<P> In a countercurrent flow system, air (or, more usually, the water containing dissolved air) is drawn in the opposite direction to the flow of blood in the gas exchanger . A countercurrent system such as this maintains a steep concentration gradient along the length of the gas - exchange surface (see lower diagram in Fig. 2). This is the situation seen in the gills of fish and many other aquatic creatures . The gas - containing environmental water is drawn unidirectionally across the gas - exchange surface, with the blood - flow in the gill capillaries beneath flowing in the opposite direction . Although this theoretically allows almost complete transfer of a respiratory gas from one side of the exchanger to the other, in fish less than 80% of the oxygen in the water flowing over the gills is generally transferred to the blood . </P> <P> Alternative arrangements are cross current systems found in birds . and dead - end air - filled sac systems found in the lungs of mammals . In a cocurrent flow system, the blood and gas (or the fluid containing the gas) move in the same direction through the gas exchanger . This means the magnitude of the gradient is variable along the length of the gas - exchange surface, and the exchange will eventually stop when an equilibrium has been reached (see upper diagram in Fig. 2). Cocurrent flow gas exchange systems are not known to be used in nature . </P> <P> The gas exchanger in mammals is internalized to form lungs, as it is in most of the larger land animals . Gas exchange occurs in microscopic dead - end air - filled sacs called alveoli, where a very thin membrane (called the blood - air barrier) separates the blood in the alveolar capillaries (in the walls of the alveoli) from the alveolar air in the sacs . </P> <P> The membrane across which gas exchange takes place in the alveoli (i.e. the blood - air barrier) is extremely thin (in humans, on average, 2.2 μm thick). It consists of the alveolar epithelial cells, their basement membranes and the endothelial cells of the pulmonary capillaries (Fig. 4). The large surface area of the membrane comes from the folding of the membrane into about 300 million alveoli, with diameters of approximately 75 - 300 μm each . This provides an extremely large surface area (approximately 145 m) across which gas exchange can occur . </P>

Where are gases exchanged in the respiratory system