Anatomy of the heart

A normal heart is a strong muscular pump. It weighs between 200 and 425 g (7 and 15 oz) and is a little larger than the size of your fist. During an average lifetime, the human heart will beat more than 2.5 billion times. The average heart beats about 100,000 times each day and pumps about 7,200 litres (1,900 gallons) of blood.

Your heart sits between your lungs in the middle of your chest, behind and slightly to the left of your breastbone. A double-layered membrane called the pericardium surrounds your heart like a sac. Blood loaded with oxygen comes from your lungs and enters your heart. To function, your heart needs a continuous supply of oxygen and nutrients, which it gets from the blood that is pumped through the coronary arteries.


Your heart and circulatory system make up your cardiovascular system. Your heart pumps blood to the organs, tissues, and cells of your body, delivering oxygen and nutrients to every cell and removing carbon dioxide and waste products made by those cells. Oxygen-rich blood is carried from your heart to the rest of your body through a complex network of arteries, arterioles and capillaries. Oxygen-poor blood is carried back to your heart through veins.

How it works

Your heart is a pump with four chambers. The upper chambers are called the left and right atria, and the lower ones are the left and right ventricles. A wall of muscle called the septum separates the left and right atria and the left and right ventricles.

The left ventricle is the largest and strongest chamber in your heart. It can push blood through the aortic valve and into your entire body.

The right two chambers of your heart (right atrium and right ventricle) pump blood from the heart to the lungs, so blood cells can pick up a fresh load of oxygen in exchange for the waste they've collected during their trip around the body. The oxygen-rich blood returns to the left chambers of the heart (left atrium and left ventricle), which then pump it around the rest of the body.

As the heart muscle relaxes, the two top chambers (the atria) fill with blood. Then, these chambers contract, squeezing blood down into the ventricles. The ventricles then contract, sending blood flowing out of the heart either to the lungs or through the body.

What happens during a heartbeat

In a normal heart, the electrical impulse that starts the heartbeat begins in a group of cells called the sinus node (or the SA node for short), in the right atrium. The SA node is often called the pacemaker of the heart. It works something like the spark plug in a car engine, producing the electrical signals that make the heart pump. The SA node generates a number of signals each minute in response to the body's needs. The resting heart rate is usually about 60 to 80 beats per minute.

After a burst of electricity is generated, it spreads out through the top half of the heart (the atria), almost like ripples spreading out from a stone dropped into a pond. This signal makes the upper chambers or atria contract. As they do, the blood inside them is squeezed out into the lower chambers of the heart – the ventricles.

Meanwhile the electrical signal that made the atria contract has reached the AV (atrio-ventricular) node, in the lower part of the right atrium. The AV node is the electrical connection between the atria and the ventricles. It holds the electrical signal for a moment, like a relay station, so the blood from the atria can be pumped into the ventricles. Then, it sends the signal to the lower chambers of the heart, making them contract. As the ventricles contract, they send blood pumping out with great force. The electrical signal has now passed through the upper and lower chambers of the heart, making them contract. This is one heartbeat. This electrical activity produces electrical waves that can be measured using a heart test called an electrocardiogram (ECG or EKG).

Last reviewed August 2009.