2.3 Transport in Animals

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WJEC AS Bio Unit 2.3: (3) Adaptations for Transport 1

AS Unit 2: Biodiversity and Physiology of Body Systems.

2.3: Adaptations for Transport.

(3) Part one – Adaptations for Transport in Animals

Syllabus Objectives:

a) The similarities and differences in the vascular systems of animal groups:

Earthworm; vascularisation, closed circulatory system and pumps, carriage of respiratory gases in

blood.

Insects; open circulatory system, dorsal tube-shaped heart, lack of respiratory gases in blood.

Fish; single circulatory system.

Mammal: double circulatory system.

b) The mammalian circulatory system including the structure and function of heart and blood vessels and the

names of the main blood vessels associated with the human heart.

c) The cardiac cycle and the maintenance of circulation to include geographical analysis of pressure changes,the role of the sino-atrial node and Purkyne/Purkinje fibres and the analysis of electrocardiogram traces to

show electrical activity.

d) The function of red blood cells and plasma in relation to transport of respiratory gases, dissociation curves

of haemoglobin of mammal (adult and foetus), including examination of microscope slides.

e) The dissociation curves of some animals adapted to low oxygen level habitats e.g. llama and lugworm

f) The Bohr effect and chloride shift.

g) The transport of nutrients, hormones, excretory products and heat in the blood.

h) The formation of tissue fluid and its importance as a link between blood and cells.

(Syllabus objective (i) – (r) = Adaptations for Transport in Plants are in booklet number (4))

Specified Practical Work

Scientific drawing of a low power plan of a prepared slide of T.S. artery and vein, including calculation

of actual size and magnification of drawing.

Dissection of mammalian heart.

Learning outcome Knowledge and

understanding, 1 5

(1 is excellent)

Revision

notes

completed

1. Explain why multicellular animals need transport mechanisms.

2.

Explain the significance of and the difference between open andclosed circulatory systems.

3.

Explain the significance of and the difference between single

and double circulations.

4.

Explain the relationship between the structure and function of

arteries, veins and capillaries.

5. Describe the passage of blood through the heart.

6.

Describe the cardiac cycle and interpret graphs showing

pressure changes during the cycle.

7. Explain the electrical control of the heartbeat.


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8. Describe the structure of blood cells.

9.

Describe the differences between blood, plasma, tissue fluid and

lymph.

10.

Describe the role of haemoglobin in the transport of oxygen andcarbon dioxide.

11.

Describe and explain the effects of raised carbon dioxide

concentration on the oxygen dissociation curve.

12.

Describe the transport of carbon dioxide in terms of the chloride

shift.

13. Describe the formation of tissue fluid and its importance in the

exchange of materials.

1.

Features of a transport system.

A. Open systems and closed circulatory systems.

Transport systems in different organisms.

When multicellular organisms develop organs of exchange such as lungs and gills, they need a transport

system to move substances over large distances, because diffusion is simply too slow. Most transport systems

consist of a series of tubes in which an efficient supply of materials is moved around under pressure. These

systems are called mass transport or mass flow systems. Plants have xylem and phloem, whereas vertebrates

have a blood system.

As organisms increase in size their surface area to volume ratio decreases to the point where diffusion through

the body surface is insufficient to meet their needs. If this is the case a specialised exchange surface is needed

to absorb nutrients and respiratory gases and to remove excretory products. These exchange surfaces are

located in specific regions of the organism. A transport system is therefore needed to take materials from the

exchange surfaces to cells, and from cells to exchange surfaces.

As well as being transported between exchange surfaces and the environment, materials also need to be

transported between different parts of the organism. As organisms have increased in size and their structures

have become more complex, the tissues and organs they are made of have become more specialised and

therefore more reliant upon one another. This makes transport systems even more essential.

Questions

1. Name the 2 main factors that influence whether or not a specialised transport medium is required.

2. Explain why larger organisms require a specialised transport medium

3. Explain why the following features are common in transport systems:

(a) A liquid based transport medium with water

(b) A closed system of tubular vessels forming a branching network

Answers

1. The surface area to volume ratio and the activity level of the organism.

2. They have a lower surface area to volume ratio so diffusion is insufficient.

3a) So that water soluble substances can be transported.

3b) So that the transport medium is distributed to all parts of the organism

Features of transport systems.

There are a number of features that are common among many transport systems. These are:

A medium to carry the materials e.g. blood. This is usually a liquid based on water because manysubstances are water soluble and water can be moved easily.


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A form of mass transport where the transport medium is moved in bulk

over large distances.

A mechanism to maintain the mass flow in one direction e.g. valves

A way of controlling the flow of the transport medium to meet the changing needs of different parts

of the organism.

A mechanism for moving the transport medium within vessels; a pump, such as the heart. This creates

a pressure difference between one part of the system and the other.

In addition some systems have:

A closed system of tubular vessels that contain the transport medium and form a branching network

so that the transport medium is distributed throughout the organism.

A respiratory pigment, which increases the volume of oxygen carried. Found in vertebrates and some

invertebrates but not insects.

Questions

4. How do animals move their transport medium?

5. How do plants move their transport medium?

Answers

4. Muscular contractions of body muscles or a specialised pumping organ

5. Passive processes such as evaporation of water

(i) Open circulatory systems.

The blood does not move around the body in blood vessels. It bathes the tissue directly while held in a cavity

called the haemocoel.

E.g. Insects. They have a long, dorsal, (top) tube shaped heart, running the length of the body. It pumps blood

out at low pressure into the haemocoel. Here materials are exchanged between the blood and body cells.

Blood the returns to the heart and the open circulation starts again.

Oxygen diffuses directly to the tissues from the tracheae so the blood does not transport oxygen and has no

respiratory pigment.


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(ii) Closed circulatory systems.

Blood moves in blood vessels.

2 types:

(a) Single circulation

Blood moves through heart once.E.g. earthworm – blood moves forward in the dorsal vessel and back in the ventral vessel.

It has 5 pairs of `pseudohearts` = thickened muscular blood vessels that pump the blood between the dorsal

and ventral blood vessels and keep it moving.

e.g.2 – fish – the ventricle of the heart pumps deoxygenated blood to the gills, where its pressure falls

Oxygenated blood returns to the atrium of the heart. Blood moves to the ventricle and the circulation stats

again.

(b) Double circulation

Mammals have a closed, double circulation.

Mammals move blood through a system of blood vessels by the pumping of the heart. Mammals have a

double circulatory system where blood passes through the heart twice in one complete circulation of the

https://www.youtube.com/results?search_query=open+and+closed+circulationhttps://www.youtube.com/results?search_query=open+and+closed+circulationhttps://www.youtube.com/results?search_query=open+and+closed+circulation


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body. A lower pressure is required at the lungs and if the blood passed straight to

the rest of the body, the pressure would be too low and slow down the circulation.

Blood is returned to the heart to increase its pressure before being circulated to the rest of the body.

Mammals have a high metabolic rate and so substances need to be delivered to the rest of the body quickly.

Organs are not in direct contact with the blood but are bathed in tissue fluid, which seeps out of the

capillaries.

The blood pigment, haemoglobin carries the oxygen.

Animal Circulation type Respiratory pigment Heart

Insect Open Haemocoel X Dorsal tube-shaped

Earthworm Closed Single √ `Pseudohearts`

Fish Closed Single √ 1 atrium and 1 ventricle

Mammal Closed Double √ 2 atria and 2

ventricles

Transport in Mammals

A. Pulmonary and Systemic circulation

Double circulatory system comprise of:

(i) The pulmonary circulation

This serves the lungs.

The right side of the heart pumps deoxygenated blood to the lungs.

Oxygenated blood returns from the lungs to the left side of the heart.

(ii) The systemic circulation

This serves the body tissues.The left side of the heart pumps oxygenated blood to tissue.

https://www.youtube.com/watch?v=q0s-1MC1hcEhttps://www.youtube.com/watch?v=q0s-1MC1hcEhttps://www.youtube.com/watch?v=q0s-1MC1hcE


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Deoxygenated blood returns from the body to the right side of the heart.

In each circuit the blood passes through the heart twice, once through the right and once through the left side.

Double circulation is more efficient than the single circulation of a fish as oxygenated blood can be pumped

around the body at higher pressure.

