The heart is a pear shaped, muscular organ in vertebrates, responsible for pumping blood through the blood vessels by repeated, rhythmic contractions. The term cardiac (as in cardiology) means "related to the heart" and comes from the Greek καρδία, kardia, for "heart."

The heart is composed of cardiac muscle, an involuntary muscle tissue which is found only within this organ. In the human body, the heart is usually situated to the left of the middle of the thorax, underneath the breastbone. The heart is usually felt to be on the left side because the left heart (left ventricle) is stronger (it pumps to all body parts).

The left lung is smaller than the right lung because the heart occupies more of the left hemithorax. The heart is enclosed by a sac known as the pericardium and is surrounded by the lungs. The pericardium is a double membrane structure containing a serous fluid to reduce friction during heart contractions. The term serous fluid is used for various bodily fluids that are typically pale yellow and transparent, and of a benign nature. The mediastinum, a subdivision of the thoracic cavity, is the name of the heart cavity.

The heart consists of four chambers, the two upper atria (singular: atrium ) and the two lower ventricles. The function of the right side of the heart is to collect de-oxygenated blood, in the right atrium, from the body and pump it, via the right ventricle, into the lungs (pulmonary circulation) so that carbon dioxide can be dropped off and oxygen picked up (gas exchange). This happens through a passive process called diffusion. The left side collects oxygenated blood from the lungs into the left atrium. From the left atrium the blood moves to the left ventricle which pumps it out to the body. On both sides, the lower ventricles are thicker and stronger than the upper atria. The muscle wall surrounding the left ventricle is thicker than the wall surrounding the right ventricle due to the higher force needed to pump the blood through the systemic circulation.

Many ilnesses of heart exist. Heart disease is an umbrella term for a number of different diseases which affect the heart.

The most common heart diseases are:

Coronary heart disease, a disease of the heart itself caused by the accumulation of atheromatous plaques within the walls of the arteries that supply the myocardium.

Ischaemic heart disease, another disease of the heart itself, characterized by reduced blood supply to the organ.

Cor pulmonale, a failure of the right side of the heart.

Hereditary heart disease, heart disease caused by unavoidable genetic factors since birth.

Hypertensive heart disease, heart disease caused by high blood pressure, especially localised high blood pressure

Inflammatory heart disease, heart disease that involves inflammation of the heart muscle and/or the tissue surrounding it.

Valvular heart disease, heart disease that affects the valves of the heart.