BODY TISSUES 297 



to ihioinbin. Jilood also contains an inhibitor, heparin, which pre- 

 vents the prothrombin conversion in circulating blood. Heparin is a 

 low-molecular-^veight carbohydrate containing glucosamine, glucu- 

 ronic acid, and sulfate groups. Its structure has not been determined 

 completely. 



The clotting process may be summarized as follows: 



thromboplastin 



1 . Prothroniinn > thrombm 



Ca+ + 



platelet accelerator 



(inhibited by heparin) 



. thrombin ^i . 



2. Fibrmogen > nbnn -|- peptide 



3. n(Fibrin) > (fibrin)„ polymer 



4. (Fibrin)„ polymer > fibrin gel 



The peptide is removed on hydrolysis by thrombin and serves as a 

 protecting or masking unit for the polymerization centers on the 

 initial fibrinogen. Thrombin also functions in the conversion of a 

 plasma globulin to a specific serum globulin. This conversion, as 

 well as (1) and (2) above, is proteolytic in nature, whereas the third 

 and fourth steps involve film formation through polymerization and 

 the entrapment of the corpuscles in the fibrin web or mesh. 



Thrombosis or intravascular clotting sometimes occurs and plugs 

 small arteries or capillaries. Such clots in many regions of the body 

 result in nothing more than a temporarily painful area. However, 

 if they occur in the heart or the brain, death may result unless the 

 clot is removed naturally or by the use of hemorrhagic drugs, heparin 

 or hirudin (the anticoagulant in the saliva of leeches and ticks). In 

 such cases it is often advisable to regulate the prothrombin synthesis 

 (stimulated by vitamin K, page 359) through the use of antivitamin 

 K, dicoumarol (page 361), or a derivative. The rodentocide, war- 

 farin, is a derivative of dicoumarol and functions through the produc- 

 tion of uncontrolled internal bleeding when consumed by rodents. 



Gas exchange. The above discussion reported the oxygen-transport 

 function of the hemoglobin of erythrocytes. Oxygen is absorbed by 

 the blood as it passes through the fine capillaries in the thin wall of 

 the lung. These capillaries are so small that the corpuscles must pass 

 through single file and thus provide ample opportunity for gas 

 exchange by simple diffusion. There is no evidence for "active trans- 

 port" across the membrane. As was stated earlier, the absorbed 

 oxygen forms a loose complex with the hemoglobin molecule. Experi- 

 ments have demonstrated that the partial pressure of the oxygen in 

 the small chambers of the lung (alveoli) is sufficient to account for 



