Figure 1. The various components of the gas transfer system in vertebrates. The path of oxygen and carbon dioxide takes four serially arranged steps: convection of the ambient medium or ventilation, diffusion through external exchange surfaces in gills or lungs, convective transfer by blood circulation, and diffusion from tissue capillaries to the cells and mitochondria.

Figure 2. Morpho-functional design of three examples of gas exchange organs in vertebrates. From left to right:

First column: gross anatomy of gill and lung. Only two adjoining gill arches are represented for the teleost gill. The bronchial tree of mammalian lung and the lung-air sac system of birds have been much simplified. Large arrows indicate the direction of flows of water and air. Small double arrows represent the movement of the walls of lung and air sacs.

Second column: schematic structure of the elementary exchange unit showing the arrangement of medium and blood flows. Two secondary lamellae (fish gill), a single alveolus (mammalian lung) and a short length of a parabronchus (bird lung) are represented with the associated blood capillaries.

Third column: simplified models summarizing the modalities of gas transfer between medium and blood. The blood–medium tissue barrier is coloured purple. In fish gills, gas exchange proceeds between two currents of water and blood running in opposite directions. In the lung of mammals, capillary blood is in contact with a pool of alveolar gas of relatively uniform composition. In bird lung, blood capillary loops exchange gases via air capillaries with the air flowing along the parabronchus. Only three capillary loops are represented. I, inspired medium; E, expired medium; A, alveolar gas; Ep, end-parabronchial air; a, arterial blood (after gas transfer); , mixed venous blood (before gas transfer).

Fourth column: profiles of variations of oxygen partial pressures (Po₂) in medium and blood during gas transfer along the elementary exchange units: secondary lamella, alveolus or parabronchus. In fish gill, countercurrent exchange and the absence of a dead space leads to a low value of Po₂ in expired water (E), maximizing the oxygen extraction (I–E) from the respired water. In mammalian lung, alveolar exchange is much less efficient and the expired air (E) is merely a mixture of alveolar gas and fresh air from the dead space, thus further reducing oxygen extraction (I–E). In bird lung, arterial blood (a) is a mixture of variously oxygenated effluents from successive capillary loops. End-parabronchial air (Ep) is efficiently deoxygenated in a stepwise manner along the parabronchus, but mixes with fresh air from the dead space as in mammals.