116 THE HEART AND CIRCULATION. 



vous system and an impulse passes by the vasodilators 

 to flush the organ. The more active a part is in func- 

 tioning the greater the number of capillaries, except 

 in the brain, which has only large vessels. The vessels 

 of the intestines contain much blood and are capable 

 of containing all the blood in the body. 



The Blood. The blood itself, which thus circulates 

 through the body, carrying nutrition to the tissues and 

 removing waste, is a complex fluid of a bright red color. 

 Its amount has been calculated to be about one-thir- 

 teenth of the body weight. One-fourth of it is gener- 

 ally in the heart, lungs, and large arteries and veins, 

 one-fourth in the liver, one-fourth in the skeletal mus- 

 cles, and one-fourth variously distributed through the 

 other organs. If there is too little blood, the vital pro- 

 cesses cannot go on as they should, while too great a 

 supply causes weakness rather than strength. So the 

 tendency is to keep the amount constant and any blood 

 added is disposed of and any blood lost is replaced. In 

 starvation it is the last tissue to be used up, for on it 

 the life of the other tissues depends. 



Composition. In composition the blood is practi- 

 cally the same in all arteries and fundamentally the same 

 everywhere, but in passing through certain organs cer- 

 tain substances are added to or taken from it, so that 

 its character changes more or less. Thus it varies some- 

 what in composition in different parts of the body, as 

 in the liver and kidneys. It has five main functions: 

 1. the conveying of fuel from the digestive tract to the 

 tissues, or force production; 2. the carrying of oxygen 

 to the tissues; 3. the carrying of tissue-building mate- 

 rials, or tissue building; 4. the distribution of heat; and 

 5. the removal of waste products. 



The blood is slightly alkaline in reaction, of a saltish 

 taste, and has a specific gravity of 1^55. Its temper- 

 ature is about 100 Fahrenheit or 37.8 Centigrade. It 

 is made up of two parts, the plasma or fluid portion and 

 the corpuscles or solid portion. The plasma, again, 



