Gas exchange, the intake of molecular oxygen (O₂) from the environment and the outflow of carbon dioxide (CO₂) into the environment, is necessary for cellular function. Gas exchange during respiration occurs largely via the movement of gas molecules along pressure gradients. Gas travels from areas of higher partial pressure to areas of lower partial pressure. In mammals, gas exchange occurs in the alveoli of the lungs, which are adjacent to capillaries and share a membrane with them.

When the lungs expand, the resultant decrease in pressure relative to the atmosphere draws oxygen into the lungs. Air entering the lungs from the environment has a higher oxygen concentration and a lower carbon dioxide concentration than the oxygen-depleted blood that travels from the heart to the lungs. Thus, oxygen diffuses from the alveoli to the blood in the capillaries, where it can be delivered to tissue. Carbon dioxide, by contrast, diffuses from the capillaries to the alveoli, where it can be expelled through exhalation.

Partial Pressure

Gas flow is determined by the pressure gradient of each gas, with each gas moving down its gradient. The pressure exerted by an individual gas in a mixture of gases is its partial pressure, and each gas moves from a higher to a lower partial pressure. Thus, the movement of O₂ and CO₂ are not directly related.

The Big Picture

Oxygen is used by the human body to convert sugar and other organic molecules into the energy compound ATP during the process of cellular respiration. A byproduct of cellular respiration is CO₂, which needs to be removed from cells or it will change the pH and damage the cells. Because oxygen is necessary to provide energy for crucial cellular functions, and CO₂ cannot be allowed to build up, the human body needs a constant flow of blood to and from all tissue to enable gas exchange.

Alveoli

The respiratory and circulatory systems structurally and functionally meet at the alveoli. Alveoli and capillaries are intertwined and physically touch, and because both are typically one cell thick, gas exchange occurs easily between the two. Even though the lungs are not large, the amount of O₂ and CO₂ that is exchanged is huge because there are so many alveoli—hundreds of millions per lung—with a surface area of around 100 m²!