CIRCULATION AND RESPIRATION 139 



is taken up mainly by the lymphatic system. Eventually this 

 fat finds way into the blood and is found as a normal constituent 

 of the plasma. Some fat is directly absorbed from the intestine 

 by the blood. Amino acids, the end products of protein digestion, 

 are absorbed as such by the blood from the intestine and are also 

 normal constituents of plasma. Those portions of sugar, fat, and 

 amino acid stored in the liver and muscles are later released into 

 the blood stream. The blood also receives water and inorganic 

 salts from the alimentary canal. It also carries oxygen and 

 endocrines. In the case of the frog and certain other animals, a 

 considerable amount of water may be taken through the skin 

 into the blood. The blood also transports wastage. 



Respiration. — In vertebrate animals and in some invertebrates 

 too, the blood supplies the tissues with molecular oxygen. The 

 source of oxygen is the air. The latter enters the body at the 

 respiratory surface, which may be a gill, lung, or the body 

 surface, from which it is then taken up, in the case of vertebrates, 

 by the hemoglobin of the red corpuscles. Hemoglobin has a 

 peculiar affinity for uniting with oxygen under certain conditions. 

 If exposed to air, 100 cc. of blood plasma will, like an equal 

 volume of water, absorb 0.56 cc. of oxygen at 20°C, at sea level. 

 The amount of oxygen absorbed depends upon the partial pres- 

 sure of the oxygen in the air, which is roughly one-fifth of the total 

 atmospheric pressure. If the atmospheric pressure is increased, 

 more oxygen will be absorbed; if decreased less. On the other 

 hand, if defibrinated blood (blood from which fibrinogen has been 

 removed, leaving corpuscles and serum) is exposed to air, it will 

 absorb practically as much oxygen as it can be made to absorb 

 under any higher pressure. Thus if the partial pressure of the 

 oxygen is doubled, only as much additional oxygen is absorbed 

 as the serum can take up, and this amount is very small. If the 

 partial pressure of oxygen is reduced one-half, only a small 

 amount is given off, and that comes from the serum. Since 

 most of the oxygen of the blood absorbed under these conditions 

 is unaffected by such pressure changes, it must be combined 

 chemically in some way with something in the corpuscles. It 

 has been shown that the substance in the corpuscles is the 

 hemoglobin of the erythrocytes. 



Oxyhemoglobin, the compound formed by oxygen and hemo- 

 globin, is not stable at low pressures, for if the oxygen pressure 

 to which it is exposed falls below 25 mm. of mercury, it dissociates 



