Biology of Man 487 



Thrombin (throm'bin) (Gr. thromhos, clot) as such does not exist in 

 significant amount in unshed blood or it might start the formation of a 

 clot in normal, circulating blood, but the thrombin is thought to exist 

 as inactive prothrombin (Gr. pro, before). In normally circulating 

 blood it is thought that heparin (hep'arin) (Gr. hepar, liver) formed 

 in the liver is combined with cephalin (thromboplastin) (sef alin) (Gr. 

 kephalos, head). Consequently, the lack of cephalin prevents the form- 

 ing of thrombin from prothrombin; hence there is no clotting in normal 

 circulating blood. When hemorrhage occurs (tissue injury and destruc- 

 tion of blood platelets), more cephalin is formed than can combine with 

 the heparin. The excess cephalin combines with the prothrombin and 

 calcium salts to form thrombin. The latter combines with a soluble 

 protein of the blood plasma known as fibrinogen (fi -brin' o jen) (L. 

 fibra, band; Gr. genos, to produce) to form the fibrin (network of in- 

 soluble, contractile threads). Fibrin collects blood corpuscles and other 

 available materials to form the clot. Vitamin K seems to be necessary 

 for the formation (in the liver) of the prothrombin which is normally 

 present in the plasma and has a life duration of only a few days. Vita- 

 min K must have bile from the liver in order to be absorbed in the 

 intestine and transported to the liver for the formation of prothrombin. 

 Vitamin K may be ingested as such or may be manufactured from foods 

 in the intestine. The formation of a clot within a blood vessel which is 

 not severed is called a thrombus (Gr. thrombos, clot). This may be due 

 to injury of the vessel wall from a blow or from toxins of bacteria which 

 injure the blood platelets. If a part of a thrombus circulates in the 

 vessels, it is called an embolus (em' bolus) (Gr. embolos, wedge). If 

 the embolus should block circulation to a vital part, serious consequences 

 may result. 



Structure and Functions of Human Lymph 



The composition of the lymph is similar to that of the blood plasma. 

 It ranges from colorless to yellowish color, has an alkaline reaction, 

 contains no blood platelets, clots slowly and not firmly, has a higher 

 percentage of waste materials than blood, contains a lower percentage 

 of nutrient materials than blood, may contain a few red blood corpuscles 

 (erythrocytes), and contains lymphocytes (white blood corpuscles). 



The lymph is derived from ( 1 ) the blood plasma by filtration through 

 the thin walls of the capillaries and (2) secretions of the endothelial 

 cells which line the numerous capillaries. 



