For the normal functioning of all organs and systems of the human body, it is vitally necessary to constantly supply them with nutrients and oxygen, as well as the timely removal of decay products and waste products. The implementation of these critical processes is ensured by constant circulation of blood. In this article we will consider the human circulatory system, and also tell how the blood from the arteries enters the veins, how it circulates through the blood vessels and how the main organ of the circulatory system, the heart, works.
The study of blood circulation from antiquity to the XVII century
The circulation of man has been of interest to many scientists for centuries. Even ancient researchers, Hippocrates and Aristotle, suggested that all organs were somehow interconnected. They believed that the blood circulation of a person consists of two separate systems that do not connect with each other. Of course, their ideas were erroneous. They were refuted by the Roman physician Claudius Galen, who proved experimentally that the blood moves the heart not only through the veins, but also through the arteries. Until the XVII century, scientists were of the opinion that blood flows from the right to the left atrium through the septum. Only in 1628 was a breakthrough made: the English anatomist William Harvey in his work “Anatomical Study of the Movement of the Heart and Blood in Animals” presented his new theory of blood circulation. He experimentally proved that it moves along the arteries from the ventricles of the heart, and then returns through the veins to the atria and cannot be infinitely produced in the liver. William Harvey was the first to quantify cardiac output. Based on his work, a modern scheme of human blood circulation was created, including two circles.
Further study of the circulatory system
For a long time, the important question remained unclear: "How does blood from arteries enter the veins." Only at the end of the XVII century, Marcello Malpigi discovered the special links of blood vessels - the capillaries that connect the veins and arteries.
In the future, many scientists (Stephen Hales, Daniel Bernoulli, Euler, Poiseuille and others) worked on the problem of blood circulation, including measuring venous, arterial blood pressure, volume of the heart chambers, elasticity of arteries and other parameters. In 1843, scientist Jan Purkine proposed to the scientific community a hypothesis that a systolic decrease in heart volume has a suction effect on the front edge of the left lung. In 1904, I.P. Pavlov made an important contribution to science, proving that there are four pumps in the heart, and not two, as previously thought. At the end of the twentieth century, it was possible to prove why the pressure in the cardiovascular system is higher than atmospheric.
Blood circulation physiology: veins, capillaries and arteries
Thanks to all the scientific research, we now know that blood constantly moves through special hollow tubes that have different diameters. They do not interrupt and pass into others, thereby forming a single closed circulatory system. In total, three types of vessels are known: arteries, veins, capillaries. All of them are different in structure. Arteries are vessels that provide blood flow to organs from the heart. Inside, they are lined with a single-layer epithelium, and on the outside they have a connective tissue membrane. The middle layer of the arterial wall consists of smooth muscles.
The largest vessel is the aorta. In organs and tissues, arteries are divided into smaller vessels called arterioles. They, in turn, branch into capillaries, which consist of a single-layer epithelial tissue and are located in the spaces between the cells. Capillaries have special pores through which water, oxygen, glucose and other substances are transported into the tissue fluid. How does blood from arteries enter the veins? It comes from the organs, deprived of oxygen and enriched in carbon dioxide, and is sent through the capillaries to the venules. Then she returns to the right atrium along the lower, upper hollow and coronary veins. Veins are located more superficially and have special semilunar valves that facilitate the movement of blood.
Circulatory Circles
All vessels, uniting, form two circles, which are called large and small. The first provides saturation of the organs and tissues of the body with oxygen-rich blood. The large circle of blood circulation is as follows: the left atrium at the same time as the right atrium contracts, thereby providing blood to the left ventricle. From there, the blood goes to the aorta, from which it continues to move along other arteries and arterioles, going in different directions to the tissues of the whole organism. Then the blood returns through the veins and goes into the right atrium.
Blood and circulation: small circle
The second circle of blood circulation starts in the right ventricle and ends in the left atrium. Through it, blood circulates through the lungs. The physiology of blood circulation in a small circle is as follows. The contraction of the right ventricle provides blood flow to the pulmonary trunk, which branches to an extensive network of pulmonary capillaries. The blood entering them is saturated with oxygen through ventilation of the lungs, after which it returns to the left atrium. We can conclude: two circles of blood circulation provide blood movement: first, it goes in a large circle to the tissues and back, and then in a small circle - to the lungs, where it is saturated with oxygen. The blood circulation of a person occurs due to rhythmic cardiac work and pressure differences in arteries and veins.
