sound is then furtherne-tuned to form words and soundsthat we can recognise by the larynx,tongue and lips. Fundamentalfrequency in males averages at 125Hz,

and at 210Hz in females. Children havea higher average pitch at around 300Hz.

The vocal cords and larynx in particularhave evolved over time to enable humans toproduce a dramatic range of sounds in orderto communicate – but how do they work?

Vocal cordsThese layers of mucousmembranes stretch acrossthe larynx and they open,close and vibrate to producedifferent sounds.

TracheaThe vocal cords are situatedat the top of the trachea,which is where air from thelungs travels up throughfrom the chest.

TongueThis muscle, situated in themouth, can affect andchange sound as it travels upfrom the vocal cords and outthrough the mouth.

EpiglottisThis is a flap of skin thatshuts off the trachea whenan individual is swallowingfood. It stops food and liquids‘going down the wrong way’.

OesophagusThis tube, situated behindthe trachea, is wherefood and liquid travels

down to the stomach.

LarynxKnown as the voicebox, this protects the tracheaand is heavily involved incontrolling pitch and volume.The vocal cords are situatedwithin the larynx.

LipsLips are essential for theproduction of specificsounds, like ‘b’ or ‘p’.

Differences between maleand female vocal cordsMale voices are often much lower thanfemale voices. This is primarily due tothe different size of vocal folds presentin each sex, with males having largerfolds that create a lower pitched sound,and females having smaller folds thatcreate a higher pitch sound. Theaverage size for male vocal cords arebetween 17 and 25mm, and femalesare normally between 12.5 and 17.5mm.From the range in size, however, malescan be seen to have quite high pitch voices, and females can have quite lowpitch voices.

The other major biologicaldifference that effects pitch is thatmales generally have a larger vocaltract, which can further lower the toneof their voice independent of vocal cordsize. The pitch and tone of male voiceshas been studied in relation to sexual

success, and individuals with lower voices have been seen to be moresuccessful in reproduction. The reasonproposed for this is that a lower tone voice may indicate a higher level oftestosterone present in a male.

The epiglottis stops foodentering the trachea

Vocal cords open whenbreathing, but are pulledtogether when speaking


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T he primary function of teeth is tocrunch and chew food. For thisreason, teeth are made of strongsubstances – namely calcium,phosphorus and various mineral salts.The main structure of the tooth is

dentine, which is itself enclosed in ashiny substance called enamel. Thisstrong white coating is the hardestmaterial to be found in the human body.

Humans have different types of teeththat function in various ways. Incisorstear at food, such as the residue foundon bones, while bicuspids have longsharp structures that are also used forripping. Bicuspids tear and crush whilemolars, which have a atter surface, grindthe food before swallowing. This aidsdigestion. Because humans have a variedarray of teeth (called collective dentition) we are able to eat a complex diet of bothmeat and vegetables. Other species, suchas grazing animals, have specic types ofteeth. Cows, for example, have large atteeth, which restrict them to a simple‘grazing’ diet.

Teeth have many functions, insome cases they aid hunting but theyalso have strong psychologicalconnotations. Both animals and humansbare their teeth when faced with anaggressive situation. Teeth are themost enduring features of the humanbody. Mammals are described as‘diphyodont’, which means they developtwo sets of teeth. In humans the teeth

rst appear at six months old and arereplaced by secondary teeth after six orseven years. Some animals develop onlyone set of teeth, while sharks, forinstance, grow a new set of teeth everytwo weeks.

With humans, tooth loss can occurthrough accident, gum disease or old age.

From ancient times healers have soughtto treat and replace the teeth with falseones. Examples of this practice can beseen from ancient Egyptian times andtoday, we see revolutionary newtechniques in the form of dental

implants, which are secured deep withinthe bone of the jaw.

EnamelThe white, outer surfaceof the tooth. This can beclearly seen whenlooking in the mouth.

CementumThe root coating, itprotects the rootcanal and thenerves. It isconnected to thejawbone throughcollagen fibres.

PulpThe pulp nourishes thedentine and keeps the

tooth healthy – the pulp isthe soft tissue of the tooth,

which is protected by thedentine and enamel.

Blood vesselsand nervesThe blood vesselsand nerves carryimportantnourishment to thetooth and aresensitive topressure andtemperature.

BoneThe bone acts

as animportant

anchor for thetooth and

keeps the rootsecure withinthe jawbone.

The troublewith teethTooth decay, also often

known as dental caries,affects the enamel anddentine of a tooth, breakingdown tissue and creatingssures in the enamel. Twotypes of bacteria – namelyStreptococcus mutans andLactobacillus – areresponsible for tooth decay.

Tooth decay occurs afterrepeated contact withacid-producing bacteria.Environmental factors havea strong effect. Sucrose,fructose and glucose causeproblems, and diet is a

factor in maintaining goodoral health.The mouth contains an

enormous variety ofbacteria, which collectsaround the teeth and gums.This is visible in the form ofa sticky white substancecalled plaque. Plaque isknown as a biolm. Aftereating, the bacteria in themouth metabolises sugar, which subsequently attacksthe areas around the teeth.

