METABOLISM IN VERTEBRATES 



move from the region of higher pressure to the region of lower pressure until 

 the pressure exerted by oxygen is equal on both sides of the membrane. This 

 is what happens during gas exchange in the lung. Oxygen in the alveolar air 

 exerts a partial pressure of about 103 mm. of mercury. Blood entering the 

 lung capillaries has a low concentration of oxygen exerting a partial pressure 

 of only about 35 mm. of mercury. Consequently, oxygen difTuses across the 

 respiratory membrane into the blood stream. The time required for the 

 blood to pass through the capillary bed of the lung allows for almost complete 

 equilibration of oxygen. 



This same mechanism results in the elimination of excess carbon dioxide 

 from the blood as it circulates through the capillaries of the lungs. During 

 its course throughout the body, the blood has picked up the carbon dioxide 

 formed during cellular oxidation. The partial pressure of carbon dioxide in 

 blood entering the lung capillaries is about 46 mm. of mercury as compared 

 with a partial pressure of 38 mm. of mercury exerted by the carbon dioxide 

 in the alveolar air. Therefore, carbon dioxide diffuses from the blood into 

 the alveolar air, and blood leaving the lung capillaries has a carbon dioxide 

 pressure of 38 mm. of mercury. 



Pulmonary gas exchange thus occurs as a result of diflfusion. Blood leaving 

 the lungs carries oxygen and carbon dioxide in concentrations equal to those 

 in the alveolar air. The first hurdle in meeting the oxygen requirements of 

 cells far removed frorrt the external supply of oxygen has been cleared. 

 Elimination of carbon dioxide through the lungs is, likewise, very important 

 in bodily maintenance. Apart from the obvious necessity of getting rid of a 

 waste product of cellular oxidation, the elimination of carbon dioxide helps 

 to maintain the normal acid-base equilibrium of the body. The amount of 

 carbon dioxide eliminated each day is equivalent in terms of acidity to a liter 

 of hydrochloric acid. 



During transport the respiratory gases are mostly in chemical combination 

 with certain constituents of the blood. This makes it possible to move 100 

 to 150 times as much oxygen and carbon dioxide as could be moved if they 

 were only dissolved in the blood. Each gas must, however, exist in simple 

 solution on its way to the carrier compound and upon its release from that 

 compound. In man about 99 per cent of the oxygen is carried in a loose 

 sort of chemical combination with hemoglobin, an iron-containing protein 

 found in red blood cells. The combination is called oxyhemoglobin and is 

 responsible for the red color of oxygenated blood. During normal ventilation 

 of the lungs, an individual has about 97 per cent of his hemoglobin saturated 

 with oxygen. With forced breathing the percentage of oxyhemoglobin rises. 

 At high altitudes, or in certain diseased conditions, considerably less satu- 

 ration of hemoglobin is possible, and the amount of oxygen supplied to the 

 cells is inadequate. Corrective measures must be taken in order to obtain 

 sufficient oxygen. Animals living at high altitudes make more red blood cells, 

 thereby supplying more oxygen carriers. In an airplane or in case of disease, 

 the concentration of oxygen in the air breathed can be raised. The increased 



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