LESS. V LOSSES OF THE BLOOD 171 



or generated in it, must be as constantly got rid of, or 

 excreted. 



Three distinct sets of organs are especially charged with 

 this office of continually removing or " excreting " waste 

 matters from the blood. They are the lungs, the kidneys, 

 and the skin (see Lesson I.). These three great organs 

 may therefore be regarded as so many drains from the 

 blood — as so many channels by which it is constantly 

 losing substance. 



On the other hand, the blood, as it passes through the 

 capillaries, is constantly giving up material by exudation 

 through the capillary walls into the surrounding tissues, 

 in order to supply them with nourishment, and thus in this 

 way also is constantly losing matter. 



The material which the blood loses by giving it up to 

 the tissues consists of complex organic bodies, such as 

 proteids, fats, carbohydrates, and various substances 

 manufactured out of these, of certain salts, of a large 

 quantity of water, and lastly of oxygen. 



The material which the blood loses by giving it up to 

 the skin, lungs and kidneys, passes away from these 

 organs as water, as carbonic acid, as peculiar organic 

 substances of which one, called urea, is much more 

 abundant than the others, and as certain inorganic salts. 

 Speaking generally we may say that these organs together 

 excrete from the blood, water, carbonic acid, urea and 

 salts. 



Another kind of loss takes place from the surface of 

 the body generally, and from the interior of the air- 

 passages. Heat is constantly being given off from the 

 former by radiation, evaporation, and conduction : from 

 the latter, chiefly by evaporation ; and the loss of heat in 

 each case is borne by the blood passing through the skin 

 and air-passages respectively. Besides this a certain 

 quantity of heat is lost by the urine and fgeces which are 

 always warm when they leave the body. 



On the side of gain we have, in the first place, the 



