The circulatory system is , in anatomy and physiology, the course taken by the blood through the arteries, capillaries, and veins and back to the heart. In humans and the higher vertebrates, the heart is made up of four chambers: the right and left auricles, or atria, and the right and left ventricles. The right side of the heart pumps oxygen-poor blood from the cells of the body back to the lungs for new oxygen; the left side of the heart receives blood rich in oxygen from the lungs and pumps it through the arteries to the various parts of the body. Circulation begins early in fetal life. It is estimated that a given portion of the blood completes its course of circulation in approximately 30 seconds.Pulmonary Circulation. Blood from the entire body is transported to the right auricle through two large veins: the superior vena cava and the inferior vena cava. When the right auricle contracts, it forces the blood through an opening into the right ventricle.
Contraction of this ventricle drives the blood to the lungs. Blood is prevented from returning into the auricle by the tricuspid valve, which completely closes during contraction of the ventricle. In its passage through the lungs, the blood is oxygenated, that is, saturated with oxygen; it is then brought back to the heart by the four pulmonary veins, which enter the left auricle.

When this chamber contracts, blood is forced into the left ventricle and thence by ventricular contraction into the aorta. The bicuspid, or mistral, valve prevents the blood from flowing back into the auricle, and the seminar valves at the beginning of the aorta stop it from flowing back into the ventricle. Similar valves are present in the pulmonary artery.Branching. The aorta divides into a number of main branches, which in turn divide into smaller ones until the entire body is supplied by an elaborately branching series of blood vessels. The smallest arteries divide into a fine network of still more minute vessels, the capillaries, which have extremely thin walls; thus, the blood is enabled to come into close relation with the fluids and tissues of the body. In the capillaries, the blood performs three functions: It releases its oxygen to the tissues, it furnishes to the body cells the nutrients and other essential substances that it carries, and it takes up waste products from the tissues. The capillaries then unite to form small veins. The veins, in turn, unite with each other to form larger veins until the blood is finally collected into the superior and inferior venae cavae from which it goes to the heart, completing the circuit.Portal Circulation. In addition to the pulmonary and systemic circulations described above, a subsidiary to the venous system exists, known as portal circulation. A certain amount of blood from the intestine is collected into the portal vein and carried to the liver. There it enters into the open spaces called sinusoids, where it comes into direct contact with the liver cells. In the liver important changes occur in the blood, which is carrying the products of the digestion of food recently absorbed through the intestinal capillaries. The blood is collected a second time into veins, where it again joins the general circulation through the right auricle. In its passage through other organs, the blood is further modified.Coronary Circulation. One exception exists to the rule that blood can pass from the right side of the heart to the left only by way of the pulmonary circulation to and from the lungs. This exception is the coronary circulation, by means of which the heart tissues themselves are supplied with nutrients and oxygen and are freed of wastes. Just beyond the seminar valves, two coronary arteries branch from the aorta. These then break up into an elaborate capillary network in the heart muscle and valve tissue. Blood from the coronary capillary circulation enters several small veins, which then enter directly into the right auricle without first passing into the vena cava.Heart Action. The action of the heart consists of successive alternate contraction (systole) and relaxation (diastole) of the muscular walls of the auricles and ventricles. During the period of relaxation, the blood flows from the veins into the two auricles, gradually distending them. At the end of this period, the auricles are completely dilated; their muscular walls contract, forcing almost the entire contents through the auriculoventricular openings into the ventricles. This action is sudden and occurs almost simultaneously in both auricles. The mass of blood in the veins makes it impossible for any blood to flow backward. The force of blood flowing into the ventricles is not powerful enough to open the seminar valves, but it distends the ventricles, which are still in a condition of relaxation. The tricuspid and mistral valves open with the blood current and close readily at the beginning of ventricular contraction.
The ventricular systole immediately follows the auricular systole. The ventricular contraction is slower, but far more forcible; the ventricular chambers are virtually emptied at each systole. The apex of the heart is thrown forward and upward with a slight rotary motion; this impulse, called the apex beat, can be detected between the fifth and sixth ribs. The heart is entirely at rest for a short time after the ventricular systole occurs. The entire cycle can be divided into three periods: in the first, the auricles contract; in the second, the ventricles contract; in the third, both the auricles and the ventricles remain at rest. In humans, with a normal heart rate of approximately 72 heartbeats per minute, the cardiac cycle has a duration of about 0.8 second. Auricular systole requires about 0.1 second; ventricular systole occupies approximately 0.3 second. Thus, the heart is completely at rest for about 0.4 second, or during perhaps half of each cardiac cycle.
With every beat, the heart emits two sounds, which are followed by a short pause. The first sound, coinciding with the ventricular systole, is dull and protracted. The second sound, made by the sudden closure of the seminar valves, is shorter and much sharper. Diseases of the heart valves may change these sounds, and many factors, including exercise, cause wide variations in the heartbeat, even in healthy people. The normal heart rate of animals varies widely from species to species. At one extreme, the heart of a hibernating mammal may beat only a few times a minute; at the other, the hummingbird has a heart rate of 2000 heartbeats per minute.Pulse. When it enters the arteries at the moment of ventricular contraction, the blood stretches the walls of the arteries. During diastole, the distended arteries return to their normal diameter, in part because of the elasticity of connective tissue and in part because of the contraction of muscles in the arterial walls. This return to normal is important in maintaining a continuous flow of blood through the capillaries during the period while the heart is at rest. The expansion and contraction of the arterial walls that can be felt in all the arteries near the surface of the skin is called the pulse.Sources of Heartbeat. The rate and strength of the heartbeat are controlled by nerves through a series of reflexes that speed it up or slow it down. The impulse to contraction, however, is not dependent on external nervous stimuli, but arises in the heart muscle itself. A small bit of specialised tissue called the sinoauricular node, embedded in the wall of the right auricle, is responsible for initiating the heartbeat. The contraction then spreads over the auricles; in the septum between the auricles, it excites another node called the auriculoventricular node. The auriculoventricular bundle conducts the impulse from this node to the muscles of the ventricles, and in this way contraction and relaxation of the heart are co-ordinated. Each phase of the cardiac cycle is associated with the production of an electrical potential that can be recorded by electrical instruments to produce a reading known as an electrocardiogram.Capillaries. Circulation of the blood in superficial capillaries can be observed under the microscope. The red blood cells can be seen moving along rapidly in the middle of the blood current, while the white cells advance more slowly along the walls of the capillaries. The capillaries present a far larger surface with which the blood comes in contact than do other blood vessels; because they consequently offer the greatest resistance to the progress of the blood, they have a great influence on the circulation. Capillaries expand when temperature rises and help to cool the blood; they contract in cold and help preserve internal heat.