gas exchange
Summary
Gas exchange is the passive diffusion of O₂ into the blood and CO₂ out of it across a thin, moist respiratory surface such as alveoli, gills or tracheae. Each gas moves down its own partial pressure gradient, and the rate rises with surface area and falls with diffusion distance.
Detail
Meaning: Gas exchange is the passive diffusion of O₂ into the blood and CO₂ out of it across a thin, moist respiratory surface. Each gas moves down its own partial pressure gradient. In the lungs, alveolar PO₂ (about 100 mmHg) exceeds that of arriving blood (about 40 mmHg), so O₂ enters; PCO₂ is slightly higher in arriving blood, so CO₂ leaves. In systemic tissues the gradients reverse.
Equation: Rate of diffusion ∝ (A × ΔP) / d, where A is surface area (m²), ΔP is the partial pressure difference (Pa), and d is the diffusion distance (m). The rate is in mol/s.
On the MCAT: You are asked to predict how a change alters exchange. Less surface area (emphysema), a thicker barrier (fluid or scarring), or lower inspired PO₂ at altitude each reduce transfer. Fish gills use countercurrent flow to keep a gradient along the whole exchange surface. Passages also link exchange to hemoglobin binding and the oxygen dissociation curve.
Common confusion: Gas exchange is not ventilation. Ventilation is bulk airflow driven by pressure changes in the thorax; exchange is diffusion across the alveolar-capillary membrane and needs no ATP. The driving force is a difference in partial pressure, and each gas follows its own gradient, not the total pressure.
Sources
- Campbell Biology
- Human Physiology: An Integrated Approach
- Biology (Raven)
Written for the MCAT and checked before it was published. For study reference only. Spotted an error? Let us know.