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RESPIRATION AND EXCRETION 



in the evaporation of sweat. In hot weather the blood vessels of 

 the skin are dilated ; in cold weather they are made smaller by 

 the action of the nervous system. The blood thus loses water in 

 the skin, and as the water evaporates, we are cooled off. The object 

 of increased perspiration, then, is to remove heat from the body. 

 With the large amount of blood present in the skin, perspiration is 

 increased; with a small amount, it is diminished. Hence, we 

 have in the skin an automatic regulator of body temperature. 



Colds and Fevers. — The regulation of blood passing through 

 the blood vessels is under control of the nervous system. If this 

 mechanism is interfered with in any way, as for example .through 

 bacterial toxins released in the body, the sweat glands may not 

 do their work, perspiration may be stopped, and the heat from 

 oxidation held within the body. The body temperature goes up, 

 and a fever results. 



If the blood vessels in the skin are suddenly cooled when full of 

 blood, they contract and send the blood elsewhere. As a result a 

 congestion may follow. Colds are, in real- 

 ity, a congestion of membranes lining cer- 

 tain parts of the body, as the nose, throat, 

 windpipe, or lungs, together with a growth 

 of bacteria which were present in the mouth 

 or throat. Some colds are communicable 

 and gain entrance to the body when the re- 

 sistance is low. 



When suffering from a cold, it is therefore 

 important not to chill the skin, as a full 

 blood supply should be kept in it and so 

 kept from the seat of the congestion. For 

 this reason hot baths (which call the blood to the skin) , the avoid- 

 ing of drafts (which chill the skin) , and warm clothing are useful 

 factors in the care of colds. Very important also are rest in bed, 

 fresh air, plenty of water to drink, and free bowel movements. 



Summary. — The organs of respiration, the lungs, are connected 

 with the outside air through the bronchial tubes, windpipe, and 

 nostrils. Inspiration is brought about by raising the ribs and 

 lowering the diaphragm. This makes the chest cavity larger and 

 thus allows air to rush into the air sacs. The lungs are the struc- 



Blood vessels : A, normal 

 B, congested. 



