|
 The cardiovascular system, which is made up of the heart and blood vessels, supplies blood to all organs and tissues of our body. With the blood, tissues receive oxygen and nutrients, which enter the body through the lungs and digestive organs.
Through the blood, carbon dioxide and substances formed in the process of exchange are removed from the tissues. The constant movement of blood is ensured by the regular activity of the heart and blood vessels.
The wall of blood vessels consists of three membranes: inner, in direct contact with the blood, middle and outer. In large arteries, the main part of the middle membrane is elastic connective tissue, and in small arteries, muscle fibers. In the wall of the arteries, branches of sensory nerves end, through which "signals" about the chemical composition of the blood and about the height of blood pressure in the arteries come to the central nervous system.
The terminal branches of the arteries are capillaries; their wall consists of only one layer of cells. This facilitates the penetration of oxygen and nutrients in the blood into the tissues, and carbon dioxide and waste decay products from the tissues into the blood.
 The vessels that carry blood to the heart are called veins; they have valves that open only in the direction of blood flow (to the heart). When walking, moving, active muscular activity, the veins are compressed, and the blood moves towards the heart. From the heart, blood enters the arteries, which, sometimes contracting, then stretching, contribute to the movement of blood.
However, the main role in maintaining the constant movement of blood in the arteries is played by the heart - a muscular sac consisting of striated muscle fibers permeated with numerous vessels and nerves - this is the myocardium (from the Greek "mis", "mios", "muscle" and " cardia "," heart "- the muscular wall of the heart). The thin, smooth shell lining the heart cavity from the inside is called the endocardium (from the Greek "endon", "inside" - the inner shell of the heart cavity), and the denser outer shell is the pericardium (from the Greek "peri", "about" - pericardial bursa, serous membrane covering the heart). The heart is divided by a continuous longitudinal septum into the right and left halves. Each half consists of two cavities: the upper - the atrium and the lower - the ventricle. Between each atrium and ventricle there is an opening with a sail or cusp valve; at the exit of the heart of large vessels - the aorta and pulmonary artery - there are semilunar valves. All valves open only in the direction in which blood moves: from the atria to the ventricles, and from the latter to the aorta and pulmonary artery.
From the right ventricle comes the pulmonary artery, which carries blood from the heart to the lungs. All blood, passing through the lungs in less than one minute, absorbs about a liter of oxygen, being freed from the same amount of carbon dioxide. From here, blood flows through the pulmonary veins into the left atrium and left ventricle. This is a small circle of blood circulation.
From the left ventricle leaves the aorta, and from it - a series of large arteries, branching in turn into smaller ones, which eventually pass into the capillaries. The latter flow into the gradually enlarging vessels - veins, through which blood returns to the right atrium, and from it to the right ventricle. This is a large circle of blood circulation, due to which tissue nutrition is provided and metabolic products are removed from them.
The heart, which usually weighs only about 300 grams, does a tremendous job.With complete physical rest, contracting about 70 times per minute, each ventricle ejects about 60-80 milliliters of blood into the arteries, which is 3 to 5 liters per minute, and with physical exertion, this figure increases significantly (up to 25 liters). The heart can do such a lot of work, firstly, because it is abundantly supplied with blood at the expense of the coronal (coronal, or coronary, vessels (from Latin - "crown" - crown, wreath) - vessels that feed the heart muscle), or coronary vessels, with a dense network of smaller branches extending from them, penetrating the entire thickness of the heart muscle. Secondly, due to the fact that the period of contraction (it lasts 0.3 seconds) is always followed by a period of relaxation (0.4 seconds), during which the heart muscle "rests" and recovers its strength.
 Contractions of the heart muscle occur involuntarily and are regulated by special nerve cells and bundles of fibers located in its thickness. The work of the whole heart and blood vessels is regulated by the central nervous system. Thanks to this, the cardiovascular system is adapted to various changes both in the body itself and in individual organs, and in the environment. Everyone is well aware of the expressions: "the heart leaps for joy", "the heart freezes from fear", "relieved, from the heart", "the heart feels", "a stone has fallen from the heart", etc., associated with various experiences: excitement, joy, anger, fear, fear, desires, etc. This happens because, under the influence of the nervous system, heart contractions sometimes become more frequent, sometimes occur somewhat less frequently.
The activity of the circulatory system is closely related to the work of the lungs, kidneys, liver and other organs.
The movement of blood, as we have already said, is provided by the activity of the heart and blood vessels. During heartbeats, blood from the heart is ejected under pressure and stretches large vessels. The presence of the muscle layer in the walls of blood vessels makes them elastic, capable of stretching and contracting. This contraction of the walls in turn helps the blood flow.
The highest pressure is noted in the aorta - 130-140 millimeters of mercury, the lowest in the capillaries - 30-40 millimeters. In small veins, it is even lower, and in large veins it becomes negative (less than atmospheric).
Blood pressure is measured with a special apparatus. In this case, two levels of blood pressure are determined. The highest level corresponds to the contraction of the heart, the so-called systole (from the Greek "systole", "contraction, contraction" - contraction of the heart) - this is the maximum, or systolic, pressure. It depends on the force of contraction of the ventricles and the amount of ejected blood. The lowest level corresponds to the relaxation of the heart, the so-called diastole (from the Greek “diastole”, “stretching” - relaxation of the heart muscle, which follows its contraction) - this is the minimum, or diastolic, pressure. It depends mainly on the vascular tone, on the resistance exerted by the walls of the arteries.
In a healthy adult, the maximum pressure in the arteries of the arm is 115-140 millimeters of mercury, and the minimum is 60-90 millimeters. With the excitement of the numerous endings of the sensory nerves embedded in the walls of blood vessels, the level of blood pressure will change. Strong mental and physical stress (excitement, joy, grief, fear), a feeling of pain, muscle work, changes in ambient temperature, atmospheric pressure lead to fluctuations in blood pressure levels. In a healthy person, these changes are short-lived, which is facilitated by the "mechanisms" that regulate blood pressure. But. there are also prolonged increases or decreases in blood pressure. In the first case, it will be hypertension, in the second - hypotension.
E. G. Paramonova - Eat right
|
