Heart Structure, Chambers, and Cardiac Cycle
The human heart is a muscular pump with four chambers, two atria that receive blood and two ventricles that pump it out, and a set of valves that keep blood flowing in one direction. In brief: the right side of the heart pumps blood to the lungs, and the left side pumps blood to the rest of the body. One heartbeat, called the cardiac cycle, includes contraction (systole) and relaxation (diastole), and a resting adult heart beats about 60 to 100 times per minute.
The heart beats around 100 000 times a day without rest, so its structure is finely adapted. This guide explains the circulatory system and the structure of the heart, names the chambers and valves, describes the stages of the cardiac cycle, works through a cardiac output calculation and corrects common mistakes.
What is the circulatory system?
The circulatory system, or cardiovascular system, transports blood around the body. It consists of the heart, the blood vessels and the blood. It delivers oxygen and nutrients to cells, removes carbon dioxide and other wastes, carries hormones and helps to regulate temperature and defend against disease.
Humans have a double circulatory system. Blood passes through the heart twice in each complete circuit: once on its way to the lungs and once on its way to the body. This keeps oxygen-rich and oxygen-poor blood separate and maintains high pressure in the body circuit. The two circuits are described in the next article.
What is the structure of the heart?
The heart is about the size of a fist and weighs around 250 to 350 grams in an adult. It lies in the chest, between the lungs, tilted slightly to the left. It is enclosed in a protective sac called the pericardium.
The wall is made mostly of cardiac muscle, called the myocardium. Cardiac muscle cells are branched, are connected by junctions that pass electrical signals quickly and have many mitochondria, so they resist fatigue. The heart muscle has its own blood supply through the coronary arteries, which branch off the aorta. Blockage of a coronary artery causes a heart attack.
A thick wall, the septum, divides the heart into a right and a left side, and blood does not mix between them in a healthy heart.
What do the four chambers do?
The heart has four chambers.
- Right atrium: receives oxygen-poor blood from the body through the vena cava (the superior and inferior venae cavae).
- Right ventricle: pumps oxygen-poor blood through the pulmonary artery to the lungs.
- Left atrium: receives oxygen-rich blood from the lungs through the pulmonary veins.
- Left ventricle: pumps oxygen-rich blood through the aorta to the body.
The atria have thin walls because they only push blood into the ventricles next to them. The ventricles have thicker walls because they pump blood further. The left ventricle has the thickest wall, about three times thicker than the right, since it must generate enough pressure to send blood to the whole body, while the right ventricle serves the nearby lungs.
What do the heart valves do?
Valves are flaps of tissue that open to let blood pass in one direction and close to prevent backflow. Two types are found in the heart.
- Atrioventricular (AV) valves lie between the atria and ventricles. The right one is the tricuspid valve (three flaps), and the left one is the bicuspid or mitral valve (two flaps). Tough cords called tendons (chordae tendineae) attach the flaps to the ventricle wall and stop them from turning inside out when the ventricle contracts.
- Semilunar valves lie at the exits of the ventricles: the pulmonary valve in the pulmonary artery and the aortic valve in the aorta. They prevent blood from falling back into the ventricles.
The “lub-dub” sounds of the heartbeat are made by the closing of the valves. The first sound is the closing of the AV valves, and the second is the closing of the semilunar valves.
Heart chambers and valves compared
| Chamber | Blood type | Blood comes from | Blood goes to | Valve at exit |
|---|---|---|---|---|
| Right atrium | Oxygen-poor | Vena cava | Right ventricle | Tricuspid |
| Right ventricle | Oxygen-poor | Right atrium | Pulmonary artery | Pulmonary |
| Left atrium | Oxygen-rich | Pulmonary veins | Left ventricle | Bicuspid (mitral) |
| Left ventricle | Oxygen-rich | Left atrium | Aorta | Aortic |
What happens during the heartbeat?
The cardiac cycle is the sequence of events in one heartbeat. At a rate of 75 beats per minute, one cycle lasts about 0.8 seconds. It has three main stages.
- Atrial systole (about 0.1 s): Both atria contract and push the remaining blood into the ventricles through the open AV valves.
- Ventricular systole (about 0.3 s): The ventricles contract. Pressure rises, the AV valves close, the semilunar valves open and blood is ejected into the arteries.
- Diastole (about 0.4 s): The whole heart relaxes. The semilunar valves close, the AV valves open and the atria and ventricles fill with blood returning in the veins.
The rhythm is started by a group of specialized cells, the sinoatrial node or pacemaker, in the wall of the right atrium. It sends electrical impulses across the atria and then, after a short delay at the atrioventricular node, down the septum to the ventricles. The nervous system and hormones, such as adrenaline, change the rate according to the needs of the body. Artificial pacemakers can substitute for a failing node.
Worked example: cardiac output
Cardiac output is the volume of blood pumped by one ventricle per minute. It equals heart rate multiplied by stroke volume, the volume pumped per beat.
- At rest: heart rate = 70 beats per minute, stroke volume = 70 mL. Cardiac output = 70 × 70 = 4 900 mL per minute, or 4.9 liters per minute.
- During exercise: heart rate = 150 beats per minute, stroke volume = 110 mL. Cardiac output = 150 × 110 = 16 500 mL per minute, or 16.5 liters per minute.
- Increase: 16.5 ÷ 4.9 ≈ 3.4, so cardiac output rises by a factor of about 3.4.
At rest, a person’s heart pumps about 5 liters per minute, roughly the total blood volume of an adult, so on average all the blood passes through the heart about once a minute. Trained athletes often have a lower resting heart rate and a larger stroke volume. These values are typical and vary between people.
How was the heart understood?
The Greek physician Galen taught that blood was made in the liver and moved back and forth. In 1628 the English physician William Harvey published De Motu Cordis, which showed that the heart pumps blood in a circuit around the body. He did not see capillaries. Marcello Malpighi observed them with a microscope in 1661, completing the picture. The first successful human heart transplant was performed by Christiaan Barnard in South Africa in 1967.
Which mistakes are common?
Mistake 1: Reversing left and right
Diagrams show the heart as if looking at a person, so the left side of the heart appears on the right of the page. Check the labels carefully.
Mistake 2: Saying all arteries carry oxygen-rich blood
The pulmonary artery carries oxygen-poor blood, and the pulmonary veins carry oxygen-rich blood. Arteries are defined by the direction of flow away from the heart.
Mistake 3: Thinking that the heart muscle is nourished by blood in its chambers
The wall is thick, and it is fed by coronary arteries.
Mistake 4: Confusing systole and diastole
Systole is contraction, and diastole is relaxation. Blood pressure readings give the systolic pressure first and the diastolic pressure second.