Transport in Mammals: Biology 9700 (Cambridge International AS & A Level)
Syllabus 8.1, 8.2, 8.3 · Strand 3 Internal transport and homeostasis
- Questions
- 10
- Total marks
- 49
- Tier mix
- 10 Core
0 of 10 questions completed
Syllabus coverage
- 8.1 4 questions completed
- 8.2 3 questions completed
- 8.3 3 questions completed
Large, active animals cannot rely on diffusion alone to supply their cells, so they need a transport system, and this topic (syllabus 8.1 to 8.3) explains how the mammalian one is built and how it works. Mammals have a closed double circulation, in which blood passes through the heart twice for each circuit of the body. You should be able to relate the structure of each vessel to its function: the thick, elastic walls of arteries that withstand high pressure, the wide lumen and valves of veins that return blood at low pressure, and the single-cell-thick walls of capillaries that allow rapid exchange and the formation of tissue fluid.
The transport of respiratory gases is a central theme. Oxygen is carried by haemoglobin, and its uptake and release are summarised by the oxygen dissociation curve, whose sigmoid shape you should be able to interpret at the partial pressures found in the lungs and in respiring tissues. Carbon dioxide is transported partly as hydrogencarbonate ions, involving carbonic anhydrase and the chloride shift, and its effect on oxygen release is described by the Bohr shift. The topic ends with the structure of the heart and the cardiac cycle, linking pressure changes during systole and diastole to the opening and closing of valves and the role of the SAN, AVN and Purkyne tissue.
The exam-style questions below are original, written to match these objectives, each with a full worked solution so you can check your reasoning step by step.
Question 1
A cross-section of the aorta, close to where it leaves the heart, shows a thick wall containing a large proportion of elastic tissue and smooth muscle in its middle layer.
Which statement best explains the function of this elastic tissue in the aorta wall?
Question 2
The cardiac cycle describes the sequence of contraction and relaxation of the heart's atria and ventricles during one heartbeat.
(a) Describe how atrial systole and ventricular diastole together complete the filling of the ventricles with blood, and explain why the atrioventricular valves are open at this point in the cycle. [2]
(b) Explain, in terms of the pressure inside the heart chambers and the main arteries, why the atrioventricular valves close at the start of ventricular systole and why the semilunar valves then open shortly afterwards. [3]
(c) State the roles of the sinoatrial node (SAN), the atrioventricular node (AVN) and the Purkyne tissue (bundle of His and Purkyne fibres) in coordinating one complete cardiac cycle. [3]
Question 3
Percentage saturation of haemoglobin with oxygen was measured across a range of partial pressures of oxygen, once in blood at rest and once in blood during vigorous exercise (when the partial pressure of carbon dioxide in the blood is considerably higher than at rest). At a partial pressure of oxygen of 4 kPa, typical of actively respiring muscle tissue, percentage saturation of haemoglobin was 40% in the resting sample but only 20% in the sample taken during exercise.
(a) Describe the shape of the oxygen dissociation curve for adult haemoglobin, and explain, in terms of the structure and behaviour of the haemoglobin molecule, why it has this shape. [4]
(b) State the name given to the effect in which a rise in the partial pressure of carbon dioxide shifts the oxygen dissociation curve to the right, and use the data above to explain why this effect benefits actively respiring muscle tissue during exercise. [4]
Question 4
(a) Describe how most of the carbon dioxide produced by respiring tissues is transported in the blood to the lungs, referring to the roles of carbonic anhydrase and the chloride shift. [4]
(b) Explain how the hydrogen ions produced during the reactions described in part (a) affect the affinity of haemoglobin for oxygen at respiring tissues, and name the effect this describes. [3]
Question 5
(a) Explain how tissue fluid is formed at the arteriole end of a capillary bed, referring to hydrostatic pressure and the (oncotic) pressure due to plasma proteins. [3]
(b) Explain how the structure of the wall of a capillary is related to its function in allowing the formation of tissue fluid and the exchange of substances with surrounding cells. [3]
Question 6
In mammals, blood passes through the heart twice during each complete circuit around the body: once through the pulmonary circulation (heart to lungs and back to the heart) and once through the systemic circulation (heart to the rest of the body and back to the heart), rather than travelling through the lungs and then on to the rest of the body in one unbroken loop.
(a) State the term used to describe this arrangement of the mammalian circulatory system. [1]
(b) Fish have a single circulation: blood is pumped from the heart through the gills and then flows on, without returning to the heart, directly to the rest of the body before eventually returning to the heart. Explain, in terms of blood pressure, why the mammalian double circulation is able to deliver blood to body tissues faster than a single circulation of this kind. [4]
Question 7
A vein returning blood from the leg to the heart has a much wider lumen relative to its overall diameter than an artery of similar overall diameter, a relatively thin wall containing little smooth muscle or elastic tissue, and a series of semilunar (pocket) valves at intervals along its length.
Which statement best explains why veins have a wide lumen and valves?
Question 8
In the placenta, oxygen diffuses from the mother's blood into the fetus's blood across a barrier of tissue, without the two blood supplies mixing directly. At a partial pressure of oxygen of 3 kPa, typical of the placenta, percentage saturation of maternal (adult) haemoglobin is approximately 30%, while percentage saturation of fetal haemoglobin at this same partial pressure of oxygen is approximately 55%.
(a) Describe, in terms of oxygen affinity, how the oxygen dissociation curve for fetal haemoglobin differs from the oxygen dissociation curve for adult haemoglobin, and use the data above to support your answer. [3]
(b) Explain how this difference in oxygen affinity allows oxygen to diffuse from the mother's blood into the fetus's blood within the placenta. [3]
Question 9
The table below shows the pressure of blood in the left atrium, the left ventricle and the aorta at four points, T1 to T4, during one cardiac cycle in a resting mammal.
| Time point | Left atrial pressure / kPa | Left ventricular pressure / kPa | Aortic pressure / kPa |
|---|---|---|---|
| T1 | 1.0 | 0.9 | 11.0 |
| T2 | 1.0 | 6.0 | 11.0 |
| T3 | 1.0 | 15.0 | 15.0 |
| T4 | 1.0 | 8.0 | 10.0 |
(a) Using the pressure values at T2, state whether the atrioventricular (bicuspid) valve and the aortic (semilunar) valve are open or closed at this point in the cardiac cycle, and explain your answer by reference to the pressure data. [3]
(b) At T4, left ventricular pressure has fallen below the aortic pressure recorded at T3. State the name given to this phase of the cardiac cycle, and explain, in terms of valve action, what is happening to the volume of blood inside the left ventricle at T4 despite the ventricle wall no longer actively contracting. [3]
Question 10
In a mammalian heart, the muscular wall of the left ventricle is considerably thicker than the muscular wall of the right ventricle, and a muscular septum completely separates the left and right sides of the heart.
Which statement best explains the functional significance of these two features?