310 THE MAINTENANCE OF THE INDIVIDUAL 



Some of the blood on its way from the heart passes to the walls 

 of the food tube and so on to its glands. From these parts it is sent 

 with its load of absorbed food to the liver. Here the portal vein 

 that carries the blood breaks up into capillaries around cells of the 

 liver, which take out the excess sugar from the blood and store it as 

 glycogen. From the liver the blood passes directly to the right auricle. 



Functions of the Blood 



The blood being the circulatory tissue plays a very important part 

 in the maintenance of the organism. Most waste products of the 

 tissues are carried by the blood from their point of origin to some 

 other region of the body which is adapted for their elimination. 

 Thus the nitrogenous waste, urea, is carried to the kidneys. Other 

 wastes are eliminated through the sweat glands of the skin or the 

 lungs. The blood stream is also concerned with the transportation of 

 oxygen from the lungs, and nutrient material from the intestines to the 

 tissues. In addition, it carries the products of one tissue to another ; 

 for example, internal secretions which are produced in glands must 

 be transported elsewhere to do their work. Secretin, already referred 

 to, will serve as an example of this type of action. 



In addition to the three transportation jobs already mentioned 

 the blood also serves to remove various waste products of metabolism 

 from the point of their formation to the organs which excrete them, 

 i.e., the lungs, skin, intestines, and kidneys. Through its accessi- 

 bility to the various organs and glands of the body, the blood may aid 

 in maintaining the normal acid-base balance of the tissues as well as 

 the water content of the body. 



We know that oxidation generates heat, which means that in 

 the human body heat is being constantly released by the working 

 cells. It is carried by the blood stream to the outside layers of the 

 body and there dissipated in the surrounding environment unless 

 special heat-regulating devices are present. Man regulates his body 

 temperature very largely by controlling the heat loss through nerve 

 impulses causing contraction of the minute blood vessels in the skin. 

 The expansion of these blood vessels, resulting from the stimulus of 

 the vasomotor center of the medulla oblongata, allows greater radiation 

 and consequent loss of heat (see page 351). What is of perhaps still 

 greater importance to man in cooling his body is the ability of sweat 

 glands to increase their action under proper nervous stimulation 

 and to pass out more sweat to be evaporated. Heat is required 



