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#48 Summary of The mammalian heart

1 The human heart, like that of all mammals, has two atria and two ventricles. Blood enters the heart by the atria and leaves from the ventricles. A septum separates the right side of the heart, which contains deoxygenated blood, from the left side, which contains oxygenated blood.

2 Semilunar valves at the entrances to the blood vessels that leave the heart (aorta and pulmonary
artery) prevent back flow of blood into the heart, and atrioventricular valves prevent backflow of blood from ventricles into the atria.

3 The heart is made of cardiac muscle and is myogenic (the muscle is self-stimulating).

4 The sinoatrial node (SAN) sets the pace of contraction for the muscle in the heart. Excitation
waves spread from the SAN across the atria, causing their walls to contract. A non-conducting barrier prevents these excitation waves from spreading directly into the ventricles, thus delaying their
contraction. Th e excitation wave travels to the ventricles via the atrioventricular node (AVN) and
the Purkyne tissue, which runs down through the septum, before spreading out into the walls of the
ventricles.

5 Both sides of the heart contract and relax at the same time. Th e contraction phase is called systole, and the relaxation phase is diastole. One complete cycle of contraction and relaxation is known as the cardiac cycle.

1. Multiple-choice test

1 Which of the following describes the mammalian circulation?

A open single circulation
B closed single circulation
C open double circulation
D closed double circulation

2 The diagram shows a vertical section through a human heart.

Which row identifies the blood vessels W, X, Y and Z?

3 Which row describes the aorta?

4 The diagrams are vertical sections through the human heart.

Which pair of arrows shows blood flow through the heart?

5 The right ventricle has much less muscle in its wall than the left ventricle.
What are the consequences of this?

1 The right ventricle develops a much smaller pressure than the left ventricle.
2 The right ventricle delivers a smaller volume of blood than the left ventricle.
3 Blood from the right ventricle travels less far than blood from the left ventricle.

A 1, 2 and 3
B 1 and 2 only
C 1 and 3 only
D 2 and 3 only

6 What are the positions of the valves on the left side of the heart when the pressure in the left ventricle is higher than the pressures in the left atrium and aorta?

7 Which of the following statements is not correct?

A Atrial muscles are connected to the ventricle muscles, except at the atrioventricular node (AVN).
B Both atria contract at the same time.
C Both ventricles contract at the same time.
D Contraction of the atria is complete before contraction of the ventricles begins.

8 Which is the correct sequence of events in a cardiac cycle, beginning with its initiation by the pacemaker?

1 A wave of electrical activity passes along Purkyne tissue.
2 A wave of electrical activity reaches the atrioventricular node (AVN).
3 A wave of electrical activity spreads from the sinoatrial node (SAN) across the atria.
4 Cardiac muscle of the walls of the atria contracts.
5 Cardiac muscle of the walls of the ventricles contracts.

A 1 → 5 → 3 → 4 → 2
B 2 → 1 → 5 → 3 → 4
C 3 → 4 → 2 → 1 → 5
D 4 → 2 → 1 → 5 → 3

9 When a heart is removed from a mammal and kept in well-oxygenated buffer solution at 37°C, it continues to beat rhythmically.
What may be concluded about the heart from this observation?

A It has an in-built mechanism for initiating contractions.
B It needs a blood supply to be able to contract.
C It needs a stimulus from a nerve to be able to contract.
D It needs a stimulus from a hormone to be able to contract.

10 The volume of blood pumped by the heart in a given period of time is called the cardiac output. It is calculated from the volume of blood pumped by one contraction of the heart (stroke volume) and the number of times the heart contracts per minute (heart rate).

cardiac output = stroke volume × heart rate

The cardiac output of a heart beating at 75 beats per minute was calculated to be 6.0dm3 per minute.
What was the stroke volume of the heart?

