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The right ventricle by its contraction drives the blood through an artery to each of the lungs, until it finally reaches the countless capillaries in the interior of these organs. Veins now receive this blood and convey it to the left auricle, whence it again enters the left ventricle. About one half minute is required to complete the circulation.

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168. Changes in the composition of the blood. The composition of the blood is continually changing in its passage through the various tissues of the body. We may, perhaps, make clearer these various changes by expressing them in tabular form as follows:

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VI. HYGIENE OF THE CIRCULATION

169. Effect of exercise on the heart. - The pulse rate is slowest when we are asleep. As the activities of the day begin, the heart beat is quickened, and after violent exercise

this organ may beat as often as twice a second. Exercise, when properly regulated, is undoubtedly beneficial to every organ of the body, for the heart should be kept in such a vigorous condition that it is ready to meet not only the ordinary requirements of everyday life, but even the strain that may come in such emergencies as necessary escape from danger or recovery from disease.

It is easily possible, however, to overstrain the heart muscle by exacting from this organ too violent or too prolonged activity (e.g. in sprinting or in long distance runs and bicycle rides). These often result in permanent thickening of the walls of the valves of the heart. Before a youth takes part in athletic contests, he should consult a competent physician as to the wisdom of his taking violent exercise.

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170. Effect of exercise on the blood vessels. one is using the muscles actively, greater oxidation of the tissues goes on, and a larger amount of blood is needed to supply the oxygen and to remove the added wastes formed by this increased oxidation. The muscular walls of the arteries relax in the organs that are specially active, thus supplying these organs with more blood. It is manifestly impossible to have an increased supply of blood in the organs of digestion, in the muscles, and in the brain all at the same time. This is the reason why it is unhygienic for an adult to exercise violently or to carry on any considerable degree of mental activity immediately after a hearty meal. Persistence in violating this rule usually results in attacks of indigestion.

171. Stopping of blood flow in wounds. One can tell when an artery has been cut by the fact that blood comes out in spurts. Since the blood is on its way from the heart, the flow can be stopped or lessened in this kind of accident

by applying pressure on the side of the wound nearest the heart.1 Thus if the finger is cut deeply and the blood jets forth, a strong cord or a handkerchief should be tied loosely about the wrist, a wad of paper, or a pebble being placed directly beneath the knot and over the artery. A pencil or piece of wood should then be run through the loop, and the knot should be twisted until the blood flow is stopped by the pressure. When blood flows evenly from a wound, it is an indication that a vein has been cut, and the pressure should be applied in a similar way on the side away from the heart. If unable to decide whether an artery or a vein has been cut, put the bandage directly over the cut.2

Bleeding from the nose may usually be stopped by holding the head erect, and by applying cold water to the bridge of the nose or to the back of the neck.

1 Every pupil should practice the method of applying a bandage in accordance with the directions given in this section.

2 For further treatment of cuts and bruises see 25.

CHAPTER VII

RESPIRATION AND THE RELEASE OF ENERGY IN MAN

I. NECESSITY FOR RESPIRATION

172. To prove that oxidation takes place in the human body.1 Laboratory study.

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A. Development of heat in the human body.

Secure two chemical thermometers that approximately agree at the room temperature. Support one of the thermometers so that it hangs free in the air; clasp the bulb of the other thermometer in the palm of the hand for several minutes.

1. Describe the experiment as it was performed.

2. Note and record the temperature as indicated on each of the thermometers.

3. What evidence have you that heat is produced in the human body?

B. Production of carbon dioxid in the human body.

Blow the breath through a tube into a bottle or test tube of lime water.

1. Describe what was done.

2. What proof have you that carbon dioxid is given off from the body?

3. What element found in foods and protoplasm must be oxidized in order to produce carbon dioxid?

1 The student should review P. B., 75 (to prove that heat energy is developed in growing seedlings) and P. B., 81 (to prove that carbon dioxid is formed during the growth of seedlings).

4. State now two evidences that oxidation is carried on in the human body..

5. What element must always be present in order that oxidation may be carried on?

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173. Examples of energy in the human body. While studying plants, we enumerated various ways in which these living organisms exhibit the energy which is developed within them (P. B., 74), and we have likewise called attention to evidences of energy in animals. In human beings the forms of energy are much more varied and striking. For example, the movements of each of the five hundred separate muscles found in the body are all due to the muscular energy developed in their protoplasm; the control of all these muscles is due to energy liberated in the nervous system (nervous energy); all the glands that produce the varied ferments owe their ability to do their work to the release of chemical energy; and when we come to deal with the highest functions, namely, feeling, thinking, and willing, it seems probable that all of them are made possible by the setting free of some form of energy. In connection with the development of all these forms of energy, heat energy, as we proved in 172, is liberated.

174. Transformations of energy. While considering the functions of green plants we found that the energy of the sun is utilized and stored in the manufacture of food materials, and thus is made available for the use of the plant. Consequently, when we take into our bodies and digest the various nutrients produced by green plants, these food substances become available as our sources of energy. But to release this stored-up energy, whether in muscle, gland, or nerve cells, oxygen is always essential. Hence, a constant supply of oxygen for the body is necessary. When this oxygen

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