BODY FLUIDS AND CIRCULATION 125 



the breaking plane and a valve, closed by blood pressure from within, 

 shuts off the blood sinus when the limb is lost (139). 



Vertebrates. In vertebrates, and in several invertebrate phyla, it is found 

 that shed blood coagulates and seals the wound. Constriction of the rup- 

 tured vessel aids the process of coagulation, since the flow of blood is 

 reduced and the blood cells have an opportunity to gather and clump 

 together. The simplest type of coagulation is due to an aggregation and 

 clumping together of blood leucocytes in the wound. This in turn may be 

 followed by the formation of a true (fibrin) clot derived from circulating 

 plasma proteins. When both mechanisms are operating, the clot or throm- 

 bus is made up partly of formed (cellular) elements and partly of fibrin- 

 clot. Before examining haemostasis in invertebrates it will be useful to 

 review briefly the situation in vertebrates, about which more is known. 



When blood vessels of a vertebrate are injured, the walls constrict and 

 platelets gather at the wound, contributing to the thrombus. At the same 

 time the platelets disintegrate and release a thromboplastic factor, which 

 participates in the chemical changes culminating in coagulation of the 

 blood plasma. Coagulation is essentially a conversion of a circulating pro- 

 tein, fibrinogen, dissolved in the plasma, into an insoluble form, fibrin, 

 which forms a gel and so closes the wound. The clotting process takes place 

 in two stages. The first phase consists of the formation of a catalyst, 

 thrombin, from a precursor, prothrombin, occurring in the blood plasma. 

 This takes place under the influence of Ca + ^ ions and a thromboplastic 

 factor, found in blood platelets, muscle and other tissues. In the second 

 phase fibrinogen is converted into insoluble fibrin through the action of 

 thrombin. For this phase the presence of Ca+ + is unnecessary. 



The same processes are involved in blood clotting in all vertebrates. 

 Prothrombin and thrombin are protein substances; the latter is classified 

 with proteolytic enzymes, and related to trypsin. The thromboplastic 

 factor (thromboplastin) is a phospholipoid (e.g. cephalin) or a conjugated 

 lipo-protein. Fibrinogen is a globulin; fibrin is a crystalline protein having 

 the form of needle-like micellae. The course of clotting is summarized as 

 follows — 



platelets dissolution 



i activation 

 plasma 



thromboplastin (cephalin-protein) 



prothrombin + Ca ++ + thromboplastin > thrombin 



fibrinogen catalysis by thrombin fibrin 



These substances are found throughout the vertebrates, but thrombin 

 and fibrinogen appear to show some specificity. Levels of prothrombin in 

 fishes are around 2-7 mg%; average values of blood fibrinogen are 0T1 % 

 in Scyliorhinus, and 0-26-0-36% in teleosts (131, 143). 



