Gasutbytet

Gasutbytet, or respiratory gas exchange, is the physiological process by which oxygen and carbon dioxide are transferred between an organism and its environment. It primarily occurs across specialized surfaces such as the alveolar membranes in the lungs of mammals, gills in many aquatic animals, or, in plants, across stomata and other gas-permeable tissues.

Mechanism and transport

Gas exchange is driven by partial pressure gradients: oxygen moves from regions of higher partial pressure to lower, and carbon dioxide moves in the opposite direction. In humans and other mammals, oxygen diffuses from inhaled air through the alveolar wall into the blood, while carbon dioxide diffuses from blood into the alveoli to be exhaled. Oxygen is largely carried in the blood bound to hemoglobin, whereas carbon dioxide is transported as bicarbonate in plasma, bound to proteins, or dissolved in plasma. The efficiency of exchange depends on the thinness and integrity of the gas–blood barrier, the surface area available for diffusion, and the match between ventilation (air reaching the alveoli) and perfusion (blood flow in the capillaries).

Influencing factors and variations

Factors affecting gasutbytet include membrane thickness, diffusion distance, surface area, temperature, and the organism’s metabolic rate. Conditions that impair the respiratory surface, such as edema, fibrosis, or airway obstruction, can reduce efficiency and lead to hypoxemia or hypercapnia. Different organisms have adaptations: lungs with extensive alveolar surfaces in terrestrial vertebrates, gills with countercurrent exchange in many aquatic species, and, in plants, stomata that regulate gas exchange for photosynthesis and respiration.

Measurement and importance

Gasutbytet is assessed clinically by arterial blood gases (PaO2, PaCO2, SaO2) and is fundamental for cellular respiration and energy production.