B. Structure and function of blood vessels.

The vessels that make up the circulatory system in mammals are divided into 3 types: arteries, veins and

capillaries. These vessels are used to transport substances over long distances. In order for materials to reach

cells, they must diffuse from the vessels quickly. This is possible because it takes place over a large surface

area, along a short diffusion pathway and there is a steep diffusion gradient.

You should know the following components of the double circulatory system (the pulmonary circulation and

the systemic circulation). You need to know the name of the blood vessels that enter and leave the heart,

lungs, kidneys and liver. In addition to this, you also need to know that the hepatic portal vein transports

blood from the intestines to the liver.

Blood vessels are named according to their structure; Arteries need to carry blood under high pressure so

away from the heart and veins low pressure after the capillary network, whereas capillaries allow fluid

movement in and out of the system.

Arteries (including Aorta) LEAVE the heart and usually ENTER the major organs.

Veins ENTER the heart and LEAVE major organs


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Questions

6. Name the blood vessel in each of the following descriptions.

a) Joins the right ventricle of the heart to the capillaries of the lungs

b) Carries oxygenated blood away from the heart

c) Carries deoxygenated blood away from the liver

d) The first main blood vessel that an oxygen molecule reaches after being absorbed from an alveolus

e) Has the highest blood pressure.

Answers

6a. Pulmonary artery

b. Aorta

c. Hepatic vein

d. Pulmonary vein

e. Aorta

(i) Basic structure of arteries and veins.

Arteries carry blood under high pressure away from the heart to organs

Arterioles are smaller arteries that control blood flow from arteries to capillaries

Capillaries are small vessels that connect arterioles to veins. The function of capillaries is to link

arterioles to veins and to take blood close to almost every cell in the body. Capillaries allow rapid

transfer of substances between cells and blood.

Veins carry blood from capillaries back to the heart under low pressure

Question

7. Use the diagram above to identify how the structure varies between an artery, vein and capillary:

Answers

7. Capillaries, arteries and veins all have endothelial cells.

Capillaries are tissues (one cell type only) whereas arteries and veins are organs.

Veins have valves but arteries and capillaries do not.

Capillaries do not have an outer layer, muscle layer or elastic layer, arteries and veins do.

Arteries have a thicker muscle layer than veins.Arteries have a thicker elastic layer than veins.

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Capillaries have a much narrower lumen

There are spaces in the lining

Arteries, veins and arterioles have the same basic layered structure. What differs between each is the

proportions of each layer in the different vessels. From the outside inwards, the layers are

Tough outer layer = tunica externa - resists pressure changes from within and outside the vessel and

so prevents over-stretching. Contains collagen fibres

Tunica media = contains a smooth muscle layer – can contract to control blood flow and maintain

blood pressure as the blood is transported from the heart so more in the arteries.

Also contains elastic fibres – allows stretching to accommodate changes in blood flow and pressure as

blood is pumped from the heart. At a certain point stretched elastic fibres recoil, pushing blood

through the artery = pulse. It also maintains blood pressure by stretching and springing back

Inner lining (endothelium) – one cell thick and surrounded by the tunica intima. It has a smooth

lining to reduce friction and thin for diffusion

Lumen – the central cavity of the blood vessel through which the blood flows

(ii) Arteries

Arteries are adapted to withstand pressure. When the heart beats, the left ventricle forces blood into the

body’s largest artery, the aorta. From here, blood enters the major arteries of the body, leading to al l the

major organs and limbs. The middle layers of the artery walls are rich in muscle and, vitally, elastic fibres. Thisgives them powerful recoil properties so they can withstand the pressure surge of each heart beat.


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The muscle layer is thick compared to veins – this means smaller arteries (arterioles) can be

constricted and dilated to control the volume of blood passing through.

The elastic layer is thick compared to veins – this keeps blood pressure high so that the blood can

reach the extremes of the body. As the heart beats, the elastic wall is stretched and then springs back

when the heart is relaxed. The stretch and recoil maintains high blood pressure and prevents surges

in pressure.

Overall the wall is thick – this resists bursting when under pressure.

There are no valves (except for pulmonary artery and aorta as they leave the heart) – the blood does

not flow backwards because of the high pressure.

Proportion of smooth muscle increase, relative to elastin that decreases, with distance from the

heart.

(iii) Arterioles

Arterioles are adapted to control blood flow. By the time blood reaches the arterioles, it has lost much of its

pressure that has been absorbed by artery walls. The walls of the arterioles do not need as many elastic fibres,

but they do have a lot of muscle fibres. This means that arterioles are capable of either:

• Vasodilation — they get larger

• Vasoconstriction — they get smaller

In this way, blood flow to certain areas of the body can be controlled. For example, vasodilation of

subcutaneous arterioles causes the skin to redden, whereas vasoconstriction causes it to go pale.

(iv) Veins

Veins are adapted to increase blood flow when pressure is low. Compared to arteries, veins have a larger

lumen and a thinner wall. This minimises friction so blood can flow more easily. The walls are made of tough

connective tissue and there are fewer elastic and muscle fibres. Veins also have valves that can open up to

prevent backflow.

The muscle layer is thin compared to arteries – they carry blood away from tissues and so they cannot

control the flow of blood to tissues. Blood flow is slower.

The elastic layer is thin compared to arteries – the pressure of the blood is low and so will not cause

the veins to burst and a recoil action cannot be created.

The overall thickness of the wall is small compared to the artery – the pressure is low which reduces

the risk of the vein bursting. Being thin means the veins can be flattened easily aiding blood flow.

For veins above the heart, blood returns to the heart by gravity. It moves through other veins by

pressure from surrounding muscle contractions.


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There are semi-lunar valves throughout – pressure is low so valves stop the

backflow of blood. Faulty functioning of valves contributes to varicose

veins and heart failure.

Open valve closed valve

(v) Capillaries

These are numerous and highly branched, providing a large surface area for diffusion. They penetrate all

organs and tissues.

Blood from capillaries collects in venules, and then into veins, which return the blood to the heart.

Capillaries allow exchange between blood and cells. They are the smallest blood vessels. Their walls (the

endothelium) are just one cell thick. The function of capillaries is to allow metabolic exchange of materials

between blood and tissue fluid so the flow of blood is much slower.

Walls consist of endothelium cells only – walls are thin so there is a short diffusion pathway and

diffusion is rapid between the blood and cells

There are many and they are branched – this increases the collective surface area

They have a narrow diameter – this means they can permeate issues so no cell is far from a capillary

(short diffusion pathway)

The lumen is narrow – red blood cells are compressed against the side of the capillary (short diffusion

pathway)

There are spaces between the endothelial cells – white blood cells can escape to deal with infections.

Capillaries are small but they cannot reach every single cell directly. Tissue fluid is the liquid thatcarries metabolic materials to the tissues.


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Complete the table below with the key points

Feature Artery Arteriole Capillary Vein

Cross-

section of

vessel

Structural

features

Blood flow

Type of

blood

Blood

pressure

Main

functions ofvessels

Adaptations

to the main

function


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Questions

8. How does the elastic tissue help to smooth the blood flow in arteries leaving the heart?

9. Why does the vein have valves within?

10. Why is the lumen of the vein so much bigger than arteries?

11. Why is the capillary only one cell thick and have minute ‘holes’ within?

Answers

8. Allows recoil and so maintains blood pressure/smooth blood flow/constant blood flow.

9. Prevent backflow of blood to tissues and so keeps it moving towards the heart.

10. It has a thinner wall and requires less contraction and pressure to move blood.

11. To provide a short diffusion pathway and to allow exchange of materials between blood and tissues.


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2. The Heart

The human heart is a muscular organ that circulates blood around the body. The heart is essentially two

separate pumps lying side by side; one dealing with oxygenated blood and the other with deoxygenated blood.

During embryonic development in mammals, the 2 separate pumps grow together to form one overallstructure; the heart.

The heart work continuously and tirelessly throughout the life of individual (hopefully).

Mammals have a double circulation. During a complete circulation of the body, blood passes through the

heart twice. It is pumped to the lungs to be oxygenated (pulmonary circulation HEART LUNGS HEART)

and then returns to the heart to be pumped to other parts of the body that use the oxygen ( systemic

circulation HEART BODY HEART).


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A. Structure

Heart Structure

The heart is divided into 4 chambers:

Right and left atria to receive blood returning from the systemic and pulmonary circulations,

respectively.

