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Cell metabolism is based on the same general principle as the combustion of any fuel, whether it be in the automobile, power plant, or a home furnace. The general combustion reaction is:
\[ CH_2O \; (fuel) + O_2 \rightarrow CO_2 + HOH \]
The same reaction occurs in the cells. The "fuel" comes from food in the form of carbohydrates, fats, and proteins. The important principle to remember is that oxygen is needed by the cell and that carbon dioxide is produced as a waste product of the cell. Carbon dioxide must be expelled from the cells and the body. The lungs serve to exchange the two gases in the blood. Oxygen enters the blood from the lungs and carbon dioxide is expelled out of the blood into the lungs. The blood serves to transport both gases. Oxygen is carried to the cells. Carbon dioxide is carried away from the cells.
Partial pressures are used to designate the concentrations of gases. Dalton's Law of Partial Pressures states that the total pressure of all gases is equal to the sum of the partial pressures of each gas. For example, the total atmospheric pressure of air is 760 mm Hg. In equation form:
P(total air) = P(O2) + P(N2) + P(CO2) + P(H2O)
760 = 160 + 594.7 + 0.3 +5.0
The partial pressures for oxygen and carbon dioxide in various locations are given in Figure 1. The movement or exchange of gases between the lungs, blood, and tissue cells is controlled by a diffusion process. The gas diffusion principle is: A gas diffuses from an area of higher partial pressure to an area of lower partial pressure.
In the lungs, oxygen diffuses from alveolar air into the blood because the venous blood has a lower partial pressure. The oxygen dissolves in the blood. Only a small amount is carried as a physical solution (0.31 ml per 100 ml). The remainder of the oxygen is carried in chemical combination with the hemoglobin in red blood cells (erthrocytes). Hemoglobin (molecular weight of 68,000) is made from 4 hemes, a porphyrin ring containing iron and globin, a 4 protein chains. Oxygen is bound to the iron for the transport process. Hemoglobin (HHgb) behaves as a weak acid (K = 1.4 x 10-8; pKa = 7.85). Oxyhemoglobin (HHgbO2) also behaves as a weak acid (K = 2.5 x 10-7; pKa = 6.6)
Because both forms of hemoglobin are weak acids, and a relationship of the numerical values of the equilibrium constants, the net reaction for the interaction of oxygen with hemoglobin results in the following equilibrium:
\[ HHgb + O_2 \rightleftharpoons HgbO_2 + H^+ \]
If 2 is increased in the blood at the lungs, the equilibrium shifts to the right and H+ ions increase. Oxyhemoglobin can be caused to release oxygen by the addition of H+ ions at the cells. The difference in pH (7.44) of arterial blood and venous blood (pH = 7.35) is sufficient to cause release of oxygen from hemoglobin at the tissue cells.
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