Page:Structure and functions of the body; a hand-book of anatomy and physiology for nurses and others desiring a practical knowledge of the subject (IA structurefunctio00fiskrich).pdf/122

 *vous system and an impulse passes by the vasodilators to flush the organ. The more active a part is in functioning 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-thirteenth of the body weight. One-fourth of it is generally in the heart, lungs, and large arteries and veins, one-fourth in the liver, one-fourth in the skeletal muscles, and one-fourth variously distributed through the other organs. If there is too little blood, the vital processes 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 practically the same in all arteries and fundamentally the same everywhere, but in passing through certain organs certain substances are added to or taken from it, so that its character changes more or less. Thus it varies somewhat 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 materials, 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 1055. Its temperature 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,