How the Heart Pumps Blood: Chambers, Valves and the Cardiac Cycle

The heart is a muscular pump about the size of a closed fist. In an average adult it beats roughly 100,000 times a day, and each beat pushes blood into vessels that together stretch about 96,000 kilometres (roughly 60,000 miles). Nothing else in the body works this hard for this long without pausing.

This article follows one drop of blood around the heart, explaining the chambers it passes through, the valves that keep it moving in one direction, and the electrical timing that makes the whole organ squeeze in a coordinated way.

Four chambers and two circuits

The heart has four chambers. The two upper chambers are the atria, which receive blood. The two lower chambers are the ventricles, which pump blood out. The right side and the left side work as two separate pumps that sit side by side inside one organ.

The right side receives oxygen-poor blood from the body and sends it to the lungs. The left side receives oxygen-rich blood from the lungs and sends it to the rest of the body. Because blood flows through the heart twice on each complete trip around the body, mammals are said to have a double circulation: a pulmonary circuit (heart to lungs and back) and a systemic circuit (heart to body and back).

The table below summarises where each chamber gets its blood and where it sends it.

Chamber	Receives from	Pumps to	Wall thickness
Right atrium	Body (vena cavae)	Right ventricle	Thin
Right ventricle	Right atrium	Lungs (pulmonary artery)	Thin
Left atrium	Lungs (pulmonary veins)	Left ventricle	Thin
Left ventricle	Left atrium	Body (aorta)	Thickest

The wall thickness column is not an accident. Each chamber's wall is sized to the job it does, as explained later.

The one-way valves

If blood could flow backwards, the heart would waste most of its effort. Four valves prevent this. Each valve is a set of flaps that open when pressure is higher behind them and snap shut when pressure is higher in front of them.

The two atrioventricular valves sit between each atrium and its ventricle. On the right side this is the tricuspid valve, with three flaps. On the left side it is the mitral valve, with two flaps. The two semilunar valves guard the exits: the pulmonary valve at the lung artery and the aortic valve at the main artery to the body.

The closing of these valves produces the "lub-dub" sound a doctor hears through a stethoscope. The first sound comes mainly from the atrioventricular valves closing as the ventricles start to squeeze. The second comes from the semilunar valves closing as the ventricles finish pumping.

The cardiac cycle: filling and squeezing

One complete heartbeat is called the cardiac cycle. It has two main phases.

Diastole: the filling phase

During diastole the heart muscle relaxes. The atria and ventricles fill with blood returning from the body and lungs. The atrioventricular valves are open, so blood flows passively from the atria into the ventricles. Most of the ventricular filling happens before the atria even contract, simply because pressure in the veins pushes blood forward.

Near the end of diastole the atria contract briefly, topping up the ventricles. This final push adds only a modest share of the total volume, which is why the ventricles can still do most of their work without it at rest.

Systole: the pumping phase

During systole the ventricles contract. Pressure inside them rises quickly, the atrioventricular valves slam shut, and the semilunar valves are pushed open. Blood is ejected into the pulmonary artery on the right and the aorta on the left.

The left ventricle must generate enough pressure to push blood through the entire body, where the resistance is high. In a healthy resting adult the aortic pressure peaks at about 120 mmHg and falls to about 80 mmHg between beats, which is the familiar "120 over 80" reading. The right ventricle works against much lower pressure in the lungs, so its wall can be far thinner.

What sets the rhythm

The heart beats without any input from the brain, although the brain and hormones can speed it up or slow it down. The rhythm starts in a small cluster of specialised cells called the sinoatrial (SA) node, located in the wall of the right atrium. These cells generate electrical impulses on their own, which is why the SA node is called the natural pacemaker.

The impulse spreads across both atria, causing them to contract. It then reaches the atrioventricular (AV) node, which holds the signal for a fraction of a second. This short delay gives the atria time to empty into the ventricles before the ventricles squeeze. After the delay the impulse travels down the bundle of His and spreads through the Purkinje fibres, which trigger the ventricles to contract from the bottom upward. Squeezing from the bottom pushes blood toward the exits.

At rest a healthy adult heart typically beats somewhere between 60 and 100 times a minute. Exercise, fear and stress raise the rate through the sympathetic nervous system, while rest and sleep allow the parasympathetic system to slow it.

Pressure, volume and cardiac output

Cardiac output is the amount of blood the heart pumps per minute. It equals the stroke volume (the amount ejected per beat) multiplied by the heart rate. A typical resting adult ejects roughly 70 millilitres per beat at about 70 beats per minute, which gives a cardiac output of around 5 litres per minute.

During hard exercise the body can demand several times that amount. The heart meets the demand by beating faster, by ejecting a larger fraction of the blood it holds, and by filling more fully during diastole.

Why the left ventricle is thickest

The left ventricle has the thickest wall of any chamber because it must generate the highest pressure. The thicker muscle contains more contractile fibres, and each fibre adds force. This is the same engineering logic that makes a pressure vessel thicker than a bag: the wall must hold the pressure without bursting or ballooning.

When the left ventricle is persistently forced to pump against high resistance, as with uncontrolled high blood pressure, the wall can thicken further. This adaptation starts as a way to keep up with the load, but over time it can make the muscle stiffer and less efficient. Doctors monitor this thickening with ultrasound because it is an early sign of strain.

How the heart feeds itself

The heart muscle works constantly, so it needs its own blood supply. The coronary arteries branch from the base of the aorta just above the aortic valve. They wrap around the outside of the heart and deliver oxygen and nutrients to the muscle.

This detail explains why blockage in a coronary artery can cause a heart attack even when the chambers themselves are healthy. The muscle downstream of the blockage starves for oxygen and can die. The heart is pumping blood for the whole body but depends on a small set of vessels for its own supply.

Key takeaways

The heart is two pumps in one organ, arranged in series so that blood visits the lungs and then the body on every trip around the circuit.

Four valves keep flow one-directional. Their closing sounds are the heartbeat a doctor hears.

A cluster of natural pacemaker cells sets the rhythm, and a short delay at the AV node lets the atria empty before the ventricles contract.

The left ventricle is thickest because it works hardest. Cardiac output depends on how much blood each beat moves and how fast the heart beats.

Understanding these steps shows why the heart is both a pump and a patient organ: it moves blood for the whole body, and it needs its own supply to keep doing so.