Right and left ventricles to force blood through the pulmonary and systemic circulations, respectively.

The right ventricle pumps blood to the lungs (a distance of a few cm) therefore it has a thinner muscular wall

than the left ventricle. The left ventricle has a thicker ventricular wall allowing it to create enough pressure to

pump blood to the extremities of the body (a distance of roughly 1.5m).


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The two sides of the heart are separate pumps (the 2 sides are separated by the

septum) and after birth there is no mixing of the blood in each of them. Nevertheless, they pump in time with

each other; both atria contract together and then both ventricles contract together.

The heat consists of cardiac muscle. This is specialised tissue with myogenic contraction = it can contract and

relax rhythmically, of its own accord and never tires.

The heartbeat is initiated within the muscle cells itself, (in the SAN), It is not dependent on nervous or

hormonal stimulation.

The heart rate is however modified by nervous and hormonal stimulation.

Atria receive blood from veins Blood flows from atria ventricles arteries

Blood from the left ventricle flows to the aorta

Blood from the right ventricle flows to the pulmonary artery

Valves

There are 4 valves in the heart that control the flow of blood in the mammalian heart; one between each

atrium and ventricle (atrioventricular) and one at the base of each artery leading away from the ventricles

(semi-lunar).

The major blood vessels associated with the heart Aorta - Largest artery carrying blood out of heart. It has a branch towards head and main flow down to


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rest of body. Transports oxygenated blood. It is connected to the left ventricle.

Pulmonary artery – Connected to the right ventricle and leaves heart and branches into 2. It takes

deoxygenated blood to the right and left lungs where oxygen is replenished and carbon dioxide is removed.

Pulmonary veins – Connected to the left atrium and take oxygenated blood back to the heart from the left

and right lungs.

Venae cavae – Connected to the right atrium. They run vertically on the right hand side of heart (2 large

veins; one from head and one from rest of body). Carries deoxygenated blood to the heart.

Coronary arteries – These are found on surface of the heart. They branch from the aorta and deliver

oxygenated blood to walls of heart. The cardiac muscle in the heart wall respires continuously to release

the energy needed for contraction. To supply the oxygen and glucose needed, the cardiac muscle has its

own blood supply – the coronary circulation. Two coronary arteries branch off the aorta just as it leaves the

left ventricle. These carry blood into arterioles and the millions of capillaries that supply the cardiac muscle

cells. The coronary arteries are narrower than many other arteries and can become blocked more easily.

Blockage of these arteries by a blood clot or atheroma leads to myocardial infarction because an area of

the heart muscle has been deprived of oxygen.

Pressure is important in blood flow

The heart consists of two pumps with output at two different pressures. Because the lungs need to have

blood flow through numerous blood capillaries (to produce large surface area) – there is a drastic drop in

pressure – NOT enough pressure to pump around the whole body. Therefore a second (stronger pump) is

needed to circulate oxygenated/deoxygenated blood around the body. Mammals therefore have a double

circulatory system for efficient gas exchange.

Questions

12. Describe and explain the differences in structure between the atria and ventricles?

13. Use the diagram of the heart to describe the route that blood takes from the body to the lungs.


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14. Use the diagram of the heart to describe the route that blood takes from the

lungs to the body

15. Suggest why it is important to prevent mixing of the blood in the two sides of the heart.

Answers

12. The atria have thin muscular walls that are elastic and stretch as they collect blood. This is because they

only pump blood a short distance to the ventricles and at quite low pressure.

The ventricles have a much thicker muscular wall. This is because they have to pump blood to the lungs or to

the rest of the body, under greater pressure.

13. Body vena cava right atrium atrioventricular valve, (tricuspid valve) right ventricle

pulmonary artery lungs

14. Lungs pulmonary vein left atrium atrioventricular valve, (bicuspid valve) left ventricle aorta

body

15. The mixing of oxygenated and deoxygenated blood would result in only partially oxygenated blood

reaching the tissues and lungs. This would mean the supply of oxygen to the tissues would be inadequate and

there would be a reduced diffusion gradient in the lungs, limiting the rate of oxygen uptake.


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B. The Cardiac Cycle

This describes the sequence of events of one heartbeat.

In a normal adult this lasts approximately 0.8seconds.There are alternating contractions (systole) and relaxations (diastole).

Cardiac cycle has 3 stages:

-

Atrial systole [about 0.1s] = contraction of the atria

- Ventricular systole [about 0.3s]= contraction of the ventricles

-

Diastole [about 0.4s]

The four chambers of the heart are continually contracting and relaxing in a definite, repeating sequence

called the cardiac cycle. In humans this sequence of events is repeated around 70 times per minute when at

rest.

When a chamber is contracting we say it is in systole.

When a chamber is relaxing we say it is in diastole.

The two sides of the heart work together; as the left atrium contracts, so does the right atrium. As the right

ventricle relaxes, so does the left ventricle. The direction of the blood flow is maintained by pressure changes

and the action of valves.

One beat of the heart pumps blood through the pulmonary and systemic circuits.

The heart has 2 pumps working in series = the ‘lub-dub’ you hear with a stethoscope, this is the noise of valves

closing in the heart during a heartbeat.

Right hand side pumps deoxygenated blood to lungs through the pulmonary artery at a blood pressure of

24mmHg (3.2 kPa). Left side pumps oxygenated blood into the aorta at 120 mmHg (15.8 kPa). NOTE –

significant difference – Lungs do not receive blood under pressure

Lungs are very spongy and blood vessels allow maximum exchange of gases in the alveoli.

Left ventricle wall is much thicker than right as it contracts and forces blood into aorta at high pressure.

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(i) Atrial systole

The walls of the atria contract. This raises the pressure of the blood in the atria above that in the ventricles and

forces open the atrioventricular valves. The blood that remains in the atria (2 percent of the total blood in the

heart) passes through the AV valves into the ventricles. The blood is only pushed a small distance so the atrial

walls are very thin. At this stage, the walls of the ventricles are relaxing.


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(ii) Ventricular systole

After a short delay that allows the ventricles to fill with blood, the ventricular walls contract. This quickly

raises the pressure of the blood in the ventricles above that in the atria, and so closes the atrioventricular

valves, (the tricuspid and bicuspid valves), preventing backflow into the atria. This creates the ‘lub’ sound of a

heartbeat.

When the pressure of the blood exceeds that in the main arteries, the semilunar valves are forced open. Blood

is ejected into the pulmonary artery and aorta. The walls of the ventricles are much thicker than those of the

atria as they pump blood much further and so genrate higher pressure tha the atria too.

The wall of the left ventricle is the thickest as it pumps blood to the extremities of the body, wheras the right

ventricle has only to pump the blood a short distane to the lungs.

(iii) Diastole

The ventricles begin to relax and so increase the volume and so the pressure of the ventricles quickly falls

below that in the main arteries. The higher pressure in these arteries closes the semi-lunar valves. This creates

the ‘dub’ sound of the heart beat. This prevents the blood re-entering the ventricles.

The aria also relax durinf diastole so Blood returns to the atria via the vena cava and pulmonary vein. This

increases the pressure in the atria. As the ventricles continue to relax, the pressure in the ventricles falls below

that in the atria. The higher pressure in the atria forces the atrioventricular valves open. Even though the atria

are not contracting, blood flows through the open valves passively ventricular filling.


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Question

16. Complete the following table to summarise the 3 main events in the cardiac cycle.

Stage Action of

atria

Result Action of

ventricles

Result

1. Atrial

systole

2. Ventricular

systole

3. Diastole

Answers

16.

Stage Action of

atria

Result Action of

ventricles

Result

1. Atrial

systole

Walls

contract

Blood is forced through AV

valves into ventricles

Walls relax Fill with blood

2. Ventricular Walls relax Blood enters through the Wall contract a) No blood leaves, but

17.


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systole veins pressure of blood increases.

b) Pressure of blood opens

semi-lunar valves and blood

is ejected to arteries.

3. Diastole Wall relax a) Blood enters atria but

cannot enter ventricles as

AV valves are closed.

b) Blood continues to enter

atria and increased pressure

opens AV valves

Walls relax a) Blood neither enters nor

leaves.

b) Blood enters from atria

by passive ventricular filling

as AV valves are open (high

to low)

(iv) Valves – their role in controlling blood flow

It is essential to think of blood flowing as a result of pressure differences (high low).

