Gas-exchange surfaces are thin and moist, with a large area. Oxygen and carbon dioxide diffuse down their respective concentration gradients. In many animals a blood supply maintains these gradients by carrying oxygen away and bringing carbon dioxide to the surface. Alveoli provide a large lung surface; gill filaments and lamellae provide a large aquatic exchange surface.
Ventilation moves air or water past a surface; gas exchange is diffusion across that surface; cellular respiration transfers energy within cells. These are related but different processes. Minute ventilation equals breathing frequency multiplied by tidal volume when units match. Loss of exchange area or thickening of a barrier reduces potential gas transfer even if breathing movements continue.
Example 1
A person takes 12 breaths per minute with a tidal volume of 0.5L. Find minute ventilation.
12×0.5=6L/min. This is ventilated air volume, not oxygen consumed.
Example 2
A lung disease thickens the alveolar barrier. Which feature of efficient exchange is reduced?
A short diffusion distance is lost. The thicker barrier makes diffusion across it slower under otherwise equal conditions.
Watch out:Breathing ventilates the lungs; cellular respiration transfers chemical energy within cells.
Animal gas exchange practice | Year 11 Biology | Ascent Arcade