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Trading with the Environment
• Every organism must exchange materials with its environment (this exchange ultimately occurs at the cellular level)
• In unicellular organisms
– These exchanges occur directly with the environment
• In multicellular organisms
– For most of the cells making up a multicellular organism direct exchange with the environment is not possible (hence the need for the circulatory and respiratory systems)
Invertebrate Circulation
• The wide range of invertebrate body size and form is paralleled by a great diversity in circulatory systems
• Gastrovascular Cavities
• Open Circulatory System
• Closed Circulatory System
Gastrovascular Cavities
• Simple cnidarians have a body wall only two cells thick that encloses a gastrovascular cavity
• Functions in both digestion and distribution of substances throughout the body
• Some cnidarians, such as jellies have elaborate gastrovascular cavities
Circularcanal
Radial canal5 cm
Mouth
Open and Closed Circulatory Systems
• More complex animals
– Have one of two types of circulatory systems: open or closed
• Both of these types of systems have three basic components
– A circulatory fluid (blood or hemolymph)
– A set of tubes (blood vessels)
– A muscular pump (the heart)
Open Circulatory System
• In insects, other arthropods, and most molluscs
– Blood bathes the organs directly in an open circulatory system
Heart
Hemolymph in sinusessurrounding ograns
Anterior vessel
Tubular heart
Lateral vessels
Ostia
(a) An open circulatory system
Blood (hemolymph) returns to the heart through ostia which have valves that close during heart contraction
In an open circulatory system blood and interstitial fluid are the same (hemolymph)
Closed Circulatory System
• In a closed circulatory system blood is confined to vessels and is distinct from the interstitial fluid
• Closed systems are more efficient at transporting circulatory fluids to tissues and cells
Interstitialfluid
Heart
Small branch vessels in each organ
Dorsal vessel(main heart)
Ventral vesselsAuxiliary hearts
(b) A closed circulatory system
Survey of Vertebrate Circulation
• Humans and other vertebrates have a closed circulatory system (often called the cardiovascular system)
• Blood flows in a closed cardiovascular system
– Consisting of blood vessels and a two- to four-chambered heart
• Arteries carry blood to capillaries
– The sites of chemical exchange between the blood and interstitial fluid
• Veins return blood from capillaries to the heart
Fishes
• A fish heart has two main chambers
– One ventricle and one atrium
• Blood pumped from the ventricle
– Travels to the gills, where it picks up O2 and disposes of CO2
Amphibians
• Frogs and other amphibians
– Have a three-chambered heart, with two atria and one ventricle
• The ventricle pumps blood into a forked artery
– That splits the ventricle’s output into the pulmocutaneous circuit and the systemic circuit (double circuit)
Reptiles (Except Birds)
• Reptiles also have double circulation
– With a pulmonary circuit (lungs) and a systemic circuit
• Turtles, snakes, and lizards
– Have a three-chambered heart (with a septum partially dividing the single ventricle)
Mammals and Birds
• In all mammals and birds
– The ventricle is completely divided into separate right and left chambers
• The left side of the heart pumps and receives only oxygen-rich blood
– While the right side receives and pumps only oxygen-poor blood
• A powerful four-chambered heart was an essential adaptation of the endothermic way of life characteristic of mammals and birds (it more efficiently delivers oxygenated blood to tissues)
Vertebrate circulatory systems
FISHES AMPHIBIANS REPTILES (EXCEPT BIRDS) MAMMALS AND BIRDS
Systemic capillaries Systemic capillaries Systemic capillaries Systemic capillaries
Lung capillaries Lung capillariesLung and skin capillariesGill capillaries
Right Left Right Left Right Left Systemic
circuitSystemic
circuit
Pulmocutaneouscircuit
Pulmonarycircuit
Pulmonarycircuit
SystemiccirculationVein
Atrium (A)
Heart:ventricle (V)
Artery Gillcirculation
A
V VV VV
A A A AALeft Systemicaorta
Right systemicaorta
Pulmonary vein
Right atrium
Right ventricle
Posteriorvena cava Capillaries of
abdominal organsand hind limbs
Aorta
Left ventricle
