L Pretest: Unit 5
Gas exchange takes place across the membranes of the alveoli and the
capillaries. How do the structures of these membranes improve the efficiency
of this process?
AIK
Bronchiole,
Alveo
Small blood
vessels (capillaris)
A. Pores in the membranes allow oxygen to move out of the alveoli's
cells and carbon dioxide to move into the cells.
O B. The shapes of the membranes slow blood flow, allowing more
time for gas exchange
O C. The shapes of the alveoli and the capillaries increase the surface
area of the membranes.
O D. The membranes have proteins that transport carbon dioxide and
oxygen against the concentration gradients.

Alveoli are tiny air sacs in the lungs, which takes the oxygen inside them. The membranes of the alveoli and the capillaries expand to take up the oxygen whereas the contract or becomes narrower when they expel out the carbon dioxide gas. Thus the shapes of the membranes of the alveoli and capillaries play an important role during exchange of gases.