BLOOD AND CIRCULATION 541 



prevent the pH of the blood from changing greatly, and they facilitate 

 the release of oxygen in the tissues of the body and the release of carbon 

 dioxide in the lungs. 



Mature mammalian erythrocytes have lost their nuclei and do not 

 survive indefinitely. Experiments which involve tagging them with 

 radioactive iron show that they have a life span of about 127 days. Red 

 cells are eventually destroyed in the spleen and liver. Cells lining the 

 blood spaces of the spleen and liver engulf or phagocytize the red cells 

 and digest them. The iron of the hemoglobin is salvaged by the liver and 

 is reused, but the rest of the molecule is excreted as bile pigment. To 

 replace those destroyed, new red cells are constantly produced in un- 

 specialized connective tissues whose cells retain their embryonic poten- 

 cies. The kidney, spleen and liver of lower vertebrates contain tissue of 

 this type. These sites are of most importance during the embryonic 

 development of mammals, but the red bone marrow is the primary 

 source of erythrocytes in the adult. 



Erythrocyte destruction and production are surprisingly rapid. From 

 the total number of red cells in the body and their average life span, 

 one can calculate that about 10,000,000 are made and destroyed each 

 second of the day and night. If the rate of production of cells or of 

 hemoglobin decreases, some type of anemia results. Anemia is charac- 

 terized by a decrease in the number of red cells per cubic millimeter 

 of blood, by a decrease in the amount of hemoglobin per red cell, 

 or both. In pernicious anemia the number of erythrocytes steadily 

 decreases. Eating large quantities of liver increases the rate of red cell 

 formation, for liver is rich in vitamin B^^ which is necessary for normal 

 erythrocyte development. A person with pernicious anemia cannot ab- 

 sorb enough Bj. even though the requisite amount may be present in 

 the diet, for the lining of his stomach does not secrete enough "intrinsic 

 factor," necessary for the absorption of Bjo. If an excess is made available 

 by giving foods especially rich in Bjo, enough can be absorbed. 



230. Platelets and Blood Clotting 



Platelets are non-nucleated blobs of cytoplasm that bud off from 

 giant cells in the bone marrow. They, and the thrombocytes of lower 

 vertebrates, are responsible for initiating blood clotting, for they break 

 down at the site of injury and release the enzyme thromboplastin. This 

 initiates a complex, and as yet incompletely understood, series of reac- 

 tions that leads to the formation of a blood clot. Apparently a plasma 

 globulin known as prothrombin, in the presence of thromboplastin and 

 calcium ions, is changed into thrombin. Thrombin in turn acts as an 

 enzyme and mediates the change of the soluble protein fibrinogen into 

 an insoluble one known as fibrin. Fibrin forms a mesh of delicate fibers 

 that entraps the blood cells, and the clot forms. Blood plasma without 

 its fibrin is known as serum, and of course will not clot. Vitamin K does 

 not enter into this series of reactions directly, but is essential for the 

 production of prothrombin in the liver. 



Clotting rarely occurs within blood vessels, since the process must 



