Anatomy and Physiology

The heart is the ‘pump’ that forces blood around the rest of the body. The heart is in the chest behind the breastbone (sternum) and between the two lungs. Although it is in the middle of the chest it lies more towards the left than the right side of the chest.

The heart is a small, hollow muscular organ that is about the size of your clenched fist. It weighs between 250g and 350g and is usually larger in men than in women. Although small it is very strong, beating over 100,000 times a day to pump 7000 litres of blood through a 60,000 mile network of arteries, veins and capillaries that make up the cardiovascular system. (The term ‘cardio’ refers to the heart, and ‘vascular’ refers to blood vessels.) This network of vessels could circle the earth more than twice.

The heart is separated into four chambers with four valves. Blood is carried to the heart by veins (the superior vena cava and the inferior vena cava). The blood first arrives into the heart chamber called the right atrium. It then passes through the tricuspid valve into the right ventricle. From there it is pumped through the pulmonary valve into the pulmonary arteries and then to the lungs.

In the lungs, some waste is removed from the blood and oxygen is added from the air in the lungs. The blood then goes back into the left atrium of the heart, through the pulmonary veins. From here it passes through the mitral valve into the left ventricle. The left ventricle is the main pumping chamber of the heart. From here, blood is pumped through the aortic valve into the main artery called the aorta and off around the body.

Although the heart is continuously filled with blood that it pumps around the body, it does not use any of that blood. It needs its own separate blood supply to provide it with oxygen and nutrients, and to take away waste products and toxins.

This individual blood supply is called the coronary circulation. It is a network of arteries and veins in the heart muscle. These arteries are very smalll – the widest is rarely bigger than 3mm wide.

Just after the aortic valve at the start of the aorta there are two branches – one leading into the right coronary artery and one to the left coronary artery. These arteries wrap around the heart itself and feed the heart muscle with blood.

The valve at the exit of each chamber prevents the blood from flowing backwards, so making sure blood flows in one direction through the heart. The valves open and close as the ventricles contract (squeeze) with every heartbeat or heart rhythm. As each chamber of the heart contracts, it pushes blood into the ventricle from the atria, or from the ventricles out of the heart through an artery.

The valves are made up of thin flaps of tissue called cusps. The cusps are attached to the wall of the heart and the ends pokes out into the ventricle. These ends are attached to cords, known as chordae tendineae, which are connected to small muscles on the inside of the heart wall. These muscles are called papillary muscles.

Pressure changes behind and in front of the valves allow the cusps to open at just the right time, and then close them tightly to prevent a backflow of blood.

An animation showing the cross section of the heart and the blood flow through the heart chambers and valves.