Structure and Functions of the Circulatory System and Heart

The circulatory system is responsible for transporting gases - carrying oxygenated blood from the heart to the body cells and deoxygenated blood back to the heart, and carrying deoxygenated blood from the heart to the lungs and oxygenated blood back - with the help of the arteries and veins.

What the circulatory system consists of

1. Heart. The heart is divided into four parts: two pumping chambers called the ventricles and two receiving chambers called the atria. The two sides of the heart are separated by a thick muscular wall, the septum.
2. Blood vessels. These include veins, arteries and capillaries. Veins bring deoxygenated blood from the cells back to the heart; arteries take oxygenated blood from the heart to the cells; capillaries are the thin blood vessels where the exchange of nutrients and oxygen takes place.
3. Blood. Blood is the fluid that carries the nutrients and oxygen transported around the body. It consists of red blood cells, white blood cells, plasma and platelets, and is red in colour because of a pigment called haemoglobin.

The circuits of circulation

1. Pulmonary circulation - between the heart and the lungs. The heart pumps deoxygenated blood to the lungs, where gases are exchanged (oxygen is picked up and carbon dioxide released), and the oxygenated blood returns to the heart. In the whole body there is only one artery that carries deoxygenated blood - the pulmonary artery - and only one vein that carries oxygenated blood - the pulmonary vein; both belong to this circuit.
2. Systemic circulation - between the heart and the rest of the body. The oxygenated blood is pumped out through the aorta, the body's biggest artery, which branches like a tree to supply every part. At each part, a network of capillaries connects the arteries and veins and allows the exchange of gases and nutrients; the deoxygenated blood then returns through smaller and then larger veins back to the heart. Both circuits run at the same time, and the heart adjusts the whole flow to the body's needs - which is why, when you run fast, your heart beats faster to meet the extra oxygen demand.
3. Coronary circulation - the heart works tirelessly and needs its own oxygen supply. The coronary arteries carry oxygenated blood to the heart muscle itself, and the coronary veins carry the deoxygenated blood back.

The heart

The heart is a cardiac muscle that makes an alternating movement of contraction and relaxation. When the heart contracts, the movement is called systole - the ventricles pump blood out of the heart; when it relaxes, the movement is called diastole - the chambers refill. The amount of blood pumped out of a ventricle in each contraction is the stroke volume, and the amount pumped out in one minute is the cardiac output.