System of vessels in an animal's body that transports essential substances (blood or other circulatory fluid) to and from the different parts of the body. It was first discovered and described by English physician William Harvey. All animals except for the most simple – such as sponges, jellyfish, sea anemones, and corals – have some type of circulatory system. Some invertebrates (animals without a backbone), such as insects, spiders, and most shellfish, have an ‘open’ circulatory system which consists of a simple network of tubes and hollow spaces. Other invertebrates have pump-like structures that send blood through a system of blood vessels. All vertebrates (animals with a backbone), including humans, have a ‘closed’ circulatory system which principally consists of a pumping organ – the heart – and a network of blood vessels.
Fish have a single circulatory system in which blood passes once around the body before returning to a two-chambered heart. In birds and mammals, there is a double circulatory system – the lung or pulmonary circuit and the body or systemic circuit. Blood is first pumped from the heart to the lungs and back to the heart, before being pumped to the remainder of the body and back. The heart is therefore a double pump and is divided into two halves. In all vertebrates, blood flows in one direction. Valves in the heart, large arteries, and veins prevent backflow, and the muscular walls of the arteries assist in pushing the blood around the body. A network of tiny capillaries carries the blood from arteries to veins. It is through the walls of capillaries that materials are transported to and from the blood.
Although most animals have a heart or hearts to pump the blood, in some small invertebrates normal body movements circulate the fluid. In the open system, found in snails and other molluscs, the blood (more correctly called haemolymph) passes from the arteries into a body cavity (haemocoel), and from here is gradually returned by other blood vessels to the heart, via the gills. Insects and other arthropods have an open system with a heart. In the closed system of earthworms, blood flows directly from the main artery to the main vein, via smaller lateral vessels in each body segment.
The human circulatory system performs a number of functions: it supplies the cells of the body with the food and oxygen they need to survive (see nutrition); it carries carbon dioxide and other waste products away from the cells; it helps to regulate the temperature of the body; and protects the body from disease. In addition, the system transports hormones, which help to regulate the activities of various parts of the body.
The human circulatory system There are three main parts to the human circulatory system. These are the heart, the blood vessels, and the blood itself.
Heart The heart is situated in the centre of the chest between the lungs and lies just under the breastbone, above the diaphragm. It has four chambers: two upper chambers – the left and right atria (or auricles) – and two lower chambers – the left and right ventricles. The chambers on the left are separated from those on the right by the septum, and blood flow between the top and bottom chambers – the atria and ventricles – is controlled by valves. The left atrium receives oxygenated blood from the lungs through the pulmonary veins. The blood then flows into the left ventricle which pumps it out into the body through the aorta. As the blood circulates the body it loses oxygen, and the deoxygenated blood is eventually returned to the heart via the right atrium and ventricle to be pumped back to the lungs for re-oxygenation. The left side of the heart is a stronger pump than the right side, because the right side only pumps blood to the lungs and back, while the left side pumps blood around the whole body.
Blood vessels There are three major types of blood vessels: arteries that carry blood from the heart; veins that return blood to the heart; and capillaries – extremely tiny vessels connecting the arteries and the veins. When blood is pumped out of the heart into the arteries it is forced out at high pressure by contractions of the muscular ventricles. Arteries therefore require strong walls to withstand the pressure of the blood flowing through them. They have elastic tissue in their walls that can stretch and recoil with the force of the blood. Artery walls also contain muscle and this determines the amount of blood that can flow through them, and the blood pressure.
As the blood travels through the body, the arteries divide into smaller and smaller vessels. These are the capillaries, and since their function is to take food, oxygen, and other materials to body cells, and to remove waste products, their walls must be thin enough to allow the passage of substances through them. As the blood continues its passage around the body, capillaries merge again to form veins through which the blood is returned to the heart. By the time blood reaches the veins its pressure has been greatly reduced and its flow is much slower. The walls of veins therefore do not need to be as thick or as elastic as those of arteries.