It is important to keep blood flowing in the right direction through the heart and around the body. This is

largely achieved by the pressure created by the heart muscle.

All the valves are one-way valves and work on essentially the same principle. Blood is a fluid; it flows from an

area of high pressure to an area of low pressure. There are however, situations within the circulatory system

when pressure differences would result in blood flowing in the opposite direction from that which is desirable.

In these circumstances the valves in the heart are used to prevent any unwanted backflow of blood. The valves

in the heart are designed to open when high pressure is forcing the blood in the ‘correct’ direction. If high

pressure forces blood in the ‘wrong’ direction, the valves are forced shut.

Valves are made up of tough, but flexible, fibrous tissues which are cusp shaped. When pressure is greater on

the convex side of cusp, they move apart to let blood flow between the cusps. When pressure is greater on the

concave side, blood collects within the bowl of the cusps. This pushes them together to prevent the flow of

blood. As the pressure generated is so great, the valves have tendons attached to the ventricular walls to

prevent them from inverting.

The valves between each atrium and ventricle are called atrioventricular valves. They prevent the backflow of

blood into the atria when the ventricles contract.

The left atrioventricular (bicuspid) valve is formed of 2 cup-shaped flaps on the left side of the heart.

17.


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The right atrioventricular (tricuspid) valve is formed of 3 cup-shaped flaps

on the right side of the heart.

Semilunar valves are found in the aorta and the pulmonary artery. They prevent backflow of blood into the

ventricles when the recoil action of the elastic walls creates a greater pressure in the vessels than in the

ventricles.

Semi-lunar are found in veins. They ensure that when veins are squeezed, blood flows back to the heart rather

than away from it.

Questions

18. Which side of the heart carries oxygenated blood?19. Why is the left ventricle more muscular than the right ventricle?

20. What is the purpose of heart valves?

21. What is the difference between the systemic and pulmonary circulatory systems?

Answers

18. Left side.

19. Left ventricle thicker than the right ventricle cos it needs to contract powerfully to pump blood all the way

round the body, under greater pressure. The right side only needs to get blood to the lungs which are nearby

and so needs less pressure.20. The atrioventricular valves link the atria to the ventricles and stop blood getting back into the atria when

the ventricles contract.

The semi-lunar valves stop blood flowing back into the heart after the ventricles contract.

21. Systemic circ = heart → body → heart.

Pulmonary circ = heart → lungs → heart.

C. Pressure and volume changes during the cardiac cycle

The graph below summarises the changes in pressure and volume in the left side of the heart during thecardiac cycle.


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(a) Atrial muscles contract = atrial systole

Thin muscular atrial walls squeeze inward onto blood, increasing pressure, push blood into the ventricles

through the atrioventricular valves.

(b) The ventricles contract = ventricular systole while atrial diastole occurs

Thick muscular ventricle walls squeeze inward onto blood, increasing pressure and push blood out of heart.

When pressure in ventricles pressure in atria = pushes atrioventricular valve shut - prevents backflow into

atria. Instead blood rushes into aorta & pulmonary artery, opening semi-lunar valves on the way.

(c) (Ventricular) diastole

Whole of heart muscle relaxes (both atrial and ventricular muscles relax) - ventricle pressure drops.

Higher pressure in atria / pulmonary artery cause semi-lunar valves to shut - prevents backflow into ventricles.

Blood from the veins (pulmonary vein / vena cava) flows back into the two atria.

Blood has very low pressure in the veins. But thin walls of atria are easily distended, providing little resistance

to the blood flow.

Atrial muscle contracts - push blood into ventricles...... Cycle starts again.

Remember the passage of blood through the two sides of the heart is coordinated.

Both atria fill at the same time, both ventricles fill at the same time, both and ventricular systole occurs in both

sides of the heart at the same time.

In together, down together and out together

A complete contraction and relaxation of the whole heart = a heartbeat.

Tips - You may be asked to explain the changes that occur at various points in the cardiac cycle from a graph

such as that shown below. Remember that:

AV valves open as soon as the pressure in the atria becomes greater than that in the ventricles; they

close as soon as the pressure in the ventricles becomes greater than that in the atria.

The semi-lunar valves open as soon as the pressure in the ventricles becomes greater than that in the

two arteries; they close as soon as the pressure on the two arteries becomes greater than that in the

ventricles.

A valve open and closes at times in the cycle when the balance of pressures on opposite sides of the

valve changes.


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Cardiac cycle – graphical representation

Exam tip: This is popular in exams. Be prepared to describe pressure changes.


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Question

22. Use the graph above to complete the table, which summarises the events that occur during one cardiac

cycle.

Atrial systole Ventricular systole Ventricular diastole

Atrial wall

Atrial pressure

Ventricular wall

Ventricular pressure

Ventricular volume

Semi-lunar valve

Atrioventricular valve

Answer

22.

Atrial systole Ventricular systole Ventricular diastole

Atrial wall Contracting Relaxing Relaxing

Atrial pressure Relatively high Relatively low Relatively low

Ventricular wall Relaxing Contracting Relaxing

Ventricular pressure Relatively low Relatively high Relatively low

Ventricular volume Increasing Decreasing Increasing

Aortic pressure Relatively low Relatively high Relatively low

Semi-lunar valve Closed Open Closed

Atrioventricular valve Open Closed Open

D. Cardiac output

The output (volume) is equal on both sides of the heart despite the varying pressure of contraction. Cardiac

output is the output from each (only consider one) ventricle per minute.

Each time the ventricles contract, they eject blood into the main arteries. The amount of blood ejected from

one ventricle is called the stroke volume and, at any one time, it is the same for both ventricles.

The other factor that affects cardiac output is heart rate – the number if beats per minute.

Cardiac output = Heart rate x the stroke volume

The units are given as dm3min

-1.

An increasein stroke volume or heart rate (or both) increases cardiac outpit.

During exercise, the cardiac output increases to deliver more blood, carrying oxygen and glucose, to the

skeletal muscles. During sleep, cardiac output decreaes from the normal resting level because the metabolic

activity of the body is low and less oxygen is needed by almost all organs.


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Questions

23. An athlete’s cardiac output is 3 dm3

per minute and her heart rate is 60 beats per minute. What is the value

of her stroke volume?

24. Match the blood vessels 1-4 with descriptions A-D:

1. Vena cava

2. Aorta

3. Pulmonary artery

4. Pulmonary vein

A. Carries blood from the right ventricle of the heart to the capillaries of the lungs.

B. Carries oxygenated blood away from the heart to the body.

C. Carries deoxygenated blood from the body to the right atrium of the heart.

D. Carries oxygenated blood from the capillaries of the lungs to the left atrium of the heart.

Answer23. SV = CO / HR = 3 / 60 = 0.05dm

3

24.

1 – C

2 – B

3 – A

4 - D

E. Control of heartbeat.

The cardiac muscle is myogenic - it naturally contracts and relaxes of its own accord, it doesn’t need nerve

impulses to contract as is the case with other muscles.

The events of the cardiac cycle must take place in the correct sequence, with the correct timing. A group of

cells in the right atrium form the sinoatrial node (SAN), which acts as a natural pacemaker. The SA node

initiates the stimulus that originates the contraction. It has the basic rhythm of stimulation that determines

the beat of the heart. In this way the heart has its own built in controlling and coordinating system - to prevent

each cell from contracting and relaxing under its own rhythm.

Chambers should only contract when they are full of blood, so the heart has a conducting pathway of

specialised muscle fibres to ensure the right sequence of events. The atria must contract first and then, when

full, the ventricles follow. This means a delay is needed to allow the ventricles to fill. The full sequence is as

follows:

https://www.youtube.com/watch?v=fZT9vlbL2uAhttps://www.youtube.com/watch?v=fZT9vlbL2uAhttps://www.youtube.com/watch?v=fZT9vlbL2uAhttps://www.youtube.com/watch?v=fZT9vlbL2uA


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1. Sinoatrial node (or SAN or pacemaker)

SAN = `specialised patch of heart muscle in wall of right atrium that initiates a wave of electrical excitation

across the atria. `

The function of the SAN is to set the rhythm for all cardiac muscle cells, by sending out a wave of electrical

activity that spreads over all atrial walls.

Atrial wall contracts, at same time as SAN - so all muscle in both atrial walls contract at the same time.

