i 4 HUMAN PHYSIOLOGY CHAP. 



nace will not burn when the drafts are closed, cutting 

 off the supply of air. Each cell in the body is a little 

 furnace in which the foods we eat are slowly burned up 

 or oxidized. If the blood contains too few red corpuscles, 

 or if we do not breathe enough oxygen, the fire must be 

 low in the cells. 



TJie white corpuscles are said to have the poiver to 

 destroy disease germs and to pick out poisons that get 

 into the blood, and so they tend to keep the blood pure 

 and wholesome. They are thought to aid in blood- 

 clotting, and they doubtless have other functions. 



15. Quantity. About one thirteenth of the weight of 

 the body is blood. A person weighing one hundred and 

 fifty-six pounds would have about twelve pounds of blood. 

 Of course, the amount varies in different persons. TJie 

 exact amount of lymph is unknown, but it is much 

 greater than that of the blood. 



One fourth of the blood is in the lungs, heart, and large 

 blood vessels ; one fourth in the liver ; one fourth in the 

 small blood vessels of the muscles and skin ; and one 

 fourth in the various other organs of the body. 



16. Clotting. When the blood is exposed to the air, 

 little threads called fibers form in the plasma, that 

 entangle the corpuscles and form a solid mass called a 

 clot. The fibers that form throughout the entire mass 

 are made of a substance called fibrinogen, that is found 

 in the plasma. By the contraction of these fibers, a 

 yellowish watery fluid called serum is squeezed out of 

 the solid mass in which the clot then floats. Blood 

 minus the clot equals blood serum. 



