Human heart and circulation: chambers, blood flow and heartbeat explained
How the human heart works: its four chambers, the path of blood through the lungs and body, the valves that keep blood moving one way, and what a heartbeat actually is.
The human heart is a muscular organ with four chambers that works as two pumps in one: the right side pumps oxygen-poor blood to the lungs, and the left side pumps oxygen-rich blood to the rest of the body, with valves keeping the blood flowing in one direction.
The four chambers
The heart has two upper chambers, the atria, and two lower chambers, the ventricles. The right atrium receives oxygen-poor blood returning from the body, and the left atrium receives oxygen-rich blood returning from the lungs. The atria pass blood down into the ventricles, which do the main pumping work.
The walls of the ventricles are thicker than those of the atria because they must push blood out of the heart. The left ventricle has the thickest wall of all, since it pumps blood around the entire body, while the right ventricle only pumps blood the short distance to the lungs. A wall of muscle between the left and right sides keeps oxygen-rich and oxygen-poor blood from mixing.
The path of blood: pulmonary and systemic circulation
Blood follows a fixed loop. Oxygen-poor blood enters the right atrium from the body through the vena cava, flows into the right ventricle, and is pushed through the pulmonary artery to the lungs. In the lungs it gives up carbon dioxide and picks up oxygen.
Oxygen-rich blood then returns through the pulmonary veins into the left atrium, flows into the left ventricle, and is pumped out through the aorta, the body's largest artery, to the tissues. After delivering oxygen and collecting waste, blood returns through veins to the right atrium and the cycle repeats. The trip to the lungs is called pulmonary circulation; the trip around the body is called systemic circulation.
Valves and the heartbeat
Valves are one-way gates inside the heart. The tricuspid valve sits between the right atrium and right ventricle, and the mitral (bicuspid) valve sits between the left atrium and left ventricle. Further valves guard the exits to the pulmonary artery and the aorta. When a ventricle contracts, its inlet valve closes and its exit valve opens, so blood moves forward instead of flowing back.
A heartbeat is this coordinated sequence: the atria contract and push blood into the ventricles, then the ventricles contract and push blood out to the lungs and body. The familiar lub-dub sound comes from the valves closing as blood moves through. The heart also supplies itself through the coronary arteries, which branch off just beyond the aortic valve and feed the heart muscle.
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Frequently asked questions
How many chambers does the human heart have?
Four: the right atrium, right ventricle, left atrium and left ventricle. The atria receive blood and the ventricles pump it out.
What is the path of blood through the heart?
Body to right atrium, right ventricle, pulmonary artery to lungs, pulmonary veins to left atrium, left ventricle, aorta to the body, then back through veins to the right atrium.
Why is the left ventricle wall the thickest?
Because it pumps oxygen-rich blood around the whole body at higher pressure, while the right ventricle only pumps blood to the nearby lungs.
What is the difference between pulmonary and systemic circulation?
Pulmonary circulation carries blood between the heart and the lungs for oxygenation. Systemic circulation carries oxygen-rich blood from the heart to the body's tissues and returns oxygen-poor blood to the heart.
What do heart valves do?
They keep blood flowing in one direction by opening to let blood through and closing to stop it flowing back, for example between the atria and ventricles and at the exits to the aorta and pulmonary artery.