Muscles of ventricles contract after atrial walls. This delay is caused by band of fibres between atria and

ventricles which does not conduct excitation wave (atrioventricular septum). The delay is required to ensure

that the ventricle does not contract too soon.

2. Atrioventricular node (or AVN).

AVN = `The only conducting area of tissue in the wall of the heart between the atria and the ventricles,

through which electrical excitation passes from the atria to the conducting tissue in the walls of the ventricles.

`

Wave from SAN can only spread to ventricles via patch of conducting fibres in the septum.

AVN picks up electrical wave from atria, there is a delay of 0.1 seconds then passes it onto bunch of conducting

fibres = bundles of His, this runs down atrioventricular septum, to the left and right bundle branches and thento the apex of the heart.

3. Bundles of His Bundles of His transmits excitation wave rapidly down to base of the atrioventricular septum,

to the apex of the heart, where it spreads outwards and upwards through ventricle walls.

The excitation is transmitted to the Purkinje, (or Purkyne) fibres in the ventricle walls, which carry it through

the muscles of the ventricle walls.


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This causes cardiac muscle wall to contract in ventricles from the bottom (apex) up

- so blood is pushed up into arteries; the aorta and the pulmonary artery. This empties the ventricles

completely. .

This table summarises the events involved in the control of the cardiac cycle.

The SAN generates an impulse; the

impulse spreads along Purkyne

fibres to all parts of the atria.

Cardiac muscle in atria contracts,

cardiac muscle in ventricles is

relaxed — blood is forced through

AV valves from atria to ventricles.

Atrial systole

The impulse is held up at the AVN,

allowing time for atria to empty.

Cardiac muscle in atria contracts,

cardiac muscle in ventricles is

relaxed — blood continues to be

forced through AV valves.

Atrial systole

The impulse is conducted along

the bundles of His through the

ventricle walls.

Cardiac muscle in atria is relaxed,

cardiac muscle in ventricles

contracts; AV valves closed; semi-

lunar valves opened – blood

ejected into main arteries.

Ventricular systole

Atriole diastole

No impulse Cardiac muscle in atria and

ventricles is relaxed – passive

ventricular following.

Atrial and ventricular diastole

Questions

25. Explain what is meant by the term ‘myogenic’

26. Explain why it is important that there is a slight delay after the atria contract.

27. Describe how the regular contraction of the atria and ventricles is initiated and coordinated by the heart

itself.

Answers

25. Heart muscle has a built-in rhythm; the heart is able to beat without nerve impulses from the brain.

26. So that the ventricles have time to fill properly.

27. Cardiac muscle is myogenic; Sinoatrial node; spreads out a wave of electrical activity across the atria; this

initiates the contraction of the atria; the impulse passes through the atrioventricular node; the impulse is

conducted along the bundle of His; to the ventricles; the ventricles contract after the atria, they contract fromthe bottom up, to force the blood up and out of the ventricles.

Fibrillation

Coordination of contraction goes wrong sometimes, then the excitation wave is chaotic – it passes through the

ventricular wall in all directions, re-stimulating areas that have already been stimulated. Small areas of

muscles contract whilst others relax.

Result = fibrillation

This causes the heart wall to flutter, rather than contracting and relaxing as a whole.


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It is nearly always fatal unless treated instantly. It is caused by either electric shock,

or damage to large areas of muscle in heart walls.

F. The Electrocardiogram (ECG) (i) The ECG

This is a method used to interpret the electrical activity, or a terrace of the voltage changes produced by the

heart, detected by electrodes on the skin, or a cathode ray oscilloscope

It is used to identify abnormalities such as the fibrillation above. A normal electrocardiogram has a distinct

pattern as below.

https://www.youtube.com/watch?v=v3b-YhZmQu8https://www.youtube.com/watch?v=v3b-YhZmQu8https://www.youtube.com/watch?v=v3b-YhZmQu8


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During the heart cycle, the heart undergoes a series of electrical current changes. These are related to the

waves of electrical activity that are created by the SAN and the heart’s response to these.

P = The P wave - this is the first part of the trace. It shows the voltage change generated by the SAN,

associated with the wave of excitation sweeping over atrial walls, causing them to contract.

The atria have less muscle than the ventricles and so the P waves are small.

The time between the start of the P wave and the start of the QRS complex = the PR interval = time taken for

the excitation to spread from the atria to the ventricles, through the AVN.

Q,R and S or the QRS Complex = depolarisation and contraction of the ventricles.

Ventricles have a more muscle than the atria and so the amplitude is bigger than that of the P wave.

T wave = repolarisation of the ventricle muscles, or the recovery of ventricle walls.

The ST segment lasts from the end of the S wave to the beginning of the T wave.

The isoelectric line = the base line of the trace and is the line between the T wave and the P wave.

ECGs are analysed to gain information on the heart rate and the rhythm.

- Heart rate can be calculated from the trace by reading on the horizontal axis. Read the time off the

axis, for one complete ECG trace. So the length of the cycle = time between equivalent points on trace

e.g. R to R, (normally approx 0.85s)

-

Therefore heart rate = 60 = 71 beats per minute (0 dp)0.85

-

The heart’s rhythm is shown by the regularity of the pattern of the trace,

(a) A person with atrial fibrillation has a rapid heart rate and may lack a P wave.


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In this scenario, the doctor would shock heart out of its fibrillation with strong

electric shock through chest wall. This will stop heart for up to 5 seconds after which normally beats again

normally.

(b) A person who has had a heart attack, or myocardial infarction, may have a wide QRS complex.

An ECG produced during a heart attack

shows less pronounced peaks and larger

troughs that are repeated.

(c) A person with enlarged ventricle walls, (hypertrophy), may have a QRS complex showing greater voltage

charge.


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(d) Changes in the height of the ST segment and T wave may be related to

insufficient blood being delivered to the heart muscle, such as with blocked coronary arteries and

atherosclerosis.

Questions

28. Use the above figure for the following questions.

a. How long does one heart beat (one cardiac cycle) last?

b. What is the heart rate represented on this graph, in beats per minute?

c. The contraction of muscles in the ventricle wall causes the pressure inside the ventricle to rise. When the

muscles relax the pressure drops again. On the diagram mark the following periods:

i. The time when the ventricle is constricting (ventricular systole).

ii. The time when the ventricle is relaxing (ventricular diastole).

d. The contraction of muscles in the wall of the atrium raises the pressure inside it. This pressure is also raised

when blood flows into the atrium from the veins, while the atrial walls are relaxed. On the diagram mark the

following periods:

i. The time when the atrium is contracting (atrial systole).

ii. The time when the atrium is relaxing (atrial diastole).

e. The atrio-ventricular valves open when the pressure of the blood in the atria is greater than that in the

ventricles. They snap shut when the pressure of the blood in the ventricles is greater than that in the atria. On

the diagram mark the point at which these valves will open and close.

f. The opening and closing of the semilunar valves in the aorta depends in a similar way on the relative

pressures in the aorta and ventricles. On the diagram mark the point at which these valves open and close.

29. Complete the gaps:


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The heartbeat is initiated in an area of the right atrium called the

............................. A wave of electrical excitation passes through conducting tissue at the

junction of the atria and ventricles called the .............................. This in turn passes the

wave to the bundle of His, which transfers it to the ................................. fibres, at the apex

of the ventricles. This cause the ventricles to contract from the base upwards and forces

blood out of the heart through the aorta and .................................

Answers

28a. 0.8 seconds.

b. 60/0.8 = 75 beats per minute.

c . 1d – atrial systole from 0 sec up until Ventricle systole (see diagram in booklet if unclear)

e and 2f

29. Sino-atrial node

Atrio-ventricular node

Purkinje fibres

Pulmonary artery

(ii) Pressure changes in the blood vessels Blood pressure highest in aorta and largest arteries. It rises and falls rhythmically with ventricular

contraction.

The higher the blood pressure, the faster the flow.

The further away from the heart that the blood travels – the lower the blood pressure and the slower

the flow.

Friction between the blood and vessel walls and the large total surface area causes a pressure drop in

the arterioles, even though they have a narrow lumen. Their pressure also depends on whether they

are constricted or dilated too.

In the capillary beds pressure drops further, as fluid leaks from the capillaries to the tissues.


