Chap. 12 CIKCULAIION and transportation BODY FLUIDS 197 



until they burst; if it contains too much salt they shrivel. Or, the salt content 

 of the plasma may be right but the proportion of other constituents may be 

 wrong. Without oxygen cells cannot liberate their energy and they die; with- 

 out sugar they starve. In a solution that contains potassium but no calcium, 

 muscle tissue twitches; with too much calcium it becomes inert. 



Plasma, Its Content and Functions. Plasma maintains a content of about 

 90 per cent water that is constantly lost and replaced. Water is lost from the 

 lungs in amounts varying with the temperature and humidity of the air and 

 the rate and depth of breathing, from the kidneys in urine, and from the 



Arteriol end of blood 



capillary in which 



pressure is high 



Lymph capillary 



helps to remove 



excess tissue fluid 



Venous end of blood 



capillary In which 



pressure is lower 



Tissue fluid 

 among cells 



Fig. 12.2. Diagram of the balancing of body fluids. When pressure in the 

 blood capillary is high fluid passes out of the blood through the capillary wall, 

 is dispersed among the cells, and becomes tissue fluid. When the pressure upon 

 the tissue fluid becomes high, the latter may enter the lymph capillary becoming 

 lymph or it may return to the blood capillaries to merge with the blood plasma. 

 Normally there is a constant balancing easily disturbed by slight chemical changes. 



sweat glands in sweat. It is increased mainly by eating and drinking. There 

 is a more or less constant demand for water, since in animals there is no 

 special storage of water, as there is of fat. There are some exceptions to this. 

 Camels, like cattle, swallow their food into a pouch, to be recalled later for 

 leisurely chewing. They are also provided with water stored in water pockets 

 opening ofl the pouch. The great mass of fat in the Jiump also provides water 

 as well as energy. 



Organic Substances. About 7 to 9 per cent of the plasma of human 

 blood consists of proteins. These take part in keeping the volume of blood 

 constant in the vessels, in giving thickness or viscosity to the blood, in holding 

 back too great seepage from the vessels, and in maintaining normal blood 

 pressure. Fibrinogen is unique among the proteins in its essential role in the 

 coagulation of blood. Serum globulin is associated with the development of 