A 0.08cm3
B 12.5cm3
C 80cm3
D 125cm3

Answers to Multiple choice test

1. D

2. B

3. B

4. A

5. C

6. C

7. A

8. C

9. A

10. C

2. End-of-chapter questions

1 Where is the mammalian heart beat initiated?

A atrioventricular node
B left atrium
C Purkyne tissue
D sinoatrial node

2 What causes the bicuspid valve to close during ventricular systole?

A a greater blood pressure in the left atrium than in the left ventricle
B a greater blood pressure in the left ventricle than in the left atrium
C contraction of muscles in the septum
D contraction of muscles in the valve

3 Figure below shows the pressure changes in the left atrium, left ventricle and aorta throughout two cardiac cycles. Make a copy of this diagram.

a i How long does one heart beat (one cardiac cycle) last?
ii What is the heart rate represented on this graph, in beats per minute?
b 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 your copy of the diagram, mark the following periods:
i the time when the ventricle is contracting (ventricular systole)
ii the time when the ventricle is relaxing (ventricular diastole).

c 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 your copy of 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).

d The atrioventricular 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 your diagram,mark the point at which these valves will open and close.
e The opening and closing of the semilunar valves in the aorta depends in a similar way on the relative pressures inthe aorta and ventricles. On your diagram, mark the point at which these valves will open and close.
f The right ventricle has much less muscle in its walls than the left ventricle, and only develops about one-quarter of the pressure developed on the left side of the heart. On your diagram, draw a line to represent the probablepressure inside the right ventricle over the 1.3 seconds shown.

The diagram shows a normal ECG. The paper on which the ECG was recorded was running at a speed of 25 mm s^-1

a Calculatethe heart rate in beats per minute.

b Thetime interval between Q and T is called the contraction time.

i Suggest why it is given this name.

ii Calculate the contraction time from this ECG.

c The time interval between T and Q is called the filling time.

i Suggest why it is given this name.

ii Calculate the filling time from this ECG.

d An adult male recorded his ECG at different heart rates. The contraction time and filling time were calculated from the ECGs. The results are shown in the table.

i Suggest how the man could have increased his heart rate for the purposes of the experiment.

ii Present these results as a line graph, drawing both curves on the same pair of axes.

iii Comment on these results.

5 The figure below shows a cross-section of the heart at the level of the valves.

a i Copy and complete the following flow chart to show the pathway of blood through the heart.

ii Explain how the valves P and Q ensure one-way flow of blood through the heart.

The cardiac cycle describes the events that occur during one heart beat. The following figure shows the changes in pressure that occur within the left atrium, left ventricle and aorta during one heart beat.

Copy and complete the table below. Match up each event during the cardiac cycle with an appropriate number from 1 to 7 on the figure. You should put only one number in each box. You may use each number once, more than once or not at all.
The firstanswer has been completed for you.

[4]

Explainthe roles of the sinoatrial node (SAN), atrioventricular node (AVN) and the Purkyne tissue during one heart beat. [5]
[Total: 13]

[Cambridge Intemational AS andA Level Biology 9700 Paper 21, Question 3, May - june 2010]

3. End-of-chapter answers

1 D
2 B
3 a i about 0.75 seconds
ii 60 ÷ 0.75 = 80 beats per minute
For b, c, d, e and f, see figure below.

4 a 1 beat = about 20 mm on the grid. 25 mm on the grid represents 1 second
so 20 mm represents 20÷25 seconds = 0.8 seconds. If one beat lasts 0.8 seconds, then in 1 second there are 1÷0.8 beats so in 1 minute there are 60÷0.8 = 75 beats.
b i this is the time during which the ventricles are contracting
ii on the grid, the distance between Q and T is about 7 mm this represents 7 ÷ 25 = 0.28 seconds

c i this is the time when the ventricles are relaxed, and are fi lling with blood
ii on the grid, the distance between T and Q is about 13 mm
this represents 13 ÷ 25 = 0.52 seconds
A quicker way of working this out is to subtract the answer to b ii from 0.8 seconds.

d i by performing varying levels of exercise
ii

iii As heart rate increases, contraction time remains constant, but fi lling time decreases.
This indicates that the increase in heart rate is produced by a shorter time interval between
ventricular contractions, rather than by a faster ventricular contraction.

The more frequent contractions increase the rate of circulation of blood around the body,
providing extra oxygen to exercising muscles.

If this was done by shortening the time over which the ventricles contract, much of the
advantage would be lost, as less blood would probably be forced out by each contraction.
By shortening the time between contractions, the amount of blood pumped out of the heart
per unit time is increased.