Left atriumPulmonary vein
Pulmonaryartery
Capillariesof left lung
Capillaries ofhead and forelimbs
Anteriorvena cava
Pulmonaryartery
Capillariesof right lung
Aorta
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11
5
4
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2
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7
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Mammalian Cardiovascular System
Trace the path of blood flow
The Mammalian Heart: A Closer Look
Aorta
Pulmonaryveins
Semilunarvalve
Atrioventricularvalve
Left ventricleRight ventricle
Anterior vena cava
Pulmonary artery
Semilunarvalve
Atrioventricularvalve
Posterior vena cava
Pulmonaryveins
Right atrium
Pulmonaryartery
Leftatrium
Parts List:
• Atria
• Ventricles
• Atrioventricular valves
• Semilunar valves
• Aorta
• Pulmonary arteries and veins
• Vena cavae
Cardiac Cycle
• Systole: The contraction, or pumping, phase of the cycle
• Diastole: The relaxation, or filling, phase of the cycle
• Heart rate: The number of beats per minute
• Cardiac output: The volume of blood pumped into the systemic circulation per minute
Semilunarvalvesclosed
AV valvesopen
AV valvesclosed
Semilunarvalvesopen
Atrial and ventricular diastole
1
Atrial systole; ventricular diastole
2
Ventricular systole; atrial diastole
3
0.1 sec
0.3 sec0.4 sec
Heart Excitation
• The sinoatrial (SA) node, or pacemaker sets the rate and timing at which all cardiac muscle cells contract
– Electrical impulses from the SA node spread rapidly through the Atria causing them to contract in unison
– The pacemaker is influenced by nerves, hormones, body temperature, and exercise
• Impulses from the SA node travel to the atrioventricular (AV) node
• At the AV node, the impulses are delayed slightly then relayed and amplified
– They travel via the Purkinje fibers and the ventricles contract in unison
Electrical Excitation
Figure 42.8
SA node(pacemaker)
AV node Bundlebranches
Heartapex
Purkinjefibers
2 Signals are delayedat AV node.
1 Pacemaker generates wave of signals to contract.
3 Signals passto heart apex.
4 Signals spreadThroughoutventricles.
ECG
Blood Vessels
• Structure
– Endothelium
– Smooth Muscle
– Connective tissue
Artery Vein
100 µm
Artery Vein
ArterioleVenule
Connectivetissue
Smoothmuscle
Endothelium
Connectivetissue
Smoothmuscle
EndotheliumValve
Endothelium
Basementmembrane
Capillary
Arteries and Veins
• Arteries have thicker walls to accommodate the high pressure of blood pumped from the heart
• In the thinner-walled veins blood flows back to the heart mainly as a result of muscle action. Veins have valves.
Direction of blood flowin vein (toward heart)
Valve (open)
Skeletal muscle
Valve (closed)
Blood Flow Velocity
• The velocity of blood flow varies in the circulatory system
– And is slowest in the capillary beds as a result of the high resistance and large total cross-sectional area
5,0004,0003,0002,0001,000
0
Aor
ta
Art
erie
s
Art
erio
les
Cap
illar
ies
Ven
ules
Vei
ns
Ven
ae c
avae
Pre
ssur
e (m
m H
g)V
eloc
ity (
cm/s
ec)
Are
a (c
m2)
Systolicpressure
Diastolicpressure
50403020100
120100806040200
Blood Pressure
• Blood pressure
– The hydrostatic pressure that blood exerts against the wall of a vessel
– Blood pressure is determined by
• cardiac output
• peripheral resistance due to variable constriction of the arterioles
• Systolic pressure
– Is the pressure in the arteries during ventricular systole
– Is the highest pressure in the arteries
• Diastolic pressure
– Is the pressure in the arteries during diastole
– Is lower than systolic pressure
Blood Pressure Measurement
Artery
Rubber cuffinflatedwith air
Arteryclosed
120 120
Pressurein cuff above 120
Pressurein cuff below 120
Pressurein cuff below 70
Sounds audible instethoscope
Sounds stop
Blood pressurereading: 120/70
A typical blood pressure reading for a 20-year-oldis 120/70. The units for these numbers are mm of mercury (Hg); a blood pressure of 120 is a force that can support a column of mercury 120 mm high.
1
A sphygmomanometer, an inflatable cuff attached to apressure gauge, measures blood pressure in an artery.The cuff is wrapped around the upper arm and inflated until the pressure closes the artery, so that no blood flows past the cuff. When this occurs, the pressure exerted by the cuff exceeds the pressure in the artery.
