Your heart is the size of your fist. It is inside the middle of your chest, slightly to the left.
The heart is a powerful pump made almost entirely of muscle.
The heart contracts (tightens) and relaxes automatically about 70 times a minute to pump blood out through your arteries.
The heart has two sides separated by a muscle wall called the septum.
The right side is smaller and weaker, and it pumps blood only to the lungs.
The stronger left side pumps blood around the body.
Each side of the heart has two chambers. There is an atrium (plural atria) at the top where blood accumulates (builds up) from the veins, and a ventricle below which contracts to pump blood out into the arteries.
Each side of the heart (left and right) ejects about 70 ml of blood every beat.
There are two valves in each side of the heart to make sure that blood flows only one way — a large one between the atrium and the ventricle, and a small one at the exit from the ventricle into the artery.
The coronary arteries supply the heart. If they become clogged, the heart muscle may be short of blood and stop working. This is what happens in a heart attack.
The heartbeat is the regular squeezing of the heart muscle to pump blood around the body.
Four heart valves make sure blood only moves one way.
The heartbeat is a sequence called the cardiac cycle and it has two phases — systole and diastole.
Systole is when the heart muscle contracts (tightens). Diastole is the resting phase between contractions, breathing and blood.
Systole begins when a wave of muscle contraction sweeps across the heart and squeezes blood from each of the atria into the two ventricles.
When the contraction reaches the ventricles, they squeeze blood out into the arteries.
In diastole, the heart muscle relaxes and the atria fill with blood again.
Heart muscle on its own would contract automatically.
Nerve signals make the heart beat faster or slower.