gasexchange

Gas exchange is the biological process by which oxygen from the environment enters the bloodstream and carbon dioxide produced by metabolism is expelled. It occurs at respiratory surfaces with large surface areas and thin barriers, such as alveolar membranes in the lungs of terrestrial vertebrates, stomata and lenticels in plants, gills in aquatic animals, and insect tracheal systems.

In animals, gas exchange happens mainly by diffusion driven by partial pressure differences. Oxygen diffuses from air or water where its partial pressure is higher into blood where it is lower; carbon dioxide diffuses in the opposite direction. In the lungs, alveoli provide a large surface area and a thin barrier adjacent to a dense capillary network. Oxygen is transported largely by binding to hemoglobin, while carbon dioxide is carried in the blood as bicarbonate, dissolved CO2, or bound to hemoglobin.

In plants, gas exchange occurs primarily through stomata, openings in leaves and stems regulated by guard cells. CO2 diffuses in for photosynthesis, while oxygen and water vapor exit as byproducts of photosynthesis and respiration. Gas exchange is influenced by stomatal conductance and environmental conditions such as light, humidity, and temperature.

In other organisms, gas exchange may occur via gills, skin, or a tracheal system, each adapted to maximize diffusion under the organism’s ecological conditions.

Key factors include partial pressure gradients, surface area, diffusion distance, membrane permeability, and perfusion of exchange surfaces. Impairments such as edema, fibrosis, or anemia can reduce effective gas exchange, leading to hypoxemia or hypercapnia. Gas exchange is essential for cellular respiration and energy production, and its efficiency affects organismal function and health.