72 THE BLOOD 



hemoglobin is discharged from cells subjected to various agents in 

 the process known as hemolysis. It can be induced experimentally 

 by increasing the concentration of salts in the fluid surrounding the 

 cells, by repeated freezing and thawing of blood, by addition of 

 chloroform or ether, and by the action of electric currents. Ilemo- 

 lyzed blood cells are said to be laked, and when this occurs in the 

 circulating blood, as it does in the case of certain pathologic condi- 

 tions, the hemoglobin is eliminated from the plasma in the kidneys 

 and passes out with the urine. 



The surface of the red cell is somewhat more dense than the 

 interior protoplasm. It is semipermeable and in osmotic equilib- 

 rium with the solutions normally present outside and is said to be 

 isotonic with them. Mammalian plasma has about the same osmotic 

 pressure as an 0.85 per cent sodium chloride solution, which is 

 isotonic with the protoplasm within the red corpuscle. Frog plasma 

 is similar to a 0.7 per cent sodium chloride solution. If distilled 

 water is added to a drop of fresh blood on a slide, microscopic ob- 

 servations show the erythrocytes increasing in size as they absorb 

 water. If the swelling is allowed to continue the cells become 

 spherical, and hemolysis occurs. This change is a result of endos- 

 mosis, since the solution has a concentration less than that of the 

 protoplasm within the red cells. Such a solution is said to be hypo- 

 tonic. A reversal of this process occurs if the external medium is 

 more concentrated, or hyj^ertonic. \Yater is then extracted from 

 the red cells which become wrinkled or crenated, and finally hemo- 

 lysis takes place. 



Oxygen diffuses through the membranes of the respiratory 

 surfaces into the tissue juice and then through the endothelial 

 walls of the capillaries. As the erythrocytes circulate slowly 

 through the capillaries in\'esting the air sacs of the lungs, or through 

 the ca])illaries in the gills and skin of aquatic animals, the hemo- 

 globin unites in a weak chemical combination with oxygen to form 

 oxyhemoglobin. In this form oxygen is carried to the various tis- 

 .sues of the body and is given up as needed. The surface area of all 

 the red cells may be computed on the basis of indi\idual cell size 

 and the estimated number of red cells in the body. In the case of 

 man, estimates from 2500 to 4500 square meters \m\e been worked 

 out, areas many times that of the body. Depending largely upon 

 the severity of service, there is a limit to the functioning of the red 

 cells. When the colloidal composition ages beyond a certain ])oint 

 a granular condition results and the red cells are broken up. These 