2 A stethoscope is used to listen for sounds of blood flow below the cuff. If the artery is closed, there is no pulse below the cuff. The cuff is gradually deflated until blood begins to flow into the forearm, and sounds from blood pulsing into the artery below the cuff can be heard with the stethoscope. This occurs when the blood pressure is greater than the pressure exerted by the cuff. The pressure at this point is the systolic pressure.
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The cuff is loosened further until the blood flows freely through the artery and the sounds below the cuff disappear. The pressure at this point is the diastolic pressure remaining in the artery when the heart is relaxed.
4
70
Blood Flow Through Capillary Beds
• Capillaries in major organs are usually filled to capacity, but in many other sites, the blood supply varies
– Amount of bloodflow through capillaries is regulated by
• Diameter of arteriole leading to the capillaries
• Precapillary sphincters
Precapillary Sphincters
• Precapillary sphincters control the flow of blood between arterioles and venules
Precapillary sphincters Thoroughfarechannel
ArterioleCapillaries
Venule(a) Sphincters relaxed
(b) Sphincters contractedVenuleArteriole
(c) Capillaries and larger vessels (SEM)
20 m
•The critical exchange of substances between the blood and interstitial fluid takes place across the thin endothelial walls of the capillaries
• The difference between blood pressure and osmotic pressure drives fluids out of capillaries at the arteriole end and into capillaries at the venule end
At the arterial end of acapillary, blood pressure is
greater than osmotic pressure,and fluid flows out of the
capillary into the interstitial fluid.
Capillary Redbloodcell
15 m
Tissue cell INTERSTITIAL FLUID
CapillaryNet fluidmovement out
Net fluidmovement in
Direction of blood flow
Blood pressureOsmotic pressure
Inward flow
Outward flow
Pre
ssur
e
Arterial end of capillary Venule end
At the venule end of a capillary, blood pressure is less than osmotic pressure, and fluid flows from the interstitial fluid into the capillary.
Fluid Return by the Lymphatic System
• The lymphatic system
– Functions
• Returns fluid to the body from the capillary beds
• Aids in body defense
– Fluid reenters the circulation
• directly at the venous end of the capillary bed
• indirectly through the lymphatic system
Blood
• Blood is a connective tissue
• Blood consists of
– several kinds of cells
– suspended in a liquid matrix called plasma
• The cellular elements
– Occupy about 45% of the volume of blood in man
Plasma
• Blood plasma is about 90% water
• Many solutes are found in plasma
– Inorganic salts in the form of dissolved ions, sometimes referred to as electrolytes
– Plasma proteins
• Plasma proteins influence blood pH, osmotic pressure, and viscosity
• Function in lipid transport, immunity, and blood clotting
Composition of mammalian plasma
Plasma 55%
Constituent Major functions
Water Solvent forcarrying othersubstances
SodiumPotassiumCalciumMagnesiumChlorideBicarbonate
Osmotic balancepH buffering, andregulation of membranepermeability
Albumin
Fibringen
Immunoglobulins(antibodies)
Plasma proteins
Ions (blood electrolytes
Osmotic balance,pH buffering
Substances transported by bloodNutrients (such as glucose, fatty acids, vitamins)Waste products of metabolismRespiratory gases (O2 and CO2)Hormones
Defense
Separatedbloodelements
Clotting
Cellular Elements
1. Erythrocytes (Red blood cells)
• Transport oxygen
2. Leukocytes (White blood cells)
• Function in defense
• Neutrophils
• Lymphocytes
• Eosinophils
• Monocytes
• Basophils
• Thrombocytes (platelets)
– Are fragments of cells that are involved in clotting
Cellular elements 45%
Cell type Numberper L (mm3) of blood
Functions
Erythrocytes(red blood cells) 5–6 million Transport oxygen
and help transportcarbon dioxide
Leukocytes(white blood cells)
5,000–10,000 Defense andimmunity
Eosinophil
Basophil
Platelets
NeutrophilMonocyte
Lymphocyte
250,000400,000
Blood clotting
Cellular elements of mammalian blood
Separatedbloodelements
Stem cells
• Erythrocytes, leukocytes, and platelets all develop from a common source
– A single population of cells called pluripotent stem cells in the red marrow of bones
B cells T cells
Lymphoidstem cells
Pluripotent stem cells(in bone marrow)
Myeloidstem cells
Erythrocytes
Platelets Monocytes
Neutrophils
Eosinophils
Basophils
Lymphocytes
Blood cells need to be constantly replaced throughout life
Blood clotting• When the endothelium of a blood vessel is damaged the clotting
mechanism begins
• A cascade of reactions converts fibrinogen to fibrin, forming a clot
Plateletplug
Collagen fibers
Platelet releases chemicalsthat make nearby platelets sticky
Clotting factors from:PlateletsDamaged cellsPlasma (factors include calcium, vitamin K)
Prothrombin Thrombin
Fibrinogen Fibrin5 µm
Fibrin clotRed blood cell
The clotting process begins when the endothelium of a vessel is damaged, exposing connective tissue in the vessel wall to blood. Plateletsadhere to collagen fibers in the connective tissue and release a substance thatmakes nearby platelets sticky.