Blood This consists chiefly of a liquid called plasma, which is made up mostly of water, but also contains proteins, glucose, amino acids, salts, hormones, and antibodies. Floating in the plasma are three kinds of solid particles: red blood cells, white blood cells, and platelets. The red blood cells are formed in bone marrow – mainly in the ribs, vertebrae, and limbs; they contain haemoglobin and carry oxygen and carbon dioxide throughout the body. The red blood cells, which have no nucleus, have a relatively short life of about four months. White blood cells are also made in the bone marrow, as well as in the lymph nodes. They do have a nucleus, often quite large, and they are able to move around and pass through the walls of capillaries into all parts of the body. Their main function is to fight infection and help protect the body from disease. This is done by the production of antibodies which counteract the effects of invading bacteria or viruses. Platelets are small fragments of cells with no nucleus. They too are produced in the bone marrow and their function is to release substances which enable blood to clot. Thus they help to prevent the loss of blood from damaged vessels.
Functions of the system The circulatory system plays an important role in many of the body's processes including respiration, nutrition, and the removal of wastes and poisons. In respiration it delivers oxygen to the body's cells and removes carbon dioxide from them. In nutrition, it carries digested food substances to the cells. Nutrients from food enter the bloodstream by passing through the walls of the small intestine into the capillaries. The blood then carries most of the nutrients to the liver, where some of these are extracted and stored for release back into the blood as and when the body needs them. Other nutrients are converted by the liver into substances which are required in the production of energy, enzymes, and new building materials for the body. Hormones, which affect or control the activities of various organs and tissues, are produced by the endocrine glands – including the thyroid, pituitary, adrenal, and sex glands – and they too are transported by the blood through the body.
Waste disposal In addition to feeding and nourishing the body, the circulatory system also helps to dispose of waste products and poisons which would prove harmful if allowed to accumulate. Carbon dioxide, produced by the body's cells as they respire, diffuses through the walls of the capillaries into the blood. The blood containing carbon dioxide is returned via the heart to the lungs and passed out of the body on expiration. In processing food, the liver removes ammonia and other wastes, together with various poisons that enter the body through the digestive system. These are converted into water-soluble substances, which are carried by the blood to the kidneys. The kidneys then filter out these wastes and expel them from the body in urine.
Temperature control As well as the heat produced generally by cells during respiration, some parts of the body, such as the liver and muscles, produce heat in the course of their activities. This heat is transported by the blood to warm other parts of the body. As the temperature of the body rises, the flow of blood into vessels in the skin increases as a result of small arteries expanding, and excess heat is conveyed to the surface where it is lost. When the temperature of the body drops the flow of blood to the skin is restricted. Thus, the circulatory system acts as a natural thermostat allowing the body to maintain an optimum and stable temperature.
Disease and disorders One of the most common diseases of the circulatory system is arteriosclerosis, a slow deterioration of arteries that results from the accumulation of fatty deposits in the arteries. If it affects the arteries supplying blood to the walls of the heart, it is called heart disease. The deposits thicken the walls of the arteries and reduce their elasticity, thus restricting the flow of blood. If a blood clot then develops in the affected vessels (thrombosis), this can further inhibit the circulation and lead to a heart attack or, if it affects the brain, a stroke – where the brain does not receive enough blood. Arteriosclerosis can also lead to increased blood pressure, or hypertension, as the heart is forced to work harder to force the blood through the arteries. This too can result in a heart attack or stroke, or in kidney failure.
There are other disorders which can result from damage or defects in the heart or blood vessels. For example, bacteria may harm or destroy the valves that control the flow of blood through the heart. Incomplete development of the heart or its blood vessels before birth may produce congenital heart disorders. Many cases of damage or defects can be corrected by surgery or alleviated with the use of drugs. Vasoconstrictors are agents which cause narrowing of the blood vessels, thus decreasing blood flow. These can be used to raise blood pressure in circulatory disorders, shock, or severe bleeding, or help to stabilize it during surgery. Vasodilators are drugs that cause widening of the blood vessels, thus increasing blood flow. These are used to lower blood pressure – for example, in hypertension.
Disorders of the blood itself, such as anaemia, can occur when the quantity of haemoglobin – the oxygen-carrying pigment in the blood – is insufficient and the blood cannot carry enough oxygen. This can lead to excessive fatigue, breathlessness, and reduced immunity to infections. Iron-deficiency anaemia results from lack of iron necessary for the production of haemoglobin. Conditions such as sickle-cell anaemia and thalassaemia are associated with abnormal forms of haemoglobin. Impaired production of red blood cells in the bone marrow can result in pernicious anaemia or leukaemia, while problems affecting the production of white blood cells can impair the body's immune system (see immunity.