130 PRINCIPLES OF ANIMAL BIOLOGY 



4. 



Coagulation. — One property possessed by blood, as a protection for 

 its own operations and the life of the organism, is its power to clot. 

 When blood vessels of small size are broken, the gap may be stopped by 

 the coagulation of the blood, thus preventing loss of excessive amounts of 

 blood. The clot consists of a tangled mass of threads of a substance 

 known as fibrin, in which are trapped multitudes of red corpuscles. The 

 fibrin is produced from fibrinogen, already mentioned as an important 

 protein component of the plasma. Conversion of fibrinogen into fibrin 

 is accomplished by the enzyme thromhase. This enzyme cannot exist in 

 the blood during normal circulation, but its forerunner, called prothrom- 

 base (page 112), is regularly present. The conversion of pi^othrombase 

 into thrombase is induced by a substance known as thromboplastin which 

 is liberated partly from the damaged tissue cells at a wound, partly 

 from the blood platelets which promptly disintegrate in exposed blood. 

 The chain of reactions here described in reverse quickly leads to the 

 precipitation of the fibrin network. Some other things are necessary to 

 that chain. Calcium ions must be present, and clotting may be pre- 

 vented in shed blood by precipitating its calcium with an oxalate or 

 citrate. Vitamin K (page 112) also aids coagulation. Clotting can 

 be artificially checked in surgical operations by injecting something 

 {heparin, for example, extracted from liver and muscle) wiiich inactivates 

 thrombase. People afflicted with hemophilia have a very slow coagula- 

 tion and bleed a long time from minor wounds. One feature of their 

 blood is the slowness with which blood platelets disintegrate, so that 

 production of thromboplastin is delayed, but there must be other factors. 



The fibrin network traps most of the blood cells, and as it contracts 

 it squeezes out a clear yellowish liquid, the serum, which is nearly identi- 

 cal with the plasma minus its fibrinogen. 



Lymph and the Lymphatic System. — As a means of fluid communica- 

 tion between all parts of an animal, the blood system alone is not quite 

 sufficient. The blood as a complete entity is confined to the blood vessels, 

 and diffusion of substances held in it, even from the capillaries, is too 

 slow to meet all needs. Moreover, the diffusion of water itself from the 

 capillaries must be a one-way movement because of the pressure of the 

 blood. Some of these inadequacies of the blood system are overcome 

 by the second of the great networks of vessels (page 122), the lymph 

 system. 



Because of the considerable pressures which are maintained in the 

 blood, there is a tendency for any of its components to escape if the}^ can 

 do so. The capillaries, with their thin walls, are the only place where this 

 is possible. The liquid part, the plasma, filters out rather readily, passing 

 into the spaces (Fig. 104) among the tissue cells. Some of the dissolved 

 parts of the plasma (chiefly proteins) are held back by the walls of the 



