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.
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.