IMPORTANT FOODSTUFFS, IMPORTANT TISSUES 143 



sodium away from the weak acid radicles enumerated above. 

 These weak acids do not ionize to any great extent, and the 

 reaction of the liquid remains near the neutral point. Acids 

 produced in pathological conditions may be neutralized in a 

 similar way. 



The hydrogen-ion concentration of blood as determined by the 

 gas chain or indicator method (see the discussion of HC1 in the 

 chapter on gastric digestion) is about N X 10~ 7 - 2 to N X 10~ 7 - 7 

 in defibrinated blood at 18-20. 



Osmotic Pressure. The osmotic pressure of the blood influ- 

 ences the passage of water to and from the cells. It is kept 

 at a uniform level by the kidneys, which excrete excessive 

 amounts of salts, urea, and sugar. These substances are the 

 main factors in determining the osmotic pressure of the blood. 

 The average depression of the freezing point in mammalian 

 blood is A = 0.5 to 0.6 C. 



Coagulation of the Blood. An enormous amount of labor 

 has been expended to clear up the mechanism of blood clotting. 

 The results still are far from conclusive. Clotting is a protec- 

 tive device to prevent excessive loss of blood after injury. If 

 blood is drawn and allowed to stand, it sets to a jelly-like mass. 

 On standing, a yellow liquid is pressed out. This is called the 

 serum. It differs from plasma in that it does not contain 

 fibrinogen, for in clotting, fibrinogen is converted into the insol- 

 uble fibrin. This fibrin forms a fine mesh work in which the 

 corpuscles are entangled. Contact with foreign bodies or with 

 the injured tissue hastens clotting. Blood platelets and calcium 

 play a role in the clotting process. In the process of clotting 

 a new substance, thrombin appears. A second new substance, 

 serum fibrinogen also has been reported. 



The following summary of the steps involved in clotting is 

 perhaps the most satisfactory explanation thus far advanced. 

 Since clotting does not occur normally in blood circulating in 

 the blood vessels, some one or more of the substances involved in 

 clotting must be absent from such blood, or be held in check 

 or in an inactive form. This substance is considered to be 