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Veins not subject to pressure changes derived from the contraction of the

ventricles, as they are so far away from them, so blood pressure is low.

Veins have a larger diameter, so blood flows faster than in capillaries despite the low pressure.

Blood does not return to the heart rhythmically. Its return is aided by the effect of the skeletal

muscles contracting around the veins.

G. Blood

Is a tissue made up of cells (45%) in a solution called plasma (55%).

(i) Red blood cells

Red blood cells or erythrocytes are red as they contain the pigment haemoglobin.

Haemoglobin function = transport oxygen from lungs to respiring tissues.

RBCs are biconcave discs. This gives a large surface area, so oxygen diffuses into them at a faster rate.

The thin centre makes them look paler in the middle. It reduces the diffusion distance and so makes gas

exchange faster.

RBCs have no nucleus and so more room for more haemoglobin and so more oxygen carried.


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(ii) White blood cells/leucocytes

Larger than erythrocytes.

2 main types:

(a) Granulocytes – have a granular cytoplasm and lobed nucleus.

They are phagocytic.

(b) Agranulocytes/lymphocytes – clear cytoplasm and spherical nucleus.They produce antibodies and antitoxins.

(iii) Plasma

Pale yellow liquid, 90% water.

Contains solutes such as:

Food molecules like glucose, amino acids, vitamins B and C, mineral ions,

Waste products, (including urea, HCO3-),

Hormones

Plasma proteins, (including albumin, blood clotting proteins and antibodies).

Plasma also distributes heat.


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Question

30. Fill in the gaps:

The blood consist of a pale yellow fluid called ................. which contains red and white

blood cells. The red blood cells or ..................... transport .......................... combined with

haemoglobin as ......................................

Answer

30. Plasma

Erythrocytes

Oxygen

Oxyhaemoglobin

3. Transport of Oxygen

A. Structure of haemoglobin.

(i) Reminder of structure from unit 1:

There is a group of haemoglobins, all chemically similar with the same general structure. All are conjugated

proteins.

Primary structure = Sequence of amino acids but there are four chains (two alpha and two beta). Secondary structure = α helix.

Tertiary structure = each chain loosely folded into a precise shape – relates to function.

Quaternary structure = 2 pairs of polypeptides (so 4 chains).

In adult haemoglobin, (HbA), there are 2α-globin and 2β-globin chains. All 4 polypeptide chains are linked to

from an almost spherical shape.

Each have a prosthetic group, which is a haem group associated with it, which contains a ferrous (Fe2+

) ion.

So in one haemoglobin molecule there are 4 haem groups.

Each Fe2+

ion can combine with a single oxygen molecule (O2). Process = oxygenation.

In total 1 haemoglobin can combine with 4 O2 molecules (8 atoms).

Exam tip – you need to be able to relate the structure of red blood cells to their function of carrying oxygen.

(ii) The role of haemoglobin.

= combines and then transports oxygen.

To do this must:

Readily associate with oxygen at surface where gaseous exchange occurs, i.e. the alveoli.

Readily dissociate from oxygen at those tissues requiring it, such as muscle.

Oxygen + Haemoglobin Oxyhaemoglobin

4O2 Hb Hb4O2

The 4 polypeptides of each haemoglobin are tightly bound together. So difficult to absorb the 1st

oxygen

molecule, onto the first haem group.Once loaded this 1st

oxygen molecule causes haemoglobin molecule to change shape, making it easier for the

2nd

oxygen molecule to attach.


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The 2nd

oxygen molecule attaching changes the shape again, making it easier for the

3rd

oxygen molecule to attach. = cooperative binding = `the increasing ease with

which haemoglobin binds its second and third oxygen molecules, as the conformation of the haemoglobin

molecule changes.`

Allows the haemoglobin to pick up oxygen rapidly in the lungs.

The 3rd

oxygen molecule does not induce a shape change, so it takes a large increase in oxygen partial pressure

to bind the 4th oxygen molecule.

Thus haemoglobin can change its affinity for oxygen under different conditions. Achieves this by changing

shape in the presence of carbon dioxide.

Different haemoglobins have slightly different sequences of amino acids and therefore slightly different

shapes. Depending on the shape, haemoglobin molecules range from those with a high affinity to those with a

low affinity for oxygen.

In presence of carbon dioxide haemoglobin binds more loosely to oxygen, so haemoglobin releases its oxygen

more easily.

Process of haemoglobin combines with oxygen = loading or associating. Happens in alveoli.Process of haemoglobin releases its oxygen = unloading or dissociating. Happens in tissues.

Region of body Oxygen

concentration

Carbon dioxide

concentration

Affinity of

haemoglobin for

oxygen

Result

Gas exchange

surface

High Low High Oxygen is attached

Respiring tissues Low High Low Oxygen is detached

(iii) Different haemoglobins

Different organisms have different haemoglobin. They differ due to how they take up and release oxygen.

Haemoglobins with a high affinity for oxygen.

Affinity = `the degree to which 2 molecules are attracted to each other.`

So here take up oxygen more easily but release it less readily.

E.g. of an organism that lives in an environment where there is l ittle oxygen, so haemoglobin must be able to

combine readily with oxygen if it is to absorb enough. Metabolic rate must not be too quick, and then it does

not matter if oxygen is not released as readily into the tissues.

Haemoglobin with a low affinity for oxygen.

Takes up oxygen less easily but release it more readily.

E.g. organism with high metabolic rate needs to release oxygen readily into its tissues. As long as there is

plenty of oxygen in the environment, then it is more important that the haemoglobin releases oxygen easily.


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Questions

31. Describe the quaternary structure of haemoglobin.

32. Explain how DNA leads to different haemoglobin molecules having a different affinity for oxygen.

33. When the body is at rest only 1 of the 4 oxygen molecules carried by haemoglobin is normally released into

the tissues. Suggest why this could be an advantage when the organism becomes more active.

34. Carbon monoxide occurs in car exhaust fumes. It binds permanently to haemoglobin in preference to

oxygen. Suggest a reason why a person breathing in car exhaust fumes might lose consciousness.

Answers

31. 2 pairs of polypeptides, (2α and 2β) link to form a spherical molecule, (globular protein). Each polypeptide

has a haem group that contains a ferrous ion.

32. Different base sequences in DNA- different amino acid sequences (different primary structure) – and so get

different tertiary/quaternary structures and shape – different affinities for oxygen.

33. If all oxygen molecules were released there would be none in reserve to supply tissues when they are more

active.

34. Carbon monoxide will gradually occupy all the sites on the haemoglobin instead of oxygen. No oxygen will

be carried to tissues, such as the brain. Cells cease to respire and to function – person loses consciousness.

B. Oxygen dissociation curves.

(i) Adult oxygen dissociation curve

Measuring oxygen concentration

Amount of gas in a mixture is measures by the pressure it contributes to the total pressure of the gas mixture =

partial pressure of the gas. The partial pressure of a gas is the pressure it would exert if it were the only one

present.

For oxygen written as pO2. For carbon dioxide = pCO2

Measured in kilopascals (kPa).The % of haemoglobin associated with oxygen at a given partial pressure of oxygen (pO 2) = % saturation.

Normal atmospheric pressure = 100kPa.

Oxygen makes up 21% of atmosphere, so its partial pressure = 21kPa.

In lungs partial pressure of oxygen = 13kPa and 98% of haemoglobin associates (binds) with oxygen.

In respiring tissue at rest pO2 = 5.3kPa, with 73% haemoglobin associated with oxygen.

In moderately respiring muscle pO2 = 2.5kPa, with 35% oxygen still associated with haemoglobin.

When a pigment is exposed to increasing partial pressures of oxygen, if it absorbed oxygen evenly, the graph

plotted would be linear. But cooperative binding means that haemoglobin exposed to increasing partial

pressure of oxygen shows a sigmoid curve, (S shaped curve).

At very low partial pressure it is difficult for haemoglobin to load oxygen but the steep part of the graph showsoxygen binding increasingly easily.

At high partial pressure of oxygen, the percentage saturation of oxygen is very high.

Graph of this = oxygen dissociation curve


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A small decrease in the partial pressure of oxygen leads to a lot of oxygen becoming dissociated from

haemoglobin.

Graph tails off at very high oxygen concentrations because haemoglobin is almost saturated with oxygen.

Where the curve is very steep a small change in pO2 causes a big change in the amount of oxygen carried by

haemoglobin.