Exam-style questions

5 a i right ventricle;
pulmonary vein; [2]
ii they open to allow blood to fl ow from atria to ventricles;
they close during ventricular systole/when ventricles contract;
reference to closure being caused by diff erences in pressure in atria and ventricles; [max. 2]

c SAN produces rhythmic pulses of electrical activity;
which spread across the muscle in the atria;
causes muscle in atria to contract;
specialised tissue, in septum/near AVN, slows spread/delays transfer to ventricles;
Purkyne tissue conducts impulses down through septum;
impulses spread upwards through ventricle walls;
causing ventricles to contract from bottom upwards;
delay of 0.1 to 0.2 s after atrial walls; [max. 5]

[Total: 13]

#47 Summary of The mammalian transport system

shahid Mandokhail AS , Summary

1. Blood is carried away from the heart in arteries, passes through tissues in capillaries, and is returned to the heart in veins. Blood pressure drops gradually as it passes along this system.

2. Arteries have thick, elastic walls, to allow them to withstand high blood pressures and to smooth out the pulsed blood flow. Capillaries are only just wide enough to allow the passage of red blood cells, and have very thin walls to allow effi cient and rapid transfer of materials between blood and cells. Veins have thinner walls than arteries and possess valves to help blood at low pressure flow back to the heart.

3. Blood plasma leaks from capillaries to form tissue fluid. This is collected into lymphatics as lymph, and returned to the blood in the subclavian veins. Tissue fluid and lymph are almost identical in composition; both of them contain fewer plasma protein molecules than blood plasma, as these are too large to pass through the pores in the capillary walls.

4. Red blood cells are relatively small cells. They have a biconcave shape and no nucleus. Their cytoplasm is full of haemoglobin.

5. White blood cells include phagocytes and lymphocytes. They all have nuclei, and are either
spherical or irregular in shape.

6. Red blood cells carry oxygen in combination with haemoglobin. Haemoglobin picks up oxygen at high partial pressures of oxygen in the lungs, and releases it at low partial pressures of oxygen in respiring tissues. A graph showing the percentage saturation of haemoglobin at diff erent partial pressures (concentrations) of oxygen is known as a dissociation curve. At high carbon dioxide concentrations, the dissociation curve shifts downwards and to the right, showing that haemoglobin releases oxygen more easily when carbon dioxide concentration is high. This is known as the Bohr effect.

7 Carbon dioxide is mostly carried as hydrogencarbonate ions in blood plasma, but also in combination with haemoglobin in red blood cells and dissolved as carbon dioxide molecules in blood
plasma.

8 At high altitudes, the partial pressure of oxygen is so low that altitude sickness can be caused, which can be fatal. The body can adapt to gradual changes, however, by producing more red blood cells and haemoglobin.

VIDEO

Video
Human circulation system

1. Multiple-choice test
1. Which description of blood vessels is correct?

A Arteries have thick walls of smooth muscle with valves at intervals.
B Arteries near the heart have large numbers of elastic fibres in their thick walls.
C Capillary walls consist of a layer of endothelium surrounded by collagen fibres.
D Small veins have thin walls made entirely of smooth muscle.

2. Which comparison of blood pressures is correct?

A The pressure in arterioles is lower than in venules.
B The pressure in capillaries is lower than in small veins.
C The pressure in small arteries is higher than in large veins.
D The pressure in the vena cava is higher than in capillaries.

3 Which statement about veins is not correct?

A Blood is forced through a semilunar valve by the contraction of smooth muscle fibres in the wall of the vein.
B Semilunar valves allow blood to move towards the heart but not away from it.
C Semilunar valves are formed from the endothelium and are moved by changes in blood pressure.
D The pressure needed for blood flow in a vein is produced by contraction of nearby skeletal muscles.

4 Which of the following describe a phagocyte?
1 lobed nucleus
2 spherical nucleus
3 small granules in the cytoplasm
4 very little cytoplasm
5 smaller than a red blood cell

A 1, 3 and 5 only
B 2, 4 and 5 only
C 1 and 3 only
D 2 and 4 only

5. Which of the following describes a molecule of haemoglobin?

A A molecule made up of four haem groups, each of which binds reversibly to an atom of oxygen.
B A molecule made up of a single haem group which binds irreversibly with a molecule of oxygen.
C A protein with quaternary structure, consisting of a single globin polypeptide attached to a haem group.
D A protein with quaternary structure, consisting of two α- and two β-globin polypeptides, each attached to a haem group.

