CIRCULATORY SYSTEMThe circulatory system is centred on theHEART, a muscular organ that rhythmically pumpsBLOODaround a complex network ofBLOOD VESSELSextending to every part of the body. Blood carries the oxygen and nutrients needed to fuel the activities of the bodys tissues and organs, and it plays a vital role in removing the bodys waste products. An average-sized adult carries about 5 litres (9 pints) of blood.

PARTS OF THE SYSTEMAll the output of blood from the left side of the heart goes into the aorta, the bodys largest artery. Other arteries branch from the aorta to supply blood to the head, limbs, and internal organs. The blood is drained from all these parts by veins into two large vessels, the inferior and superior venae cava, which deliver the blood back to the right side of the heart.HEARTThe heart contracts tirelessly more than 2.5 billion times over an average lifetime to pump blood around the body. These contractions are triggered by electrical impulses that originate in a specialized area of heart tissue. The signals spread through the muscle in the wall of the heart via a network of conducting fibres.INSIDE THE HEARTThe heart has two upper chambers, called atria, and two lower chambers, called ventricles. Blood from the body arrives in the right atrium. This blood is low in oxygen, and is shown here in blue. The blood passes to the right ventricle, which pumps it to the lungs to pick up more oxygen. The left atrium receives oxygen-rich blood (red) back from the lungs. This passes to the left ventricle, which pumps it by way of the aorta to the body.

HEART VALVESAt the exit of each heart chamber lies a valve, which ensures the one-way flow of blood through the heart and into the circulation. These valves are made of flaps that open to allow blood to pass through but snap tightly shut to prevent backflow. The valves have three flaps, except for the valve between the left atrium and left ventricle, which has two.BLOOD VESSELSIf an adults blood vessels were laid end to end, they would stretch out over 100,000 km (62,500 miles). There are three main types of vessel. Arteries carry blood from the heart to the bodys tissues, while veins carry blood back from the tissues to the heart. Small arteries are called arterioles and small veins are referred to as venules. The third and smallest type of vessel, capillaries, form a network connecting the smallest arterioles with the smallest venules.

TYPES OF BLOOD VESSELArteries have relatively thick elastic walls that enable them to withstand the high pressure of blood pumped from the heart. By the time the blood has been forced through capillaries and arrived in veins, its pressure has dropped, so veins have thinner walls.

BLOODBlood is composed of a straw-coloured fluid, plasma, and huge numbers of blood cells that float in the plasma. Of the two main types of blood cell, red blood cells carry oxygen to the bodys tissues, and white blood cells help defend the body against infection. Blood also transports nutrients, proteins needed for blood clotting, and waste products.

BLOOD CLOTTINGIf a blood vessel is damaged, a clot forms to stop blood leaking. First, platelets stick together to form a plug that stops the leak. At the same time, a complex sequence of chemical events in the blood leads to the production of long strands of a protein called fibrin. These bind the blood cells and debris together to form a gel-like clot that gradually solidifies. The solid clot remains until the blood vessel has been repaired.BLOOD CELLSA drop of blood contains millions of red cells, and each cell contains 250 million molecules of a substance called haemoglobin. In the lungs, oxygen binds to haemoglobin, but in the tissues the oxygen is released again. Several types of white blood cell exist, and all are important to the bodys immune system. Platelets are tiny cells that are needed for blood clotting.