MULTICELLULAR ANIMALS 175 



terially in health, for it contains substances, chiefly carbonates, that 

 form acid if excess alkali enters, or form alkali if excess acid is taken 

 in. Such substances are termed buffers; in the blood plasma they 

 are most important in maintaining the chemical balance of the 

 system. The osmotic pressure of the human blood is given as equal 

 to between 5 and 7 atmospheres. The viscosity varies within limits, 

 depending on the amount of liquid taken into the body or lost. It 

 is generally thought that foods and wastes are carried by the blood 

 in solution in the plasma. The plasma also contains salts of Ca, Na, 

 and K, the immunity bodies, antitoxins and bacteriolysins, and 

 minute quantities of substances secreted by glands. An important 

 feature of the blood is its ability to form solid masses or clots. Nor- 

 mally this occurs only when the blood is exposed to air. The blood 

 plasma contains small blood platelets, which enclose a substance 

 known as prothrombin, that, on contact with surfaces, particularly 

 oil-free surfaces in the presence of air and calcium ions, becomes 

 chemically changed into a different substance, thrombin. Thrombin 

 then acts on another plasma constituent, fibrinogen, converting it 

 into an insoluble protein, fibrin. Fibrin in air-exposed blood forms 

 masses of fibres; these entrap the blood cells, the whole forming a 

 blood clot. Calcium is necessary for the clotting reaction for it will 

 not take place if the blood calcium salts are removed. The forma- 

 tion of the clot is therefore the result of a series of enzyme actions 

 set in motion by the bursting of the blood platelets. Clotting is a 

 very important defense mechanism; in cases in which it is lacking, 

 even a minor injury results in continuous and profuse bleeding. 

 Inability of the blood to form clots, haemophilia, is a hereditary- 

 character in some human families. Its inheritance is sex linked; 

 mothers are not usually bleeders but their sons may be (Chapt. IX). 

 Transportation of Gases by the Blood. The chief func- 

 tion of the red blood cells is the transportation of oxygen from the 

 respiratory apparatus to the tissue cells and the conveyance away of 

 the carbon dioxide. The hemoglobin that colors the erythrocytes is 



