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
Animation
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.
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
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.
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]
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 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]
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]
iii Bohr effect; [1]
iv causes morerelease of oxygen (than if this effect did not occur);
in respiring tissues;
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;
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]
ii lymph [1]
[Total: 11]
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