1 The platelets form a plug that providesemergency protectionagainst blood loss.
2 This seal is reinforced by a clot of fibrin when vessel damage is severe. Fibrin is formed via amultistep process: Clotting factors released fromthe clumped platelets or damaged cells mix withclotting factors in the plasma, forming an activation cascade that converts a plasma proteincalled prothrombin to its active form, thrombin.Thrombin itself is an enzyme that catalyzes the final step of the clotting process, the conversion of fibrinogen to fibrin. The threads of fibrin become interwoven into a patch (see colorized SEM).
3
Cardiovascular Disease
• Cardiovascular diseases
– Disorders of the heart and the blood vessels
– Account for more than half the deaths in the United States
Atherosclerosis
• Atherosclerosis: Is caused by the buildup of cholesterol within arteries
(a) Normal artery (b) Partly clogged artery50 µm 250 µm
Smooth muscleConnective tissue Endothelium Plaque
Cardiovascular Diseases
• Hypertension, or high blood pressure
– Promotes atherosclerosis and increases the risk of heart attack and stroke
• Heart attack (Myocardial Infarction)
– Is the death of cardiac muscle tissue resulting from blockage of one or more coronary arteries
• Stroke
– Is the death of nervous tissue in the brain, usually resulting from rupture or blockage of arteries in the head
• Aneurysms
– Dilation of blood vessel walls
Gas Exchange
• Gas exchange
– Supplies oxygen for cellular respiration and disposes of carbon dioxide
• Animals require large, moist respiratory surfaces for the adequate diffusion of respiratory gases between their cells and the respiratory medium, either air or water
Invertebrates-1
• In some invertebrates the gills have a simple shape and are distributed over much of the body
(a) Sea star. The gills of a sea star are simple tubular projections of the skin. The hollow core of each gillis an extension of the coelom(body cavity). Gas exchangeoccurs by diffusion across thegill surfaces, and fluid in thecoelom circulates in and out ofthe gills, aiding gas transport. The surfaces of a sea star’s tube feet also function in gas exchange.
Gills
Tube foot
Coelom
Invertebrates-2
• Many segmented worms have flaplike gills
– That extend from each segment of their body(b) Marine worm. Many
polychaetes (marine worms of the phylum Annelida) have a pair of flattened appendages called parapodia on each body segment. The parapodia serve as gillsand also function incrawling and swimming.
Gill
Parapodia
Invertebrates-3
• The gills of clams, crayfish, and many other animals are restricted to a local body region
(d) Crayfish. Crayfish and other crustaceanshave long, feathery gills covered by the exoskeleton. Specialized body appendagesdrive water over the gill surfaces.
(c) Scallop. The gills of a scallop are long, flattened plates that project from themain body mass inside the hard shell.Cilia on the gills circulate water around the gill surfaces.
Gills
Gills
Fish Gills
• The effectiveness of gas exchange in some gills, including those of fishes
– Is increased by ventilation and countercurrent flow of blood and water
Countercurrent exchange
Gill arch
Water flow Operculum
Gill arch
Blood vessel
Gillfilaments
Oxygen-poorblood
Oxygen-richblood
Water flowover lamellaeshowing % O2
Blood flowthrough capillariesin lamellaeshowing % O2
Lamella
100%
40%
70%
15%
90%
60%
30% 5%
O2
Countercurrent flow of blood and water maintains a concentration gradient down which oxygen diffuses from water to blood over entire length of capillary.
