IB Biology HL B3.1.2 Properties of Gas Exchange Surfaces Topic Practice

a. alveoli provide a large surface area (for easy diffusion of gases);

b. thin walls/ 1 -cell thick walls /type 1 pneumocytes/cells lining the alveoli are (extremely) thin (to facilitate gas exchange);

c. a moist surface on the cells (to dissolve the gases/oxygen and carbon dioxide);

d. type II pneumocytes secrete surfactant to reduce surface tension/prevent sides of alveolus sticking together;

e. blood capillaries have thin walls/1-cell thick walls (for short path/easy diffusion of gases);

f. good supply of blood / blood flow through capillaries around alveoli to maintain the concentration gradients of gases/ higher oxygen concentration in alveoli than in blood capillaries/ higher $\mathrm{CO}_{2}$ concentration in blood capillaries that in alveoli;

g. ventilation/breathing maintains concentration gradients of gases (between air in alveoli and blood capillaries) (to facilitate diffusion);

Mpb. Refers to thin walls, not membranes.

Mpd. Requires function of surfactant.

3 marks