Respiratory gaseous exchange occurs between the interior of an organism and the environment. In all animals with a vascular system, in which the blood transports respiratory gases, there is a twofold gaseous exchange within actively respiring tissues and the respiratory organs. In humans, the respiratory organs are the lungs, and there gaseous exchange takes place in the alveoli.
The quantity of gas diffusing through the alveolar tissue depends on pressure differences. There are pressure gradients between the air in the alveoli and the blood. The steeper the gradient, the more rapid the gaseous exchange. The steepness of the pressure gradient corresponds to the difference between the partial pressure of the respiratory gas in the alveolar air and its partial pressure in the blood (the pressure exerted by a particular component of a mixture of gases is called the partial pressure of that gas). Inspired air contains 21 vol% of oxygen which corresponds to a partial pressure of 21.3 kPa. After mixing with the air in the lungs, this partial pressure falls to about 13 kPa. In contrast, the oxygen partial pressure of the venous blood delivered to the lung capillaries by the lung arteries is about 5kPa. After passage through the lung capillaries, the oxygen partial pressure of the blood rises to about 13 kPa. Because the total volume of oxygen in the lungs is large compared with the volume that diffuses into the blood, the oxygen partial pressure of the alveolar air decreases only slightly. In the pulmonary vein to the heart, the oxygen concentration decreases somewhat, because it is mixed with "short circuited" blood, which passed through the lungs, but does not take part in gaseous exchange. The final oxygen partial pressure of arterial blood is about 12 kPa.
In the organism, carbon dioxide diffuses in the opposite direction to oxygen. The carbon dioxide partial pressure of inspired air increases from 0.03-5.3 kPa on mixing with pulmonary air. By gaseous exchange, the carbon dioxide partial pressure of the blood decreases from 6 to 5 kPa during passage through the lung capillaries. Carbon dioxide diffuses much more rapidly than oxygen, so that carbon dioxide exchange is completed before the oxygen exchange.
Gaseous exchange in the tissues also depends on diffusion. The direction of diffusion of the two respiratory gases is the reverse of that in the lungs. In tissue cells, oxygen partial pressure varies between 0 and 4 kPa, that of carbon dioxide between 6.7 and 8 kPa. The rate of gaseous exchange in tissues depends on two factors, the metabolic activity of the tissue and the dilation of blood vessels. Metabolic activity can alter the pressure gradients of oxygen and carbon dioxide between the tissue the blood while the dilation of the blood vessels causes an increase in blood flow through the tissue and enhances the rate of gaseous exchange by increasing the diffusion factor.
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