How Does Human Blood Circulate?

To understand thoroughly how human blood circulates, we need three concepts: systemic circulation, pulmonary circulation and coronary circulation. All are closely related to a small but vital organ in our body—the heart.

The heart is a fist-sized muscular pump located in the middle mediastinum between the two lungs, slightly to the left. Two-thirds of it lies to the left of the body's midline and one-third to the right.

This muscular pump is divided into left and right halves by a muscular septum; the left and right sides do not communicate. The septum dividing them is called the cardiac septum. Each side is further divided into an atrium and a ventricle: on the left, the left ventricle and left atrium; on the right, the right ventricle and right atrium. Our heart therefore has four chambers in total.

Although the ventricle and atrium on each side communicate, between them on each side there are two valve-like structures that ensure blood flows through the heart in a fixed direction.

On the left, the structures preventing backflow are the mitral valve and aortic valve; on the right, the tricuspid valve and pulmonary valve. They all lie at the junction between the atrium and ventricle on each side—the atrioventricular orifice.

These valves open and close with the contraction and relaxation of the heart muscle. When open, blood flows from the atrium on that side into the ventricle; when closed, blood no longer flows between atrium and ventricle. If any of the four valves malfunctions, blood regurgitates within the heart, producing various related diseases—collectively called heart disease. The causes of heart disease are complex: primary, secondary and congenital, and cannot be summarized simply.

The heart has this structure because it must supply nutrients and oxygen to cells throughout the body and transport metabolic wastes, carbon dioxide and water to the kidneys, lungs and skin for excretion. It must therefore maintain a certain circulatory order to function in an orderly way.

Our blood is pumped from the ventricles through the aorta into the systemic arterial system, then collected by veins throughout the body and returned to the atria. Arteries carry blood away from the heart; veins carry blood back to the heart. Arterial blood is oxygen-rich; venous blood is carbon-dioxide-rich.

What flows in an artery is not necessarily arterial blood, and what flows in a vein is not necessarily venous blood. The left ventricle pumps out oxygen-rich arterial blood; the right ventricle pumps out carbon-dioxide-rich venous blood.

The arterial blood pumped by the left ventricle is distributed by the aorta into arteries of all levels and then into capillaries, supplying oxygen and nutrients throughout the body. As the body's cells take up oxygen and nutrients, these are consumed; meanwhile the metabolic wastes, carbon dioxide and water produced by the cells enter the blood, making it venous blood. Venous blood is carried from capillaries into veins of all levels and finally flows into the right atrium. This is systemic circulation, also called the greater circulation.

The venous blood in the right atrium then flows, during the contraction and relaxation of the myocardium, into the right ventricle. The right ventricle pumps this carbon-dioxide-rich venous blood into the pulmonary trunk, which distributes it into smaller pulmonary arteries and finally to the alveolar capillaries. Oxygen inhaled from the air is absorbed by the blood in the alveolar capillaries, while the venous blood releases carbon dioxide there, once again becoming oxygen-rich arterial blood.

The oxygenated arterial blood then passes from the pulmonary capillaries into the pulmonary venous system and finally enters the left atrium through the upper and lower pulmonary veins of both lungs. This is pulmonary circulation, also called the lesser circulation.

The oxygenated arterial blood entering the left atrium then flows, during myocardial contraction and relaxation, from the left atrium into the left ventricle. A new cycle begins: the left ventricle pumps it into the aorta, from which it is distributed throughout the body. This cycle repeats, sustaining life. One complete circulation pumps about 5 liters of blood and takes only one minute.

The familiar heartbeat sounds are produced by closure of the mitral, tricuspid, pulmonary and aortic valves. The heart beats about 100,000 times a day on average and about 2.5 billion times over a normal lifespan.

Because a fetus cannot breathe on its own in the womb, its cardiac structure is slightly different.

In the fetal heart there is a temporary channel between the right and left atria—the foramen ovale—which allows blood to flow from the right to the left atrium but not in reverse. After birth, the foramen ovale in a healthy person gradually closes, leaving a mark called the fossa ovalis. If it fails to close after birth, venous blood from the right atrium can enter the left atrium, causing hypoxemia. Patent foramen ovale is a congenital heart disease; severe cases require surgery.

The fetal cardiovascular system also has a temporary channel called the ductus arteriosus, an important vascular structure connecting the aorta and pulmonary artery. In a healthy person it closes within days after birth, becoming the ligamentum arteriosum. If it remains patent six months after birth, it also causes mixing of arterial and venous blood and is another congenital heart disease.

The heart itself also needs blood to supply nutrients for its normal work. Its own blood supply is called coronary circulation. Although the heart accounts for only about 0.5% of body weight, total coronary blood flow is 4%-5% of cardiac output, so coronary circulation is of vital importance.

The arteries of the heart are called coronary arteries, divided into the left and right coronary arteries, arising respectively from the left and right aortic sinuses. They branch extensively: the main branches of the left coronary artery include the anterior interventricular branch and the circumflex branch; the right coronary artery gives off branches to the sinoatrial node, right margin, posterior interventricular septum, right circumflex, right atrium and atrioventricular node. These branches reach every part of the heart, supplying nutrients and oxygen.

Cardiac veins are divided into superficial and deep systems. Superficial veins arise from various parts of the myocardium and form networks and trunks under the epicardium; most venous blood is collected by the coronary sinus and drained into the right atrium, although a few small veins open directly into the cardiac chambers.

Any stenosis or blockage anywhere in the coronary circulation causes various types of heart disease. Once the heart itself cannot function normally, our blood circulation fails, and in severe cases life is endangered.

These three circulations work together so that our blood circulates without ceasing.