alveolus

The tiny air sacs, called alveoli, in the lungs are covered with a network of blood capillaries, allowing oxygen to enter the blood. The alveoli of an adult have a total surface area of 70 sq m/750 sq ft.

Gas exchange in the alveolus. Blood enters the lungs low in oxygen and high in carbon dioxide. The carbon dioxide diffuses from the blood to the alveolus, where carbon dioxide levels are low. Oxygen diffuses from the alveolus, where oxygen levels are high, into the blood where the level is low, so blood leaves the lungs high in oxygen and low in carbon dioxide.

One of the many thousands of tiny air sacs in the lungs. By the process of diffusion oxygen is transported from air in the alveoli into the blood, while carbon dioxide diffuses out of the blood and into the air in the alveoli. This process is called gas exchange. To aid this process, the alveoli have very thin walls, which are folded in order to increase their surface area and allow rapid gas exchange, and are surrounded by blood capillaries. The air in the alveoli is replenished as a result of breathing or ventilation. Smoking and exposure to other pollutants can cause the loss of the folds in the alveoli, which causes difficulties in breathing and the condition called emphysema.