PHYSIOLOGY OF ORGANS _ 149 



plasma. Hemoglobin readily enters into chemical combination with 

 oxygen producing a compound known as oxyhemoglobin. This substance 

 carries the bulk of the oxygen carried by the blood, a much smaller 

 proportion of the oxygen being carried in solution in the plasma. Under 

 normal pressure of oxygen in the atmosphere (equal to about 152 mm. 

 of mercury) hemoglobin is combined with oxygen practically to satura- 

 tion, and under higher pressures of oxygen no more is taken up. As the 

 pressure of oxygen is reduced the oxyhemoglobin gives up some of its 

 oxygen but the proportion lost is not great until the pressure of the 

 oxygen is lowered to about 70 mm. Below this pressure it gives up its 

 oxygen rapidly. Howell (Textbook of Physiology) states: 



"The arterial blood enters the capillaries with its hemoglobin nearly saturated 

 with oxygen- — about 19 cc. to each lOOcc. of blood. After it leaves the capillaries 

 the venous blood contains only about 12 volumes of oxygen to each 100 cc. of 

 blood. In the passage of the capillaries, which takes only about one second, 

 the blood loses, therefore, about 35 per cent, or more of its oxygen. The physical 

 theory of respiration furnishes data to show that this loss is due to a dissociation 

 of the oxyhemoglobin, owing to the fact that in passing through the capillaries 

 the blood is brought into exchange with a surrounding medium — ^lymph, cell 

 liquid — in which the oxygen pressure is very low." 



Physical Theory of Respiration. — The movement of gases through 

 membranes into tissues and liquids of the body or from these into the 

 surrounding air or water is explained by the theory of diffusion of gases. 

 If two volumes of gas, or two volumes of gas in solution be separated 

 by a permeable membrane molecules of the gas pass through the mem- 

 brane in either direction. If the tension or pressure of gas on one side 

 of the membrane is greater than on the other, a greater number of mole- 

 cules of gas pass through the membrane toward the place of lower pressure 

 than in the reverse direction. If permitted to do so the passage of mole- 

 cules continues until an equal number of molecules is passing through 

 the membrane in either direction; that is, until the pressure is the same 

 on both sides. An equilibrium is thus established and the condition is 

 the same as if no molecules were passing through the membrane. 



In the air of the lungs or in the water bathing the gills the tension 

 of the oxygen is greater than it is in the blood plasma or in the hemo- 

 globin of the venous blood in the capillaries on the other side of the 

 membrane. It has been estimated that in the lungs of man the tension 

 of oxygen in the air of the alveoli is about 100 mm. of mercury and the 

 pressure of oxygen in the veins is 37.6 mm. According to the theory 

 diffusion must take place from the place of higher tension to the place 

 of lower tension. The blood is in constant circulation and the nearly 

 saturated blood is borne away, blood poor in oxygen following in its 

 place. The direction of diffusion of oxygen in the lungs is, therefore, 

 always inward toward the venous blood. 



