The mammalian circulatory system is adapted for the long-distance transport of the respiratory gases, oxygen and carbon dioxide. The system is described as a closed double circulation.

1. (a)Name: • the two different circulations of the double circulatory system of mammals • the main vein returning deoxygenated blood to the heart.[3]
2. (b)In a closed circulation, blood is kept within vessels at all times. Name the type of blood vessel that connects capillaries and veins.[1]
3. (c)Fig. 1.1 is a diagram of a section through the heart. [Figure 1.1] On Fig. 1.1: • add a label line and the letter L to show the artery that takes blood from the heart to the lungs • add a label line and the letter R to show the valve that closes when the right ventricle is in systole.[2]
4. (d)The entry of carbon dioxide into red blood cells results in the production of hydrogencarbonate ions. This involves the enzyme carbonic anhydrase. Complete the passage summarising the production of hydrogencarbonate ions by: • writing the correct biological term in the spaces provided • writing the molecular formula for two of the terms in the spaces in brackets. Carbonic anhydrase has an overall spherical shape and is known as a ..................................................... protein. The enzyme acts within the cell so can be described as an ..................................................... enzyme. When blood passes into the capillary network through actively respiring tissues, carbon dioxide (CO2) diffuses into red blood cells and carbonic anhydrase catalyses a reaction where ..................................................... (...........................) is combined with CO2 to form ..................................................... (H2CO3), which rapidly forms ..................................................... ions (.................................) and hydrogencarbonate ions (HCO3¯).[5]

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