TL;DR:
The heart is two pumps side by side: the right side sends blood to the lungs, the left side sends it to the body.
Atria receive blood, ventricles push it out, and valves keep it moving one way.
Cardiac output is heart rate × stroke volume. For example,
72 × 70 mL = 5040 mL/min.
A common exam question: 'Trace a drop of blood from the vena cava and back to it.' The trick is that the drop passes through the heart twice on one lap. Once you see why, the path is easy to remember.
Two pumps in one organ
The heart has four chambers. The two upper chambers are atria (singular atrium), which receive blood. The two lower ones are ventricles, which push blood out. A wall called the septum keeps the left and right sides apart.
The right side handles blood that has been around the body and is low in oxygen. It sends that blood to the lungs. The left side receives oxygen-rich blood from the lungs and sends it to the body. These two loops are the pulmonary circuit (heart to lungs and back) and the systemic circuit (heart to body and back).
Why valves matter
Blood only moves forward because of valves, flaps that open under pressure from behind and snap shut if blood tries to flow backward. Each time a chamber squeezes, pressure rises and the valve ahead opens. When it relaxes, the valve closes.
The sounds of a heartbeat are largely the valves closing. There are four valves: between the right atrium and ventricle (tricuspid), between the right ventricle and the pulmonary artery (pulmonary valve), between the left atrium and ventricle (mitral, or bicuspid), and between the left ventricle and the aorta (aortic valve).
Worked example: the path of one drop of blood
Start where blood returns from the body, low in oxygen, and follow it step by step.
Blood enters the right atrium through the vena cava (the large veins).
It passes the tricuspid valve into the right ventricle.
The right ventricle contracts and pushes blood through the pulmonary valve into the pulmonary arteries, toward the lungs.
In the lungs the blood picks up oxygen and gives up carbon dioxide.
Oxygen-rich blood returns by the pulmonary veins to the left atrium.
It passes the mitral valve into the left ventricle.
The left ventricle contracts hard and pushes blood through the aortic valve into the aorta.
The aorta branches to the body. Tissues take up oxygen. Blood returns to the vena cava, and the lap starts again.
Count the chambers: right atrium, right ventricle, left atrium, left ventricle. That is four chambers and four valves on one lap, and the blood passes through the heart twice, once on each side.
Vessel | Carries | Oxygen level |
|---|---|---|
Pulmonary artery | Heart to lungs | Low |
Pulmonary vein | Lungs to heart | High |
Aorta | Heart to body | High |
Vena cava | Body to heart | Low |
One heartbeat, in two phases
A heartbeat has a relaxing phase and a squeezing phase. In diastole the chambers relax and fill. Blood flows from the atria into the ventricles, and the atria squeeze at the end to top them up. In systole the ventricles squeeze, the valves between atria and ventricles snap shut, and the valves leading out open.
Both ventricles contract at almost the same moment, so the lungs and the body each receive a pulse of blood together. Between beats, the outflow valves close and stop blood from falling back, and the cycle repeats.
Why the left ventricle is thicker
The left ventricle pumps blood around the whole body, which needs much more pressure than the short trip to the lungs. Its wall is therefore thicker and more muscular than the right ventricle's. If a diagram shows one wall clearly thicker, it is the left.
Putting a number on it: cardiac output
Cardiac output is the volume of blood the heart pumps per minute. It equals heart rate times stroke volume, the volume pushed out per beat. Say a resting heart beats 72 times a minute and ejects 70 mL each beat (a made-up but plausible set of numbers).
Cardiac output = 72 × 70 mL = 5040 mL/min, which is about 5.0 L/min. Check the units: beats per minute × mL per beat = mL per minute. If heart rate rises to 144 with the same stroke volume, output doubles to 144 × 70 = 10080 mL/min.
How to check your answer
Your path should cross the heart twice and the lungs once.
Valves and chambers should alternate: chamber, valve, chamber, valve.
Oxygen level should change in the lungs and in the body, not in the heart itself.
Left and right labels on diagrams are from the patient's point of view, so they look reversed on the page.
Common mistakes
Saying all veins carry deoxygenated blood. The pulmonary veins carry oxygen-rich blood.
Saying all arteries carry oxygen-rich blood. The pulmonary arteries carry oxygen-poor blood. The rule is about direction, not oxygen.
Skipping a valve. Name them. Examiners look for them.
Thinking the blood in the right side mixes with the left. The septum keeps them separate.
A quick self-test
Cover the list above and try this. Name the chamber that receives blood from the lungs. Name the vessel that carries blood out to the body. Say which valve lies between the right atrium and right ventricle. If you can answer all three without looking, you have the structure; if not, redraw the path once and repeat.
Where you will use it next
Heart structure leads straight to exercise, blood pressure and the control of heart rate by the nervous system. For the electrical side, see how neurons send signals. This article is general science, not medical advice about your own health.
Frequently asked questions
Why do atria have thin walls?
They only push blood a short distance into the ventricles, so they do not need to generate much pressure.
What does the heart's own blood supply do?
The heart muscle is fed by its own small arteries, the coronary arteries, branching off the aorta.
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