Gas exchange is the process by which oxygen and carbon dioxide (the respiratory gases) move in opposite directions across an organism's respiratory membranes, between the air or water of the external environment and the body fluids of the internal environment. Oxygen is needed by cells to extract energy from organic molecules, such as sugars, fatty acids, and amino acids. Carbon dioxide is produced in the process and must be disposed.
Principles of Gas Exchange
The random movement of molecules is called diffusion. Although individual molecules move randomly, a substance can have directed movement, or net diffusion. The net diffusion of a substance occurs because of a difference in its concentration, or gradient , along its course. Within an animal's body as oxygen is used up and carbon dioxide produced, the concentration gradient of the two gases provides the direction for their diffusion. For example, as air or water nears the respiratory membrane, the oxygen concentration on the outside of the membrane is higher than on the internal side so oxygen diffuses inward. The concentration gradient for carbon dioxide is in the opposite direction, and so net diffusion of carbon dioxide keeps it diffusing out of the body.
The solubility of the respiratory gases in water is low, and the solubility of oxygen is only about one-twentieth that of carbon dioxide. Special transport molecules within body fluids increase the oxygen content by holding oxygen molecules within circulating fluids. These molecules are called respiratory pigments and include hemoglobin , which is red, and hemocyanin, which is blue. These molecules combine with oxygen at the respiratory membrane, where oxygen concentrations are relatively high and easily release the oxygen in deeper tissues, which are low in oxygen.
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