The oxygen affinity of haemoglobin is high at high partial pressure of oxygen and oxyhaemoglobin does not

release its oxygen.

Oxygen affinity reduces as the partial pressure of oxygen decreases and oxygen is readily released, meeting

respiratory demands. A very small decrease in the oxygen partial pressure leads to a lot of oxygen dissociating

from haemoglobin.

There are a large number of oxygen dissociation curves because there are many types of haemoglobin and any

1 type of haemoglobin molecule can change under different conditions.

All have roughly the same shape but remember:

The further to the left the curve is – the greater the affinity of haemoglobin for oxygen, so it takes oxygen up

result but releases it less easily.

The further to the right the curve is – the lower the affinity of haemoglobin for oxygen, so it takes up oxygen

less readily but releases it more easily.

If the relationship between oxygen partial pressure and % saturation of haemoglobin with oxygen were

linear:

At higher partial pressure of oxygen, haemoglobin’s oxygen affinity would be too low and so oxygen

would be readily released and would not reach the respiring cells.

At lower partial pressure of oxygen, haemoglobin’s affinity would be too high and oxygen would not

be released in respiring tissues, even at low oxygen partial pressures.

(ii) Different lives – Different haemoglobins.

(a) High altitude adaptations in mammals.

At high altitude – temp, humidity and pressure decreases. Oxygen partial pressure is lower, reducing theamount of taken up by blood.

Can lead to inadequate amounts of oxygen getting to respiring cells.


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If person goes up mountain slowly, then body adjusts = altitude acclimatisation.

Get increase in haemoglobin content and increase in density of red blood cells in blood.

With more haemoglobin the carrying capacity of haemoglobin increase but blood becomes thicker and

requires more pressure to pump it around body.

Those that live at higher altitudes are born with higher red blood cell counts and have oxygen dissociationcurves shifted to the left of a normal curve.

Advantage because it increases the oxygen saturation of haemoglobin at low oxygen partial pressure that

occur at high altitude.

Disadvantage – oxygen is unloaded less readily.

(b) The dissociation curve of foetal haemoglobin

Foetus in uterus gets oxygen by diffusion from mum’s placenta. Foetus has foetal haemoglobin (HbF). This has

two α-globin chains and two δ-globin chains. This means there are variations in amino acid sequences produce

haemoglobin with different properties. This gives the foetal haemoglobin a higher affinity for oxygen than

maternal blood, at the same partial pressure of oxygen.

Their blood flows very close in the placenta, so oxygen transfers to the foetus’s blood at any partial pressure ofoxygen, the percentage saturation of the foetus’s blood is higher than the mother’s. So foetal haemoglobin has

oxygen dissociation curve to left of maternal one.

(c) Transport of oxygen in other animals.

Lugworm

Live in the sand at the beach. Not very active, living head down, in a U shape burrow. Is covered by sea water,

which circulates in its burrow. Oxygen diffuses into the lugworm’s blood, from the water and the haemoglobin

transports it to the tissues respiring. This means that its haemoglobin loads oxygen very readily but onlyreleases it when partial pressure of oxygen is very low. The haemoglobin will be 90% saturated.

When the tide goes out it does not have fresh supply of oxygenated water. So water contains less and less

oxygen, as lugworm uses it up.

Organisms with access to low concentrations of oxygen have haemoglobin with a high affinity for oxygen than

human haemoglobin. The curve is to the left of the human one.


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Llama

Llamas live at high altitude. Here the atmospheric pressure is lower and so the partial pressure of oxygen isalso lower. It is therefore difficult to load haemoglobin with oxygen. Its haemoglobin has a high affinity for

oxygen at all partial pressures, so loads oxygen more readily in the lungs and releases oxygen when the oxygen

partial pressure is low, in its respiring tissues.

Another solution to the problem of low oxygen availability occurs in people living at high altitude, e.g. in the

Andes and in athletes who train at high altitudes. They make more red blood cells, allowing more oxygen to becarried around the body.

Questions

35. Explain why a lugworm can survive at these low concentrations of oxygen while a human cannot.

36. How is the lugworm able to obtain sufficient oxygen from an environment that contains so little?

37. Suggest 1 feature of a lugworm’s way of life that helps it to survive in an environment that has little

oxygen.

38. Haemoglobin usually loads oxygen less readily when the concentration of carbon dioxide is high, (the Bohr

shift). The haemoglobin of lugworms does not exhibit this effect. Explain why to do so could be harmful.

39. Suggest a reason why lugworms are not found higher up the seashore.

Answers

35. At this partial pressure it is still 90% saturated. This is enough for a sedentary animal like the lugworm. For

a human this low partial pressure would mean a much lower % saturation, more like 10%, not enough to keep

cells alive.

Haemoglobin has a high affinity for oxygen, so pick up oxygen easily and release it less readily.

36. The dissociation curve is shifted to the left. This means it is fully loaded with oxygen, even when there is

little in the environment available.

37. Lugworm is not very active. So requires little oxygen.

38. Respiration produced carbon dioxide. This builds up in burrow. If lugworm exhibited the Bohr shift effect, it

would not be able to absorb much oxygen when it was present in very low concentrations.

39. Higher part of beach is uncovered for longer period of times, so lugworm would receive less frequent fresh

sea water, during long times without fresh sea water, the lugworm would use up all oxygen and die.Higher up the beach, there will be drier sand and so the burrow will have less water in it and so less oxygen.


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Diving mammals

E.g. whales and seals.

Would expect these, to have haemoglobin with high affinity for oxygen because they dive in deep water but

not so, as they take in air before they dive at the surface, so they don’t require high affinity for oxygen.

Small mammals

Have a large surface area to volume ratio. So lose heat quickly. So to maintain temperature they have high

metabolic rate to generate heat.

Active = higher demand for oxygen, so have haemoglobin with a lower affinity for oxygen than human

haemoglobin.

So oxygen dissociation curve of a mouse is to the right of humans.

Questions

40. The oxygen dissociation curve of the mouse is shifted to the right of humans. What difference does this

make to the way oxygen is unloaded from mouse haemoglobin compared to that of a human?

41. What advantage does this have for the maintenance of body temp in mice?

Answers

40. It unloads more readily.

41. Oxygen is more readily released from haemoglobin to the tissues. This helps tissue respire more and

produce more heat, which helps maintain the body temp of a mouse.

Birds and fish.

Flight in birds and swimming in fish both need energy. Flight muscles in wings need lots of oxygen to respire, to

keep them airborne. So during flight they have a very high metabolic rate, to produce the energy to oppose

gravity in air that gives little support.

Fish have a different problem – they expend a lot of energy swimming because water is very dense anddifficult to move through.

(d) Haemoglobin in root nodules

Get haemoglobin in some plants and also symbiotic bacteria e.g. root nodules of leguminous plants like peas

and beans. They have special haemoglobin like molecule = lepthaemoglobin.

Nodules contain bacteria – they absorb nitrogen from air → ammonia = nitrogen fixation, using enzyme –

nitrogenase.

Must be in anaerobic conditions, but plant roots need aerated soil.

So leguminous plants have evolved the lepthaemoglobin from cytochrome molecules that are present in all

cells. Lepthaemoglobin absorbs the oxygen in the root nodules and creates an oxygen free atmosphere for thenitrogen fixation bacteria.

Questions

42. Is the oxygen dissociation curve of a pigeon to the right or left of a human? Why?

43. Mackerel swim in the surface water of the sea. They swim fast to avoid predators. Plaice move slowly on

the sea-bed, camouflaged from predators. Both are approx. same mass. Sketch a graph to show the positions

of the oxygen dissociation curves for these 2 fish.

44. What is the effect of increased carbon dioxide concentration on oxygen dissociation?

45. How does this change the saturation of haemoglobin with oxygen?

46. A rise in temperature shifts the oxygen dissociation curve right. How does this enable exercising muscle to

work more efficiently?

Answers


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42. Shifted to the right so more oxygen is readily released to the tissues, so

haemoglobin supplies more oxygen to respiring muscles.

43. Sigmoid curves. Plaice to the left of mackerel.

44. The curve is shifted to the right.

45. Haemoglobin has become less saturated.

46. Exercising muscle releases heat, shifting the curve to the right. This causes haemoglobin to release more

oxygen for muscular activity and increased respiration. Hence supply can meet demand.

