THE CIRCULATORY SYSTEM 117 



external influences. ' The total amount of blood is about one-four- 

 teenth of the body weight. 



The composition of the blood is very uniform, being 66 per cent, 

 plasma and 34 per cent, corpuscles. 



The blood-plasma is the fluid part of the blood. It is nine- 

 tenths water. The proportion of water and solids is little increased 

 by drinking and only slightly reduced by thirst. Its constancy was 

 explained when the functions of the kidneys were studied. Four 

 different albuminous bodies form about four-fifths of the total 

 solids of plasma. They are fibrinogen, nucleoprotein, serum-albu- 

 men, and serum-globulin. 



The clotting of blood is due to the fibrinogen. This substance 

 is transformed into fibrin by the action of the enzyme thrombin. 

 Thrombin is derived from decomposing blood-plates, the minute 

 extremely sensitive bodies present in normal blood. Freshly 

 drawn blood may be defibrinated so it will not clot by whipping 

 with a stiff wire or shaking with glass beads. The fibrin separates 

 out as an elastic, fibrous mass. 



The blood-serum is the clear liquid which is pressed out by the 

 contraction of the blood-clot. It differs chiefly from the plasma in 

 containing no fibrinogen. The color of serum is amber-yellow. 

 Sodium chlorid is present in about 0.6 per cent, and is the principal 

 inorganic constituent. 



The blood-corpuscles are of two kinds, white and red. They 

 are present in the proportion of about one to five hundred respect- 

 ively. The white corpuscles, or leukocytes, are somewhat larger 

 than the red. By- virtue of their ameboid movement they have 

 the ability to pass through intact vessel walls without causing 

 harm. They are active agents in protecting the body against dis- 

 ease-producing micro-organisms. The red corpuscles, or erythro- 

 cytes, are biconcave disks, without nuclei, measuring 45^^ inch 

 in diameter. They originate in the red marrow of bones, in the 

 liver, and in the spleen. In very thin layers they appear yellow 

 in color, but under ordinary conditions are red from the hemo- 

 globin, a pigment, which they contain. Hemoglobin is a protein 

 substance, making up about 90 per cent, of the solid part of the 

 erythrocytes! It has the remarkable property of combining with 

 oxygen in the lungs and of releasing this gas when it comes into a 

 situation where there is a relative lack of the element, as is- found 

 in the tissues. 