6 Which of the following word equations, showing reactions in a red blood cell, includes a mistake?

A    haemoglobin + oxygen oxyhaemoglobin
B oxyhaemoglobin + hydrogen ions haemoglobinic acid
C carbon dioxide + water carbonic acid
D    haemoglobin + carbon dioxide carboxyhaemog

7 The red blood cell count of humans increases when they remain at high altitudes.
What is the effect of this?

A    It increases the Bohr effect.
B It compensates for the lack of oxygen at high altitudes.
C It reduces the amount of haemoglobin per red blood cell.
D It increases the percentage saturation of haemoglobin with oxygen.

8. The graph shows dissociation curves for haemoglobin at two different concentrations of carbon dioxide.

What may be concluded from the graph?

A P is at a higher concentration of carbon dioxide than Q.
B P is at a lower pH than Q.
C Q shows haemoglobin that is more saturated with oxygen than P.

D Q shows haemoglobin with a lower affinity for oxygen than P.

9. The diagram shows dissociation curves for adult haemoglobin, fetal haemoglobin and myoglobin. Myoglobin only releases oxygen when concentrations are very low. Fetal haemoglobin has a higher affinity for oxygen than adult haemoglobin does.

10 The statements describe blood, tissue fluid and lymph in a capillary bed.

W lacks large plasma proteins and red blood cells and has a higher water potential than Z.
X is at a lower pressure than Y and contains red blood cells and large plasma proteins.
Y is at a higher pressure than W and contains red blood cells and large plasma proteins.
Z is at a lower pressure than Y and lacks red blood cells.

Which row identifies W, X, Y and Z?

Answers to Multiple choice test

1. B

2. C

3. A

4. C

5. D

6. D

7. B

8. D

9. D

10. D

2. End-of-chapter questions

1 The diagram shows the changes in blood pressure as blood flows through the blood vessels in the human systemic circulatory system.

The micrograph shows an artery and a vein.

3. Constructa table comparing the structure of arteries, veins and capillaries. Include both similarities and differences, and give reasons for the differences which you describe.

4. Constructa table comparing blood plasma, tissue fluid and lymph.

5. Explain how the structure of haemoglobin enables it to carry out its functions. (You may wish to remind you about the various levels of structure of a protein molecule such as Hb)

6. The following statements were all made by candidates in examination answers. Explain what is wrong with each statement.

a Oxyhaemoglobin gradually releases its oxygen as it passes from the lungs to a muscle. The b The strong walls of arteries enable them to pump blood around the body.
c Each red blood cell can combine with eight oxygen atoms.
d Red blood cells have a large surface area so that many oxygen molecules can be attached.

7 Carbon dioxide is transported in the blood in various forms.
a Describe how carbon dioxide molecules reach red blood cells from respiring cells. [:
The figure shows part of a capillary network and some cells of the surrounding tissue.

b State three ways in which the blood at Y differs from the blood at X other than in the concentration of carbon dioxide.

An enzyme in red blood cells catalyses the reaction between carbon dioxide and water as blood flows through respiring tissues.

c i Name the enzyme that catalyses this reaction. [1]
ii Explain the significance of this reaction in the transport of carbon dioxide. [3]
d The figure below shows the effect of increasing the carbon dioxide concentration on the oxygen dissociation curve for haemoglobin.

i State the percentage saturation of haemoglobin with oxygen at a partial pressure of 5 kPa of oxygen when the partial pressure of carbon dioxide is:

1.0 kPa

1.5 kPa [1]

ii The percentage saturation of haemoglobin with oxygen decreases as the partial pressure of carbon dioxide increases.Explain how this happens. [2]

iii Name the effect of increasing carbon dioxide concentration on the oxygen dissociation curve. [1]

iv Explain the importance of the effect of carbon dioxide on haemoglobin as shown in the figure. [3]

[Total: 16]

[Cambridge International AS and A Level Biology 9700 Paper 21, Question 2, June 2011]

8. Mammalh save a closed, double circulation.

a State what is meant by the term double circulation. [1]

The figure below shows part of the circulation in a mammalian tissue. The central part is enlarged to show a capillaray,cell supplied by the capillary, and vessel Z.

b Explainwhy the wall of the artery is thicker than the wall of the vein. [2]
c Suggest one role for the pre-capillary sphincter muscle shown in the figure. [1]
d With reference to the figure, describe the role of capillaries in forming tissue fluid. [3.]
e i Describe three ways in which plasma differs from tissue fluid. [3]
ii Name the fluid in vessel Z. [1]

[Total:11]

[Cambridge InternationalAS andA Level Biology 9700 Paper 2, Question 4, November 2008]

3. End-of-chapter answers

1 C
2 D

5 Points that could be made include:

• The haemoglobin molecule is a protein with quaternary structure. Hydrogen bonds, ionic bonds and van der Waals forces hold the protein

in its three-dimensional shape, which is important for its function.