(iii) The effects of carbon dioxide concentration – The Bohr Shift

In the presence of a higher concentration of carbon dioxide, haemoglobin has a reduced affinity for oxygen.

Haemoglobin gives up or releases oxygen more readily at higher partial pressures of carbon dioxide.

At any oxygen partial pressure, the haemoglobin is less saturated with oxygen, so the data points on the

dissociation curve are all lower. This shift in the graph’s position – the Bohr shift.

Bohr shift = `The movement of the oxygen dissociation curve to the right at a higher partial pressure of carbon

dioxide, because at a given oxygen partial pressure, haemoglobin has a lower affinity for oxygen.`

At a given partial pressure of oxygen, when the partial pressure of carbon dioxide is higher, haemoglobin has a

lower affinity for oxygen.

Cells respire and produce carbon dioxide. This raised the Pco2, so increases the rate of oxygen unloading and

curve shifts down.

The greater the conc. of carbon dioxide the more readily the oxygen is released. This accounts for the

unloading of oxygen from oxyhaemoglobin in respiring tissues, where the partial pressure of carbon dioxide is

high and oxygen is needed.

Lungs

= gaseous exchange surface. Here low conc. of carbon dioxide, so affinity of haemoglobin for oxygen is

increased, which coupled with the high conc. of oxygen means oxygen is readily loaded by haemoglobin. The

reduced carbon dioxide level has shifted the oxygen dissociation curve to the left.


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Respiring tissues

E.g. muscles – with higher levels of CO2. The affinity of haemoglobin for oxygen is reduced. Added to the low

concentration of oxygen in the muscles means oxygen is readily unloaded from haemoglobin into muscle cells.

The increased CO2 has shifted the oxygen dissociation curve to the right.

Loading, transport and unloading of oxygen.

The greater the concentration of carbon dioxide, the more readily haemoglobin releases its oxygen. This is

because dissolved CO2 is acidic and the low pH causes haemoglobin to change shape.

At gas-exchange surface carbon dioxide is constantly removed.

pH increases here due to low levels of CO2.

High pH changes shape of haemoglobin so that oxygen is loaded more readily.

This change in shape increases the affinity of haemoglobin for oxygen, so it is not released whilst

being transported to the tissues.

In tissue CO2 levels high due to respiration.

Carbon dioxide dissolves in water to form carbonic acid.

This reaction releases hydrogen ions

So pH of blood drops. Lower pH changes shape of haemoglobin so it has a low affinity for oxygen.

Haemoglobin releases oxygen into respiring tissues.

The higher the rate of respiration → the more CO2 tissues produce → the lower the pH → the greater the

haemoglobin shape change → the more readily oxygen is unloaded → the more oxygen is available for

respiration.

Humans – haemoglobin carries 4 oxygen molecules. Normally when resting only 1 of the 4 is unloaded at

respiring tissues, so the haemoglobin that returns to the lungs still is 75% saturated.

In an actively respiring tissue, then the 3 remaining oxygen molecules can be unloaded as well.

Or partial pressure of oxygen (kPa)

kPa 12 = Haemoglobins molecule is loaded with oxygen in the lungs.


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kPa 6 = Haemoglobin molecule in a resting tissue unloads 25% of its oxygen.

kPa 2 = Haemoglobins molecule in an active tissue unloads 75% of its oxygen.

The partial pressure of oxygen at which haemoglobin is 95% saturated = the loading pressure.

The partial pressure of oxygen at which haemoglobin is 50% saturated = the unloading pressure.

(iv) Myoglobin

Is a muscle protein.

Its oxygen dissociation curve is far to the left of haemoglobin.

Normally respiring tissue obtains oxygen from haemoglobin.

Oxyhaemoglobin only unloads its oxygen when the oxygen partial pressure is very low.

E.g. when exercising heavily.

So Myoglobin is described as an oxygen store.

4.

Transport of carbon dioxide

A. The 3 methods

1. In solution in the plasma (approx 5%).

2.

As the hydrogen carbonate ion, HCO3-(approx 85%)

3.

Bound to haemoglobin as carbamino-haemoglobin (approx 10%)

Some carbon dioxide is transported in the red blood cells but most is converted into hydrogen carbonate in

the red blood cells and then it diffuses into the plasma.

B. The chloride shift The following describes the reactions in red blood cells:

CO2 diffuses into red blood cells

Carbon dioxide + water → carbonic acid (H2CO3). Catalysing enzyme = carbonic anhydrase.

Carbonic acid dissociates → hydrogen ions and hydrogen carbonate (HCO3-) ions.

HCO3- ions diffuse out of red blood cells into blood plasma.

To balance the outflow of negative ions and maintain electrochemical neutrality, chloride ions diffuse into the

red blood cells from the plasma = chloride shift

H+ ions cause oxyhaemoglobin to dissociate into haemoglobin and oxygen.

The hydrogen ions are picked up by haemoglobin → haemoglobinic acid, (HHb). This removes hydrogen ions

and so pH of red blood cells does not fall.

Oxygen diffuses out of red blood cells into respiring tissues.


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Haemoglobin here is acting as a buffer for the blood, resisting the changes in blood

pH when carbonic acid is formed. It does this by removing hydrogen ions from solution, so preventing pH

falling.

The formation of haemoglobinic acid forces haemoglobin to unload oxygen, causing the Bohr shift. So the

higher the partial pressure of carbon dioxide; the lower the affinity of haemoglobin for oxygen.This is why the behaviour of haemoglobin is different in the lungs than the tissues.

5. Intercellular or Tissue fluid

Exam tip: make sure you can describe the differences between plasma, tissue fluid and blood.

What is tissue fluid?

Capillaries adapted to allow exchange of materials by:

-

Thin, preamble walls.

-

Large surface area for exchange of materials.

-

Slow blood flow to allow time for exchange.-

Blood flows close to every cell of the body in the capillary networks in all organs. However, it is tissue fluid, not

blood which carries glucose, amino acids, fatty acids, salts and oxygen to the cells. As well as supplying the

cells with materials tissue fluid also receives carbon dioxide and other waste materials from the tissues. In this

way, tissue fluid is the means by which materials are exchanged between blood and cells. Tissue fluid is

formed from blood plasma, when it is forced thorough the capillary walls, in every capillary network. It flows

around the cells, bathing them in a fluid that provides a constant environment. The constant pH and

temperature of the tissue fluid help to provide optimum conditions for enzyme activity in the cells.

The diffusion of solutes in and out of the capillaries relates to the blood’s hydrostatic pressure and solute

potential.


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Formation of tissue fluid

Tissue fluid forms because the capillary walls are permeable to most molecules and the pressure of the bloodentering the capillaries is high enough to force materials across the capillary walls. However, plasma protein

molecules are too large to leave and so are not found in tissue fluid.

As tissue fluid leaves the blood, it carries with it dissolved oxygen and nutrients. These enter the cells from the

tissue fluid by diffusion, active transport, or facilitated diffusion. Most of the tissue fluid that bathes the cells is

returned to the blood in the capillary networks carrying with it dissolved carbon dioxide and other metabolic

waste products. The remainder drains into the lymphatic system. The lymphatic vessels carry lymph towards

the heart. Lymph is returned to the blood where veins from the head and neck join with those from the arm.

Lymph is formed from excess tissue fluid.


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What is Hydrostatic pressure?

Two main factors are involved in the formation of tissue fluid:

Due to the contraction of the ventricles, there is a high hydrostatic pressure in the blood. This acts

outwards on the wall of any vessel carrying the blood. If the wall is permeable, liquid is forced out ofthe vessel.

The other factor is water potential of blood plasma. The plasma contains many dissolved substances

including proteins, which are too large to leave the capillaries. Therefore it has a low water potential,

which draws water into the blood plasma in the capillaries by osmosis.

At the arterial end of a capillary bed:

Blood is under pressure from the heart pumping and muscle contraction in artery and arteriole walls. High

hydrostatic pressure pushes liquids outwards from the capillaries to the spaces surrounding the cells.


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Plasma is a solution and has a low solute potential, due to the colloidal plasma

proteins. It tends to pull water back into the capillary by osmosis.

At the arterial end of the capillary the high hydrostatic effect outweighs the effect of the plasma’s solute

potential and

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2.3 Transport in Animals