196 



THF INTF.RNAL F.NVIRONMENT OF TlIF BODY 



Part 111 



Hiinuin Blood 



(General Composition. Blood is composed of fluid and cells. It is as much 

 a tissue as bone; in blood the substance between the cells is fluid; in bone it 

 is solid. When a tube is filled with blood and whirled in a centrifuge, the cells 

 are thrown down to the bottom of the tube. The blood is thus separated into 

 a mass of cells constituting about 45 per cent of the whole blood, and a 

 clear pale yellowish fluid, the plasma, composing the other 55 per cent (Fig. 



12.1). 



Plasma and lymph and tissue fluid are fluids which come and go, join 

 together, are separated, and join again, over and over, continually sharing 



Human blood contains 



Fluid 



(plasma) 



55% 



< 



r 



Solids 



(cells) 



45% 



< 



^ 



Water 



Proteins and 

 ottier substances 



Salt and 

 other minerals 



Cells 



Fig. 12.1. Fluid and solids of human blood. (Redrawn and modified from Public 

 Affairs Pamphlet No. 145. New York, Public Affairs Committee, Inc., 1948.) 



their contents (Fig. 12.2). Plasma is complex because via the tissue fluid it 

 receives contributions of every kind from all cells of the body (Table 12.1). 

 When blood is under sufficient pressure in the capillaries, the excess fluid 

 seeps through the walls and becomes the only fluid that is in actual direct 

 contact with cells. When there is greater pressure on the tissue fluid outside 

 than on the blood inside the capillaries, the excess fluid goes back into the 

 capillaries or the lymph vessels from whence it is ultimately returned to the 

 blood. Thus these fluids continually pass to and fro; taking food and other 

 substances to the cells; removing their useful secretions and waste products; 

 distributing the heat of their oxidations; keeping them wet; providing them 

 with necessary salts, acids, and gases; and guiding their behavior by hormones. 

 The life of all cells is dependent on the continuity of this environment, and 

 its delicately balanced content must not change unless it is altered specifically 

 and in a way useful to the whole animal. If the plasma does not contain 

 enough salt the osmotic pressure rises and water enters the blood corpuscles 



