THE TRANSPORTATION SYSTEM 



507 



from solution. Therefore, calcium must be 

 essential for clotting. The smooth paraffin 

 is similar to the lining of the blood vessels; 

 therefore, rough surfaces such as often 

 formed in ragged injuries must aid in the 

 formation of clots. 



If freshly drawn blood is stirred vigor- 

 ously with stiff bristles, fibrin shortly be- 

 gins to collect on the bristles, and if the 

 stirring continues all of the fibrin can thus 

 be removed, leaving what appears to be 

 perfectly normal blood except for its failure 

 to clot. This is called defibrinated blood. 

 There must have been a precursor protein, 

 much like fibrin, that was in solution in the 

 blood but which became insoluble with 

 the stirring. This is known as fibrinogen. 

 The problem is : Why did fibrinogen become 

 fibrin? What initiated the chain reaction? 



Something must happen when the blood 

 is taken out of the vessels, because clots 

 do not normallv form within the circulatorv 

 system itself. Careful chemical analysis has 

 revealed another substance, thrombin, in 

 the plasma which is responsible for the con- 

 version of fibrinogen to fibrin. Obviouslv, 

 thrombin cannot occur freely in the blood 

 stream, or clots would form within the 

 vessels. This happens only rarely and even 

 then only under pathological conditions. 

 Again experimentation has revealed a pre- 

 cursor, prothrombin, that is normally pres- 

 ent in the blood. The next problem is: What 

 starts prothrombin to form thrombin? 



Anothei: careful search has demonstrated 

 that the platelets and probably other tis- 

 sues of the body contain a substance or 

 group of substances, called thromboplastin, 

 which are essential in starting this long 

 chain of reactions. It was stated earlier that 

 calcium was also essential, because if it is 

 removed from the blood no clot forms. Just 

 what the relationship is between these sub- 

 stances is not clear at present. Probably 

 when a hemorrhage occurs, the broken cells 

 in the vicinity of the injury, together with 

 the disintegrated platelets, release throm- 

 boplastin. Thromboplastin, reacting with 



calcium in tlie blood in some unknown 

 manner, produces prothrombin which con- 

 verts to thrombin and this, in turn, changes 

 fibrinogen to fibrin. Summarized: 



Hemorrhage — > thromboplastin 

 thromboplastin + calcium -|- prothrombin 



— ^ thrombin 

 thrombin + fibrinogen — > fibrin 

 fibrin + red blood cells -^ clot 



Earlier in the chapter on nutrition there 

 was mention made that vitamin K had 

 something to do with blood clotting. If it 

 is formed in too low levels in the digestive 

 tract the clottincr time increases dano;er- 

 ously, so much so that surgery is inadvis- 

 able until large quantities of this vitamin 

 are given to restore the normal clotting 

 time. The above discussion does not include 

 anything concerning this substance. It is 

 now thought that vitamin K is associated 

 with prothrombin formation, and hence its 

 effect on clotting time. 



A blood-clotting defect that has been 

 known for a long time and has been the 

 concern of certain royal families because of 

 its sex-linked inheritance is hemophilia ( see 

 p. 602). The "blood" of the once royal 

 house of Spain was "tainted" so that this 

 defect appeared frequently in the sons, be- 

 ing passed to them by their mothers who 

 did not suffer from tlie disease themselves. 

 The slightest wound could be fatal in such 

 afflicted people because the blood clotted 

 very slowly. Analysis of their blood deter- 

 mined that it was normal in all respects 

 except that the platelets did not disinte- 

 grate as readily as in normal blood. Just 

 why they are less fragile or what to do 

 about it still remains an vmsolved mystery. 



Red blood corpuscles 



Red blood corpuscles are the most nu- 

 merous and most conspicuous of the formed 

 elements of the blood ( Fig. 19-4 ) . A blood 

 smear will reveal them as tiny, circular, 

 biconcave disks without nuclei. All verte- 

 brates except mammals possess nucleated 

 erythrocytes; just why nuclei are absent in 



