OxfordAQA International A Level Biology 3.2.6 The Circulation of Blood and the Structure of the Mammalian Heart Exam Questions

About OxfordAQA International A Level Biology 3.2.6 The Circulation of Blood and the Structure of the Mammalian Heart

This Unit 2 topic covers the mammalian blood system and the heart as a pump. You need to relate the structures of arteries, veins and capillaries to their functions, explain how tissue fluid forms and returns to the blood, and describe the gross structure of the heart. You also need to explain the pressure and volume changes in the cardiac cycle and how the atrioventricular and semilunar valves open and close. Cardiac output calculations are common. It links to gas exchange, heart disease and Required practical 5 on heart or pulse rate. Examiners often give a pressure graph and ask when valves open or close.

What you need to know

• Blood vessels Arteries have thick walls with elastic tissue and smooth muscle to withstand and maintain high pressure. Veins have thin walls, a wide lumen and valves to prevent backflow. Capillaries have walls one cell thick, giving a short diffusion distance for exchange.
• Tissue fluid At the arterial end of a capillary, hydrostatic pressure forces water and small solutes out to form tissue fluid. At the venous end, hydrostatic pressure is lower and water returns by osmosis because plasma proteins lower the water potential of the blood. Excess fluid drains into the lymph system.
• Heart structure The heart has right and left atria and ventricles. The left ventricle has a thicker muscular wall because it pumps blood around the whole body. Blood enters through the vena cava and pulmonary vein and leaves through the pulmonary artery and aorta.
• Cardiac cycle Atrial systole pushes blood into the ventricles. Ventricular systole raises pressure, closing the atrioventricular valves and opening the semilunar valves. In diastole the heart relaxes and the semilunar valves close as pressure in the arteries exceeds that in the ventricles.
• Cardiac output Cardiac output = heart rate x stroke volume. Heart rate can be found as 60 divided by the length of one cycle in seconds.

Common mistakes in 3.2.6 The Circulation of Blood and the Structure of the Mammalian Heart

• Saying arteries have muscle to pump blood. Muscle in artery walls constricts or dilates the vessel; the heart pumps blood.
• Saying valves open because of muscles. Valves open and close because of differences in pressure on either side.
• Explaining the return of tissue fluid without plasma proteins. Plasma proteins give the blood a lower water potential, so water returns by osmosis.
• Forgetting to convert the time of one cardiac cycle into beats per minute before calculating cardiac output.

Try three 3.2.6 The Circulation of Blood and the Structure of the Mammalian Heart exam-style questions