Gas exchange is the biological process by which oxygen is absorbed into the bloodstream and carbon dioxide is removed from it. This process operates via passive diffusion, moving gases from an area of high partial pressure to an area of low pressure across a semi-permeable membrane. In the human body, gas exchange occurs at two distinct sites: externally between the lungs and the blood (pulmonary exchange), and internally between the blood and the working muscle tissues (tissue exchange).
External gas exchange, known as alveolar capillary exchange, takes place within the lungs where the alveoli meet the surrounding capillary network. Deoxygenated blood arriving at the lungs from the right side of the heart has a low partial pressure of oxygen and a high partial pressure of carbon dioxide. Conversely, the fresh air inside the alveoli possesses a high partial pressure of oxygen and a low partial pressure of carbon dioxide. This stark difference creates a steep pressure gradient. Oxygen diffuses across the thin respiratory membrane from the alveoli into the blood, binding to hemoglobin in red blood cells, while carbon dioxide simultaneously diffuses out of the blood into the alveoli to be exhaled.
Internal gas exchange, or tissue capillary exchange, occurs at the site of the active skeletal muscles. Here, the partial pressure gradients are reversed because the working muscles constantly consume oxygen and produce carbon dioxide during cellular respiration. The arterial blood arriving at the muscle tissue is highly oxygenated, meaning it has a high partial pressure of oxygen and a low partial pressure of carbon dioxide. The muscle cells have a low partial pressure of oxygen and a high partial pressure of carbon dioxide. Consequently, oxygen diffuses out of the capillary blood down its pressure gradient into the muscle cells to be used for energy production, while carbon dioxide diffuses out of the muscle cells into the bloodstream to be transported back to the heart.
During exercise, the rate of gas exchange at both sites increases dramatically to meet the elevated metabolic demands of the athlete. As muscles work harder, they consume oxygen and generate carbon dioxide far more rapidly, which drastically lowers the partial pressure of oxygen and raises the partial pressure of carbon dioxide within the muscle tissue. This accentuates the partial pressure gradients at both the muscles and the lungs. Because the difference in pressures becomes much wider, the rate of diffusion accelerates significantly, ensuring a faster delivery of oxygen to the working fibers and a swifter removal of fatiguing waste products.