THE BLOOD AND CIRCULATION 317 



ing bacteria. The formation of immunizing substances in 

 the blood is attributed to leucocytes. Other corpuscles aid 

 in the absorption of fats while others aid in forming a clot 

 when a blood-vessel is wounded. 



Blood plasma.- -The plasma is the fluid part of the blood. 

 It supports the red and white corpuscles. It is composed of 

 much water, a substance called fibrin, certain salts, absorbed 

 nutrients in the form of serum albumin, and certain wastes 

 (urea and acids) from the tissue cells. 



Blood clotting. Blood exposed to the air forms a clot 

 by the mixture of some of the white blood corpuscles and fi- 

 brin of the plasma. If a bowl of blood is stirred vigorously 

 the fibrin may be separated from the serum and blood cor- 

 puscles. Blood clot or coagulation protects against the 

 loss of blood when a blood-vessel is wounded. 



The amount of blood in the body and its distri- 

 bution. A grown person is provided with about six quarts 

 of blood which equals about one-thirteenth the weight of 

 the body. This amount is constant under normal conditions, 

 because the amount of food and oxygen given up to the tissues 

 is balanced by the amount of waste received. The supply of 

 blood to the heart and lungs, the liver and the skeletal 

 muscles equals about seventy-five per centum of the whole 

 amount in the body. It is distributed to the tissues accord- 

 ing to their needs. An active gland or an active muscle 

 requires more than a resting gland or muscle. During di- 

 gestion and absorption a large supply goes to the stomach. 



Effect of food, fresh air, exercise and rest upon the 

 blood.- -The healthy condition of the blood depends upon 

 abundance of fresh air to supply it with oxygen; nutritious 

 "food and plenty of water to maintain the proper composition 

 of the blood plasma and to supply iron for the red corpuscles; 

 a sufficient amount of exercise so that all the arteries and 

 capillaries may be thrown into activity; and sleep and rest 

 that the demands of the tissues may not be too great a drain 



