212 Tin; INII.RNAL INVIRONMINI Ol III! IU)I)Y Part III 



vessels. The ehanges are climaxed by the formation of gel fibrin, the strands 

 of which hold the cell mass together in the clot (Fig. 12.11). 



Some of the substances that take part in forming fibrin, such as calcium, 

 prothrombin, and fibrinogen are present in circulating blood; the others, 

 thrombokinase and thrombin are formed during the clotting process. At the 

 beginning of this process the exceedingly delicate platelets are injured and 

 in most cases the cells of the blood and the vessel walls as well. The substance 

 liberated by the decomposing platelets and broken cells is thrombokinase, 

 an cnzymelike clot-hastcner. The newly formed thrombokinase unites with 

 the calcium and prothrombin already in the blood and produces thrombin. 

 This second newly formed substance unites with fibrinogen, also already in 

 the blood and produces the fibrin whose strands hold together the cell mass 

 called the clot. The process may be summarized as follows: 



Thrombokinase + calcium -\- prothrombin = thrombin 

 Thrombin -f fibrinogen = fibrin 



Fibrin -\- cell mass =^ clot 



The existence of prothrombin in healthy circulating blood depends in turn 

 upon the presence of the antihemorrhagic vitamin K (Table 11.1). This is 

 taken in with food and with the aid of bile is absorbed in the intestine, then 

 goes to the liver where it takes part in the formation of prothrombin. If 

 vitamin K is missing, the formation of prothrombin is prevented, clotting does 

 not take place, and bleeding results. 



Abnormal Blood Clotting 



Hemophilia is an inherited defect in blood clotting. Persons who suffer 

 from it are known as bleeders (Chap. 20). 



Thrombosis is coagulation of the blood in any part of the circulatory sys- 

 tem. A coronary thrombosis is the stoppage of a coronary artery by a blood 

 clot; the coronary arteries originate near the base of the aorta and supply 

 the walls of the heart. Occasionally a fragment of a clot, called an embolus, 

 breaks ofT, is carried free in the circulation, and becomes lodged in the brain 

 or heart. In the brain a clot results in loss of memory, speech, and paralysis 

 of various parts of the body. 



The Lymphatic System 



Lymph originates from plasma that, except for its proteins, filters through 

 the walls of blood vessels. Outside them it becomes tissue fluid occupying 

 any spaces there may be among the tissues. As it fills these spaces and as 

 the pressure in them rises, it filters through the walls of the lymph vessels 

 and becomes lymph. Lymph capillaries end blindly. The ready entrance of 

 tissue fluid into lymph capillaries is due to their extreme thinness and deli- 



