The Circulation : Part 4

Physiology and Hygiene for Secondary Schools
by Francis M. Walters

(Page 8 of 29)

In the pulmonary capillaries the blood gives up carbon dioxide and receives oxygen, changing from a dark red to a bright red color. In the systemic capillaries it gives up oxygen, receives carbon dioxide and other impurities, and changes back to a dark red color.

In addition to the two main divisions of the circulation, special circuits are found in various places. Such a circuit in the liver is called the portal circulation, and another in the kidneys is termed the renal circulation. To some extent the blood supply to the walls of the heart is also outside of the general movement; it is called the coronary circulation.

Blood Pressure and Velocity. - The blood, in obedience to physical laws, passes continuously through the blood vessels, moving always from a place of greater to one of less pressure. Through the contraction of the ventricles, a relatively high pressure is maintained in the arteries nearest the heart. This pressure diminishes rapidly in the small arteries, becomes comparatively slight in the capillaries, and falls practically to nothing in the veins. Near the heart in the superior and inferior vene cave, the pressure at intervals is said to be negative. This means that the blood from these veins is actually drawn into the right auricle by the expansion of the chest walls in breathing.

The velocity of the blood is greatest in the arteries, less in the veins, and much less in the capillaries than in either the arteries or the veins. The slower flow of the blood through the capillaries is accounted for by the fact that their united area is many times greater than that of the arteries which supply, or the veins which relieve, them. This allows the same quantity of blood, flowing through them in a given time, a wider channel and causes it to move more slowly. The time required for a complete circulation is less than one minute.

Summary of Causes of Circulation. - The chief factor in the circulation of the blood is, of course, the heart. The ventricles keep a pressure on the blood which is sufficient to force it through all the blood tubes and back to the auricles. The heart is aided in its work by the elasticity of the arteries, which keeps the blood under pressure while the ventricles are in a state of relaxation. It is also aided by the muscles and elastic tissue in all of the blood vessels.

These, by keeping the blood vessels in a state of "tone," or so contracted that their capacity just equals the volume of the blood, enable pressure from the heart to be transmitted to all parts of the blood stream. A further aid to the circulation is found in the valves in the veins, which enable muscular contraction within the body, and variable pressure upon its surface, to drive the blood toward the heart. The heart is also aided to some extent by the movements of the chest walls in breathing. The organs Of circulation are under the control of the nervous system.

Hygiene of the Circulation

Care of the Heart. - The heart, consisting largely of muscle, is subject to the laws of muscular exercise. It may be injured by over-exertion, but is strengthened by a moderate increase in its usual work. It may even be subjected to great exertion without danger, if it be trained by gradually increasing its work. Such training, by giving the heart time to gain in size and strength, prepares it for tasks that could not at first be accomplished.

In taking up a new exercise requiring considerable exertion, precautions should be observed to prevent an overstrain of the heart. The heart of the amateur athlete, bicyclist, or mountain climber is frequently injured by attempting more than the previous training warrants. The new work should be taken up gradually, and feats requiring a large outlay of physical energy should be attempted only after long periods of training.

Since the heart is controlled by the nervous system, it frequently becomes irregular in its action through conditions that exhaust the nervous energy. Palpitations of the heart, the missing of beats, and pains in the heart region frequently arise from this cause. It is through their effect upon the nervous system that worry, over study, undue excitement, and dissipation cause disturbances of the heart. In all such cases the remedy lies in the removal of the cause. The nervous system should also be "toned up" through rest, plenty of sleep, and moderate exercise in the open air.

Effect of Drugs. - A number of substances classed as drugs, mainly by their action on the nervous system, produce undesirable effects upon the organs of circulation. Unfortunately some of these are extensively used, alcohol being one of them. If taken in any but small quantities, alcohol is a disturbing factor in the circulation. It increases the rate of the heart beat and dilates the capillaries. Its effect upon the capillaries is shown by the "bloodshot" eye and the "red nose" of the hard drinker. Another bad effect from the use of much alcohol is the weakening of the heart through the accumulation of fat around this organ and within the heart muscle. The use of alcohol also leads in many cases to a hardening of the walls of the arteries, such as occurs in old age. This effect makes the use of alcohol especially dangerous for those in advanced years.

Tobacco contains a drug, called nicotine, which has a bad effect upon the heart in at least two ways: 1. When the use of tobacco is begun in early life, it interferes with the growth of the heart, leading to its weakness in the adult. 2. When used in considerable quantity, by young or old, it causes a nervous condition both distressing and dangerous, known as "tobacco heart."

Tea and coffee contain a drug, called caffeine, which acts upon the nervous system and which may, on this account, interfere with the proper control of the heart. In some individuals the taking of a very small amount of either tea or coffee is sufficient to cause irregularities in the action of the heart. Tea is considered the milder of the two liquids and the one less liable to injure.

D.C. Heath and Co. - Publishers
Original copyright 1909