Merck Manual

Please confirm that you are not located inside the Russian Federation

honeypot link

Biology of the Heart


The Manual's Editorial Staff

Reviewed/Revised Mar 2022 | Modified Sep 2022
Get the full details
Topic Resources

What is the heart?

Every minute, your heart beats about 70 times and pumps about 1 gallon (4 liters) of blood. Your heart beats faster and pumps harder during exercise, when your body needs more oxygen. When you check your pulse, you're measuring your heart rate, or the number of beats per minute.

Blood traveling through your body delivers oxygen and nutrients to tissues and organs. Waste from those tissues and organs is carried by blood to the lungs and kidneys for removal from the body.

Overview of the Heart and Blood Vessels

How does the heart work?

The heart is a pump for blood. It's actually 2 pumps connected together—one on the right side of the heart and one on the left side.

  • The pump on the right side gets blood from your body and pumps it through your lungs where it picks up oxygen

  • The pump on the left side gets oxygen-filled blood from your lungs and pumps it throughout your body

In order to pump blood, your heart has:

  • Four hollow spaces (chambers) for the blood to flow through

  • Four heart valves to make sure blood flows in the right direction

  • An electrical system to tell the heart muscle when to contract

  • Blood vessels to feed the heart muscle itself

A Look Into the Heart

A Look Into the Heart

This cross-sectional view of the heart shows the direction of normal blood flow.

What are the heart chambers?

The heart has 4 compartments (chambers), two on the right and two on the left. The chambers of your heart relax, fill with blood, and then contract to pump the blood out.

  • The two upper chambers (the right atrium and left atrium) let blood into the heart

  • The two lower chambers (the right and left ventricles) pump blood out

What are heart valves for?

Your heart has 4 valves that control the flow of blood. The valves open to let blood out of one chamber and into the next chamber or blood vessel. The valves close to keep blood from flowing backward into the wrong chamber.

When you put your head on someone's chest and listen to their heartbeat, you're hearing the sound of the heart valves opening and closing.

What is the heart's electrical system?

Your heart should always have a regular, rhythmic beat, like the ticking of a clock:

  • Your heart rhythm is controlled by pacemaker cells in your heart

  • Pacemaker cells send out regular electrical signals to your heart muscle to make it contract

  • The signals are carried through tissue called the conduction system

There are special pacemaker cells in a part of your heart called the SA node. The pacemaker cells have their own natural rhythm of 60 to 100 signals per minute. Nerves from your brain can send messages to the cells telling them to speed up or slow down.

Your heart's conduction system has tiny strips of tissue sort of like electrical wires. The conduction system carries the pacemaker signals to the rest of your heart. The conduction system includes a gateway called the AV node. The AV node controls how signals pass from the upper chambers of your heart (atria) to the lower chambers (ventricles). When the conduction system is working properly, the signals get to each of your heart muscle cells at just the right time. Your heart then gives a good, strong beat that pumps blood properly.

Why does the heart need blood vessels?

Like all muscles, the heart needs a steady supply of blood to work. You might think that, because the heart is full of blood, it doesn't need a separate blood supply. However, blood that pumps through the heart doesn't feed the heart muscle. Instead the heart muscle is fed by its own blood vessels.

  • The heart's blood vessels are called coronary arteries

Coronary is a word for heart.

What can go wrong with the heart?

You can have problems with any part of your heart:

quiz link

Test your knowledge

Take a Quiz!