Tracheae
Air sacs
Spiracle
(a) The respiratory system of an insect consists of branched internaltubes that deliver air directly to body cells. Rings of chitin reinforcethe largest tubes, called tracheae, keeping them from collapsing.
Enlarged portions of tracheae form air sacs near organs that require a large supply of oxygen. Air enters the tracheae through openings
called spiracles on the insect’s body surface and passes into smaller tubes called tracheoles. The tracheoles are closed and contain fluid(blue-gray). When the animal is active and is using more O2, most ofthe fluid is withdrawn into the body. This increases the surface area of air in contact with cells.
Tracheal Systems in Insects
• The tracheal system of insects
– Consists of tiny branching tubes that penetrate the body
• The tracheal tubes supply O2 directly to body cells
Airsac
Body cell
Trachea
Tracheole
TracheolesMitochondria
Myofibrils
Body wall
(b) This micrograph shows crosssections of tracheoles in a tinypiece of insect flight muscle (TEM).Each of the numerous mitochondriain the muscle cells lies within about5 µm of a tracheole.
2.5 µm
Air
Lungs
Lungs are found in
• most terrestrial vertebrates (amphibians have relatively small lungs in general and some lack them entirely. The skin of amphibians supplements gas exchange in the lungs)
• spiders
• land snails
• a few fish (lungfish)
Mammalian Respiratory Systems: A Closer Look• A system of branching ducts conveys air to the lungs
• air inhaled via the nostrils passes through the pharynx into the trachea, bronchi, bronchioles, and dead-end alveoli, where gas exchange occurs Branch
from the pulmonary vein (oxygen-rich blood) Terminal bronchiole
Branch from thepulmonaryartery(oxygen-poor blood)
Alveoli
Colorized SEMSEM
50 µ
m
50 µ
m
Heart
Left lung
Nasalcavity
Pharynx
Larynx
Diaphragm
Bronchiole
Bronchus
Right lung
Trachea
Esophagus
Ventilation
• Breathing ventilates the lungs
– The alternate inhalation and exhalation of air
• An amphibian such as a frog
– Ventilates its lungs by positive pressure breathing, which forces air down the trachea
How a Mammal Breathes
• Mammals ventilate their lungs by negative pressure breathing, which pulls air into the lungs
Air inhaled Air exhaled
INHALATIONDiaphragm contracts
(moves down)
EXHALATIONDiaphragm relaxes
(moves up)
Diaphragm
Lung
Rib cage expands asrib muscles contract
Rib cage gets smaller asrib muscles relax
How a Bird Breathes
• Besides lungs, bird have eight or nine air sacs
– That function as bellows that keep air flowing through the lungs
INHALATIONAir sacs fill
EXHALATIONAir sacs empty; lungs fill
Anteriorair sacs
Trachea
Lungs LungsPosteriorair sacs
Air Air
1 mm
Air tubes(parabronchi)in lung
Control of Breathing in Humans
• The main breathing control centers
– Are located in two regions of the brain, the medulla oblongata and the pons
PonsBreathing control centers Medulla
oblongata
Diaphragm
Carotidarteries
Aorta
Cerebrospinalfluid
Rib muscles
In a person at rest, these nerve impulses result in
about 10 to 14 inhalationsper minute. Between
inhalations, the musclesrelax and the person exhales.
The medulla’s control center also helps regulate blood CO2 level. Sensors in the medulla detect changes in the pH (reflecting CO2
concentration) of the blood and cerebrospinal fluid bathing the surface of the brain.
Nerve impulses relay changes in
CO2 and O2 concentrations. Other sensors in the walls of the aortaand carotid arteries in the neck detect changes in blood pH andsend nerve impulses to the medulla. In response, the medulla’s breathingcontrol center alters the rate anddepth of breathing, increasing bothto dispose of excess CO2 or decreasingboth if CO2 levels are depressed.
The control center in themedulla sets the basic
rhythm, and a control centerin the pons moderates it,
smoothing out thetransitions between
inhalations and exhalations.
1
Nerve impulses trigger muscle contraction. Nerves
from a breathing control centerin the medulla oblongata of the
brain send impulses to thediaphragm and rib muscles, stimulating them to contract
and causing inhalation.
2
The sensors in the aorta andcarotid arteries also detect changesin O2 levels in the blood and signal the medulla to increase the breathing rate when levels become very low.
