CIE A Level Biology Topic 8: Transport in Mammals Exam Questions

About CIE A Level Biology Topic 8: Transport in Mammals

Transport in mammals covers the double circulatory system and how blood carries oxygen and carbon dioxide. In the Cambridge 9700 syllabus you need to relate the structure of arteries, veins and capillaries to their functions, describe the components of blood and the formation of tissue fluid, and explain how haemoglobin loads and unloads oxygen, including the oxygen dissociation curve and the Bohr shift. You also need the role of carbonic anhydrase and the chloride shift in carbon dioxide transport. The second half covers the structure of the heart, the cardiac cycle with pressure changes, and the initiation of the heartbeat by the sinoatrial node.

What you need to know

• Blood vessels Arteries have thick walls with elastic fibres and smooth muscle to withstand and smooth high pressure. Veins have thin walls, a wide lumen and valves. Capillaries are one cell thick for short diffusion distances.
• Tissue fluid At the arterial end of a capillary, hydrostatic pressure forces fluid out; at the venous end, lower water potential of the blood draws water back. Excess tissue fluid drains into lymph.
• Haemoglobin Haemoglobin binds oxygen cooperatively, giving an S-shaped dissociation curve. The Bohr shift moves the curve right at high carbon dioxide concentration, so oxygen is released more readily in respiring tissues.
• Carbon dioxide transport Most carbon dioxide is carried as hydrogencarbonate ions. Carbonic anhydrase in red blood cells catalyses carbonic acid formation; H+ binds to haemoglobin to form haemoglobinic acid and chloride ions move in (chloride shift).
• Heart structure The left ventricle wall is thicker than the right because it pumps blood around the whole body. Atrioventricular and semilunar valves prevent backflow.
• Cardiac cycle and control The SAN acts as a pacemaker; the wave of excitation spreads over the atria, is delayed at the AVN, then passes down the Purkyne tissue so the ventricles contract from the base upwards.

Common mistakes in Topic 8: Transport in Mammals

• Saying the left ventricle is thicker to pump more blood; it pumps the same volume but at higher pressure over a longer distance.
• Stating that valves open because the heart opens them; valves open and close because of pressure differences.
• Describing the Bohr effect as haemoglobin binding carbon dioxide instead of oxygen; carbon dioxide lowers pH and reduces the affinity of haemoglobin for oxygen.
• Forgetting the delay at the AVN; it allows the atria to empty before the ventricles contract.
• Saying tissue fluid contains red blood cells and plasma proteins; these are too large to leave capillaries.

Try three Topic 8: Transport in Mammals exam-style questions