In adults the heart is about the size and shape of a clenched fist. One end of the heart is pointed (apex); the other is broad (base) and is deeply cleft at the division between the two atria. One groove running down the front and back shows the division between the two ventricles; a circular, deeper groove marks off the atria above from the ventricles below. The capacity of each cavity is somewhere between 90 and 180 millilitres.

The heart lies within a strong fibrous bag, known as the pericardium. Since the inner surface of this bag and the outer surface of the heart are both covered with a smooth, glistening membrane faced with flat cells and lubricated by a little serous fluid (around 20 ml), the movements of the heart are accomplished almost without friction. The main thickness of the heart wall consists of bundles of muscle fibres, some of which run in circles around the heart, and others in loops, first round one cavity, and then round the corresponding cavity of the other side. Within all the cavities is a smooth lining membrane, continuous with that lining the vessels which open into the heart. This investing smooth membrane is known as epicardium; the muscular substance as myocardium; and the smooth lining membrane as endocardium.

Important nerves regulate the heart's action, especially via the vagus nerve and the sympathetic system (see NERVOUS SYSTEM). In the near part of the atria lies a collection of nerve cells and connecting fibres, known as the sinoatrial node or pacemaker, which forms the starting-point for the impulses that initiate the beats of the heart. In the groove between the ventricles and the atria lies another collection of similar nerve tissue, known as the atrioventricular node. Running down from there into the septum between the two ventricles is a band of special muscle fibres, known as the atrioventricular bundle, or the bundle of His. This splits up into a right and a left branch, one for each of the two ventricles, and the fibres of these distribute themselves throughout the muscular wall of the ventricles and control their contraction.

There is no direct communication between the cavities on the right side and those on the left; but the right atrium opens into the right ventricle by a large circular opening, and similarly the left atrium into the left ventricle. These are protected by valves to prevent backflow. Into the right atrium open two large veins, the superior and inferior venae cavae, with some smaller veins from the wall of the heart itself, and into the left atrium open two pulmonary veins, one from each lung. One opening leads out of each ventricle – to the aorta from the left ventricle, and to the pulmonary artery from the right.

Before birth, the heart of the FETUS has an opening (foramen ovale) from the right into the left atrium, through which the blood passes; but when the child first draws air into his or her lungs this opening closes and is represented in the adult only by a depression (fossa ovalis).

The heart contains four valves. The mitral valve consists of two triangular cusps; the tricuspid valve of three smaller cusps. The aortic and pulmonary valves each consist of three semilunar-shaped segments. Two valves are placed at the openings leading from atrium into ventricle, the tricuspid valve on the right side, the mitral valve on the left, so as completely to prevent blood from running back into the atrium when the ventricle contracts. Two more, the pulmonary valve and the aortic valve, are at the entrance to these arteries, and prevent regurgitation into the ventricles of blood which has been driven from them into the arteries. The noises made by these valves in closing constitute the greater part of what are known as the heart sounds, and can be heard by anyone who applies his or her ear to the front of a person's chest. Murmurs heard accompanying these sounds indicate defects in the valves, and may be a sign of heart disease (although many murmurs, especially in children, are ‘innocent’).

At each heartbeat the two atria contract and expel their contents into the ventricles, which at the same time are stimulated to contract together, so that the blood is driven into the arteries, to be returned again to the atria after having completed a circuit in about 15 seconds through the body or lungs. The heart beats from 60 to 90 times a minute, the rate in any given healthy person being about four times that of the respirations. The heart is to some extent regulated by a nerve centre in the MEDULLA, closely connected with those centres which govern the lungs and stomach, and nerve fibres pass to it in the vagus nerve. The heart rate and force can be diminished by some of these fibres, by others increased, according to the needs of the various organs of the body. If this nerve centre is injured or poisoned – for example, by lack of oxygen – the heart stops beating in human beings; although in some of the lower animals (e.g. frogs, fishes and reptiles) the heart may, under favourable conditions, go on beating for hours even after its entire removal from the body.