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5
3
4
Control of Breathing in Humans
• The centers in the medulla
– Regulate the rate and depth of breathing in response to pH changes in blood and in the cerebrospinal fluid
– The medulla adjusts breathing rate and depth to match metabolic demands
• Sensors in the aorta and carotid arteries
– Monitor O2 and CO2 concentrations in the blood
– Exert secondary control over breathing
Gas Transport
• The metabolic demands of many organisms
– Require that the blood transport large quantities of O2 and CO2
The Role of Partial Pressure Gradients
• Gases diffuse down pressure gradients in the lungs and other organs
• Diffusion of a gas
– Depends on differences in a quantity called partial pressure
• A gas always diffuses from a region of higher partial pressure to a region of lower partial pressure
• In the lungs and in the tissues O2 and CO2 diffuse from where their partial pressures are higher to where they are lower
Inhaled air Exhaled air
160 0.2O2 CO2
O2 CO2
O2 CO2
O2 CO2 O2 CO2
O2 CO2 O2 CO2
O2 CO2
40 45
40 45
100 40
104 40
104 40
120 27
CO2O2
Alveolarepithelialcells
Pulmonaryarteries
Blood enteringalveolar
capillaries
Blood leavingtissue
capillaries
Blood enteringtissue
capillaries
Blood leaving
alveolar capillaries
CO2O2
Tissue capillaries
Heart
Alveolar capillaries
of lung
<40 >45
Tissue cells
Pulmonaryveins
Systemic arteriesSystemic
veinsO2
CO2
O2
CO 2
Alveolar spaces
12
43
Respiratory Pigments
• Respiratory pigments
– Are proteins that transport oxygen
– They greatly increase the amount of oxygen that blood can carry
– The respiratory pigment of almost all vertebrates is the protein hemoglobin, contained in the erythrocytes
• Like all respiratory pigments
– Hemoglobin must reversibly bind O2, loading O2 in the lungs and unloading it in other parts of the body
Heme group Iron atom
O2 loadedin lungs
O2 unloadedIn tissues
Polypeptide chain
O2
O2
Figure 42.28
• Hemoglobin also helps transport CO2 and assists in buffering
• Carbon dioxide from respiring cells
– Diffuses into the blood plasma and then into erythrocytes and is ultimately released in the lungs
Tissue cell
CO2Interstitialfluid
CO2 producedCO2 transportfrom tissues
CO2
CO2
Blood plasmawithin capillary Capillary
wall
H2O
Redbloodcell
HbCarbonic acidH2CO3
HCO3–
H++Bicarbonate
HCO3–
Hemoglobinpicks up
CO2 and H+
HCO3–
HCO3– H++
H2CO3Hb
Hemoglobinreleases
CO2 and H+
CO2 transportto lungs
H2O
CO2
CO2
CO2
CO2
Alveolar space in lung
2
1
34
5 6
7
8
9
10
11
To lungs
Carbon dioxide produced bybody tissues diffuses into the interstitial fluid and the plasma.
Over 90% of the CO2 diffuses into red blood cells, leaving only 7%in the plasma as dissolved CO2.
Some CO2 is picked up and transported by hemoglobin.
However, most CO2 reacts with water in red blood cells, forming carbonic acid (H2CO3), a reaction catalyzed bycarbonic anhydrase contained. Withinred blood cells.
Carbonic acid dissociates into a biocarbonate ion (HCO3
–) and a hydrogen ion (H+).
Hemoglobin binds most of the H+ from H2CO3 preventing the H+ from acidifying the blood and thuspreventing the Bohr shift.
CO2 diffuses into the alveolarspace, from which it is expelledduring exhalation. The reductionof CO2 concentration in the plasmadrives the breakdown of H2CO3 Into CO2 and water in the red bloodcells (see step 9), a reversal of the reaction that occurs in the tissues (see step 4).
Most of the HCO3– diffuse
into the plasma where it is carried in the bloodstream to the lungs.
In the HCO3– diffuse
from the plasma red blood cells, combining with H+ released from hemoglobin and forming H2CO3.
Carbonic acid is converted back into CO2 and water.
CO2 formed from H2CO3 is unloadedfrom hemoglobin and diffuses into the interstitial fluid.
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The Ultimate Endurance Runner
• The extreme O2 consumption of the antelope-like pronghorn
– Underlies its ability to run at high speed over long distances
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