180 The Vertebrate Organ Systems 



it is known as an embolus. This can be dangerous inasmuch as the 

 embolus may ultimately clog up a vital vessel and even cause death. 



There are at least three factors which normally prevent the clotting 

 of blood within the blood vessels. Their nature can in part be surmised 

 by reviewing the clotting reaction. First it may be noted that thrombin 

 is present in the blood as prothrombin, an inactive form. Further 

 assurance of its remaining inactive is due to the presence of another 

 material, an anticoagulant called antiprothrombin. This is in some way 

 tied to the prothrombin. A second factor which prevents clotting is 

 the fact that thrombin is not formed until thrombokinase is released 

 by either the disintegration of the platelets or the injured tissues. The 

 thrombokinase apparently forms a union with antiprothrombin to free 

 the prothrombin. Prothrombin is then converted to active thrombin 

 by the essential calcium ions in the blood. There is one other important 

 factor in this prevention of clotting. That is the fact that the walls 

 of the blood vessels are very smooth and offer no surfaces upon which 

 clotting can be initiated. The truth of this can be demonstrated if 

 freshly drawn blood is carefully placed in a container lined with wax. 



Occasionally there are individuals in whom clotting does not occur 

 after injury of tissues. These rare individuals have the unusual in- 

 herited disease, hemophilia. From studies of their blood it appears that 

 the defect is due to the unusual stability of the platelets. They fail to 

 disintegrate when tissue is injured, thus thrombokinase is not released 

 to initiate clotting. Clotting also is at times delayed by a lack of suffi- 

 cient vitamin K in the blood. It is usually avoided by administering 

 the vitamin. 



Blood Types. — For many years, it has been a well-known fact 

 that blood can be transfused from one individual to another. Formerly 

 these transfusions often resulted in the death of the recipient. Now 

 it is known that at times the blood of different individuals is incom- 

 patible, that is, when the blood is mixed, the red blood cells clump 

 together to prevent the flow of blood through the capillaries. This 

 clumping is known as agglitfinafion and involves two factors. One of 

 these, the agglutinin , is found in the plasma; the other, the agglutinogen, 

 is found in the red blood cells. (Fig. 53.) 



Two agglutinogens are of concern to us. These are named A and 

 B. They may occur individually, in combination, or not at all, giving 

 four possible types : A, B, AB, and O. The agglutinins of the plasma 

 occur in the same types and are listed as a, b, ab, and o. An individual 



