Alveoli have a large surface area in order to rapidly exchange gases, which is the primary function of the lungs. There are approximately 300 million alveoli found inside the lungs.
Gas exchange between the air that humans breathe in and the blood within the circulatory system is vital to survival. Gas exchange describes the exchange of oxygen and carbon dioxide across the respiratory membrane. The alveoli are tiny, circular air sacs which are important because they significantly increase the surface area of the lungs. Due to the many surface walls of the alveoli, the lungs have a surface area that is approximately the size of a tennis court.
This large surface area allows for rapid gas exchange. Carbon is diffused into the alveoli and oxygen is diffused out from the alveoli into the blood. Each alveolus is connected to a web of capillaries which bring oxygen-deficient red blood cells close to the alveoli so that they may absorb diffused oxygen. This intricate and expansive capillary network allows for the continuous exchange of large volumes of gas.
The alveoli have several additional features that work to improve the efficiency of the lungs. Alveoli are moist so that gases dissolve before diffusing through them. Gases are able to diffuse through alveoli at a fast pace due to their thin walls.