The biologicalstructures that are so versatile they enable usto eat a well varied diet

All

aboutteeth


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The ancient Egyptians had severe problems with their teeth. They invented the world’s first dental bridgeDID YOU KNOW?

ToothanatomyThe tooth is a complex structure. Theenamel at the surface of the tooth is highly visible while the dentine is a hard butporous tissue found under the enamel.The gums provide a secure hold for thetooth, while the root is anchored rightinto the jawbone. In the centre of the tooththere is a substance called ‘pulp’ whichcontains nerves and blood vessels, thepulp nourishes the dentine and keeps thetooth healthy.

Tooth formation begins before birth.Normally there are 20 primary teeth(human baby teeth) and later, 28 to 32permanent teeth, which includes the wisdom teeth. Of the primary teeth, tenare found in the maxilla (the upper jaw)and ten in the mandible (lower jaw), whilethe mature adult has 16 permanent teethin the maxilla and 16 in the mandible.

Wisdom teethUsually appear between theages of 17 and 25, and oftenerupt in a group of four.

Inside yourmouthThe upper and lower areas of the mouthare known as the maxilla and themandible. The upper area of the mouth

is attached to the skull bone and is oftencalled the upper arch of the mouth, while the mandible is the v-shaped bonethat carries the lower set of teeth.

Canine teethLong, pointed teeth that areused for holding and tearing atthe food within the mouth.

First and secondpremolar teeth

The premolar or bicuspids arelocated between the canine

and molar teeth. They areused for chewing.

Lateral and central incisorsIncisor comes from the Latin word ‘tocut’, they are used to grip and bite.

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Regular check-ups help keepteeth healthy

Maxilla A layout of the upper area

of your mouth

Mandible A look inside your lower jawbone

3rd molar orwisdom tooth

3rd molar orwisdom tooth

2nd molar

1st molar

1st bicuspid

2nd bicuspid

Canine

Central incisors

Lateral incisors

2nd molar

1st molar

1st premolar

2nd premolar

Canine

Lateral incisors

Central incisors

Eruptionof teethThe approximateages at which thepermanent teethbegin to erupt

Age 6First molar

Age 7Central incisor

Age 9First premolar

Age 10Second premolar

Age 11CanineAge 12Second molar

Age 17 to 21or not at allThird molar(wisdom teeth)


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T he human neck is a perfect blend of formand function. It has several specic tasks(eg making it possible to turn our heads to

see), while serving as a conduit for other vitalactivities (eg connecting the mouth to the lungs).

The anatomical design of the neck wouldimpress modern engineers. The exibility of thecerv ical spine allows your head to rotate, ex andtilt many thousands of times a day.

The muscles and bones provide the strengthand exibility required, however the reallyimpressive design comes with the trachea,

oesophagus, spinal cord, myriad nerves and the vital blood vessels. These struc tures must all ndspace and function perfectly at the same time.They must also be able to maintain their shape while the neck moves.

These structures are all highly adapted toachieve their aims. The trachea is protected by aring of strong cartilage so it doesn’t collapse, whileallowing enough exibility to move whenstretched. Above this, the larynx lets air move overthe vocal cords so we can speak. Farther back, theoesophagus is a muscular tube which food anddrink pass through en route to the stomach.Within the supporting bones of the neck sits the

spinal cord, which transmits the vital nervesallowing us to move and feel. The carotid arteriesand jugular veins, meanwhile, constantly carryblood to and from the brain.

Explore one of the most complex and functional areas of the human body Anatomy of the neck

They are connected at the bottom of the skulland at the top of the spinal column. The rstvertebra is called the atlas and the second iscalled the axis. Together these form a specialpivot joint that grants far more movement thanother vertebrae. The axis contains a bonyprojection upwards, upon which the atlasrotates, allowing the head to turn. The skull sitson top of slightly attened areas of the atlas,providing a safe platform for it to stabilise on,and allowing for nodding motions. These bonyconnections are reinforced with strong muscles,adding further stability. Don’t forget that thisamazing anatomical design still allows the vitalspinal cord to pass out of the brain. The cord sitsin the middle of the bony vertebrae, where it isprotected from bumps and knocks. It sends outnerves at every level (starting right from the top)granting control over most of the body.

How does the headconnect to the neck?

We show the major features that are packed intothis junction between the head and torso

Get it in the neckLarynxThis serves two mainfunctions: to connect themouth to the trachea, andto generate your voice.

CartilageThis tough tissueprotects the delicateairways behind,including the larynx.

Carotid arteryThese arteries transmitoxygenated blood from

the heart to the brain.There are two of them

(right and left), in case onebecomes blocked.

Verteb

Documents

How It Works Book of the Human Body 6th Edition