Ok since you don't have any answers to choose from I would say Functions is the right answer. Usually we're supposed to explain why but I'm not sure how to for this question, sorry.
Mutations and re combinations
The Major function of the respiratory system is to exchange oxygen and carbon dioxide. Inhaled oxygen enters the lungs and reaches the alveoli, the layers of the cells lining the alveoli and the surrounding capillaries are each only one cell thick and are in very close contact with each other, the barrier between air and blood averages about 1 micron in thickness. Oxygen diffuses quickly through the air-blood barrier into the blood in the capillaries. Similarly, carbon dioxide passes from the blood into the alveoli and is then exhaled.
Chronic lymphocytic leukaemia where your body build up adnormal white cells which causes cancer of the cells. Also HIV destroys these cells.
