Q.The heart is made up of

The circulatory system's job is transportation: it carries oxygenated blood from the heart out to the body's cells, and deoxygenated blood back to the heart, and then on to the lungs and back again — all powered by the heart and carried through arteries and veins.

The HEART itself has four parts: two pumping chambers (the ventricles) and two receiving chambers (the atria), with the two sides of the heart kept separate by a thick muscular wall called the septum. BLOOD VESSELS come in three kinds: veins (which bring deoxygenated blood from the cells back to the heart), arteries (which carry oxygenated blood from the heart out to the cells), and capillaries (the very thin vessels where the actual exchange of nutrients and oxygen with the tissues happens). BLOOD itself is the fluid doing the carrying — made of red blood cells, white blood cells, plasma and platelets, and coloured red by the pigment haemoglobin.

The system runs as three circuits. Pulmonary circulation sends deoxygenated blood from the heart to the lungs (via the pulmonary artery — the one artery in the body that carries deoxygenated blood) to pick up oxygen, returning oxygenated blood to the heart via the pulmonary vein (the one vein that carries oxygenated blood). Systemic circulation then pumps that oxygenated blood out to the entire body through the aorta — the body's biggest artery — reaching every tissue via a branching network of capillaries, before deoxygenated blood is collected back through the veins. Coronary circulation is the heart supplying itself: coronary arteries bring oxygenated blood to the heart muscle itself, and coronary veins carry the deoxygenated blood back.

The heart's own pumping action alternates between systole (contraction, when the ventricles push blood out) and diastole (relaxation, when the atria fill ready for the next beat). The amount of blood pumped out of a ventricle in a single contraction is the stroke volume; the amount pumped out in a full minute is the cardiac output — which is why, when you run, your heart rate (and so your cardiac output) rises to meet your body's higher oxygen demand.