• The primary structure of each polypeptide chain determines how the chain will fold and where the bonds will form, thus determining its three- dimensional shape.

• The haemoglobin molecule has R groups with small charges on its outer surface (hydrophilic R groups), which help to make it soluble in water. This allows it to dissolve in the cytoplasm of a red blood cell.

• Each haemoglobin molecule is made up of four polypeptide chains, each with a haem group at its centre. Each haem group can bind reversibly with one oxygen molecule.

• When one oxygen molecule binds with one of the haem groups, it slightly changes the shape of the haemoglobin molecule so that it becomes easier

for more oxygen molecules to bind with the other haem groups.

6 a The word ‘gradually’ is not correct. The partial pressure of oxygen is high in the lungs and low in muscle. It does not change gradually as the blood flows from the lungs to the muscle, because it is only when it gets to the muscle that the blood is in contact with anything that is using oxygen. While it is inside an artery, it remains fully oxygenated. The blood is only exposed to a low partial

pressure of oxygen once it enters a capillary inside a respiring tissue, such as a muscle. Capillary walls, unlike those of arteries, are thin and easily permeable to oxygen.

b Arteries do not pump blood. Their strong walls, which are also elastic, enable the artery to expand and recoil as pulses of high-pressure blood pass through. The recoil of the artery wall does help to give the blood a further ‘push’ in between these pulses, but this is not ‘pumping’ and is due only to elasticity, not to muscle contraction.

c This should say: Each haemoglobin molecule can combine with eight oxygen atoms. A red cell is huge compared with a haemoglobin molecule. One red cell contains well over 200 million haemoglobin molecules.

d Red blood cells do have a large surface area, but oxygen does not attach to their surface. The large surface area allows more oxygen to diffuse in and out at any one time, therefore increasing the rate at which the cell can take up and release oxygen. Once inside the cell, the oxygen does not attach to its surface, but to the haemoglobin molecules within its cytoplasm.

Exam-style questions

7 a reference to diffusion;

down concentration gradient;

through the wall of a capillary; [max. 2]

b lower pressure;

lower concentration of oxygen; lower concentration of glucose;
lower water potential;

lower concentration of proteins/amino acids/fatty acids/other named nutrient;

higher concentration of carbon dioxide/urea; [max. 3]

  c i    carbonic anhydrase;    [1]
ii   hydrogencarbonate ions diffuse out of red blood cells;
(hydrogencarbonate ions) are transported in solution in blood plasma;
conversion of CO2 to hydrogencarbonate reduces concentration of CO2 in the blood;
which maintains diffusion gradient for CO2 to diffuse into the blood from respiring tissues;    [max. 3]

d i 73%, 62%; [1]

ii presence of carbondioxide causesaffinity of haemoglobin for oxygen to decrease;

hydrogen ions (from the dissociation of H2CO3) bind with haemoglobin;

cause change in shape of Hb molecule; [max. 2]
iii Bohr effect; [1]

iv causes morerelease of oxygen (than if this effect did not occur);
in respiring tissues;

where demand for oxygen is high/where production of carbon dioxide is high; [3]

[Total: 16]

8 a blood goes through heart twice on onecomplete circuit of the body; [1]

b has more smooth muscle/elastic tissue;

to withstand higher (blood) pressure;

to withstand fluctuating (blood) pressure; [max. 2]

c to prevent blood flowing into the capillary bed/to divert blood to other capillary beds; [1]
d permeable walls/reference to pores in walls;
allow water/dissolved ions/dissolved substances (from plasma) to pass out;
do not allow large protein molecules/cells to pass out;
reference to greater hydrostatic pressure inside capillary than in tissue fluid; [max. 3]
e i (plasma contains) more proteins;

has lower water potential;

has lower, carbon dioxide/HCO3 concentration;

has greater glucose concentration;

has greater oxygen concentration; [max. 3]
ii lymph [1]

[Total: 11]