Circulatory system: heart
The human circulatory system includes, in addition to arterial, venous vessels and capillaries, the heart. It is a muscular organ, hollow inside and having a conical shape. The heart, located in the chest cavity, is freely located in the pericardial sac, consisting of connective tissue. The bag provides constant hydration of the surface of the heart, and also supports its free contractions. The heart wall is formed of three layers: endocardium (internal), myocardium (middle) and epicardium (external). The structure of the heart muscle is somewhat reminiscent of striated muscle, but has one distinctive feature - the ability to automatically contract regardless of external conditions. This is the so-called automation. It becomes possible due to special nerve cells that are in the muscle and produce rhythmic excitations.
Heart structure
The internal structure of the heart is as follows. It is divided into two halves, left and right, with a solid partition. Each such half has two sections - the atrium and ventricle. They are connected by an opening equipped with a flap valve that opens towards the ventricle. In the left half of the heart, this valve has two wings, and in the right - three. Blood flows to the right atrium from the upper, lower hollow, as well as coronary veins of the heart, and from the four pulmonary veins to the left. The right ventricle gives rise to the pulmonary trunk, which, being divided into two branches, transports blood to the lungs. The left ventricle directs blood along the left aortic arch. On the borders of the ventricles, pulmonary trunk and aorta are lunar valves with three valves on each. They close the lumens of the pulmonary trunk and aorta, as well as pass blood into the vessels and prevent the reverse flow of blood into the ventricles.
Three phases of the heart muscle
The alternation of contractions and relaxation of the heart muscles allows blood to circulate in two circles of blood circulation. There are three phases in the work of the heart:
- atrial contraction;
- ventricular contraction (otherwise systole);
- relaxation of the ventricles and atria (aka diastole).
The cardiac cycle is the period from one to the other atrial contractions. All cardiac activity consists of cycles, and each of them consists of systole and diastole. The heart muscle contracts about 70-75 times in one minute (if the body is at rest), that is, about 100 thousand times in one day. At the same time, it pumps over 10 thousand liters of blood. Such a high working capacity is created by increased blood supply to the heart muscle, as well as a large number of metabolic processes in it. The nervous system, in particular its vegetative department, regulates the work of the heart. Some sympathetic fibers enhance contractions during irritation, while others - parasympathetic ones, on the contrary, weaken and slow down cardiac activity. In addition to the nervous system, the work of the heart is also regulated by the humoral one. For example, adrenaline accelerates its work, and an increased content of potassium inhibits it.
Heart rate
Pulse refers to rhythmic fluctuations in the diameter of blood vessels (arterial), which are caused by cardiac activity. The movement of blood through the arteries, including the aorta, is carried out at a speed of 500 mm / s. In thin vessels, capillaries, blood flow is significantly slowed down (up to 0.5 mm / s). Such a low speed of blood movement through the capillaries allows you to give all the oxygen and nutrients to the tissues, as well as take their waste products. In the veins, as you approach the heart, blood flow increases.
What is blood pressure?
This term refers to the hydrodynamic pressure of blood in arteries, veins, capillaries. Blood pressure appears due to the implementation of its activity by the heart, which pumps blood into the vessels, and they resist. Its value in different types of vessels varies. Blood pressure increases with systole and decreases during the diastole period. The heart throws a portion of blood, which stretches the walls of the central arteries and aorta. This creates high blood pressure: the maximum systolic values are 120 mm Hg. Art., and diastolic - 70 mm RT. Art. During diastole, the stretched walls are compressed, thereby pushing the blood further through the arterioles and beyond. When blood moves through the capillaries, a gradual decrease in blood pressure to 40 mm RT occurs. Art. and below. When capillaries pass into venules, blood pressure is only 10 mmHg. Art. This mechanism is caused by friction of blood particles on the walls of blood vessels, which gradually delays the flow of blood. In the veins, a drop in blood pressure continues. In the vena cava it becomes even slightly lower than atmospheric. This difference between the negative pressure in the vena cava and the high pressure in the pulmonary artery and the aorta ensures continuous human circulation.
Blood pressure measurement
Finding the value of blood pressure can be done in two ways. The invasive method involves the introduction of a catheter connected to the measuring system into one of the arteries (usually the radial). This method allows you to continuously measure pressure and obtain highly accurate results. The non-invasive method involves the use of mercury, semi-automatic, automatic or aneroid sphygmomanometers for measuring blood pressure. Typically, pressure is measured on the arm, slightly above the elbow. The obtained value shows what pressure is in this artery, but not in the whole body. Nevertheless, this indicator allows us to conclude about the magnitude of the blood pressure in the subject. The value of blood circulation is enormous. Without a continuous movement of blood, normal metabolism is impossible. Moreover, the life and functioning of the body is impossible. Now you know how the blood from the arteries enters the veins, and how the blood circulation process occurs. We hope you find this article helpful.