Learning Center > Major Bodily Systems > Circulatory System

The circulatory system (or cardiovascular system) is an organ system that moves substances to and from cells; it can also help stabilize body temperature and pH (part of homeostasis).

The human circulatory system has three main parts: (1) the heart, (2) the blood vessels, and (3) the blood. A watery fluid called lymph, and the vessels that carry it, are sometimes considered a part of the circulatory system.

The heart, the lungs, and the blood vessels work together to form the circle part of the circulatory system. The pumping of the heart forces the blood on its journey. On average, the body has about 5 liters of blood continually traveling through it by way of the circulatory system.

Basic functions of the human circulatory system:

• Delivery of oxygen and nutrients to all parts of the body.
• Collection of metabolic wastes and delivery to the excretory organs, e.g. kidneys.
• Role in the immune system of defense against infection.
• Transport of hormones.

The body's circulatory system really has three distinct parts: pulmonary and systemic circulation, and portal circulation. The lungs (pulmonary), and the rest of the system (systemic) are the primary systems, complemented by the portal (or portal vein) circulation. Each system must be correctly working independently in order for them to all work together.

Pulmonary circulation
The right ventricle pumps deoxygenated blood into the pulmonary arteries. These arteries carry the blood to the lungs, where it passes through a capillary network close to air-filled alveoli. This enables the release of carbon dioxide and the uptake of oxygen from the air. The now oxygenated blood returns to the left atrium in the pulmonary veins.

Systemic circulation
Oxygenated blood from the lungs returns to the heart via the pulmonary veins, flows into the left atrium and then into the left ventricle, which then pumps the blood through the aorta, the major artery which supplies blood to the body. Smaller arteries branch off the aorta. Also called visceral circulation, the splanchnic circulation is the part of the systemic circulation that supplies the digestive organs. The major arteries of the splanchnic circulation branch directly off the aorta and include the celiac artery (celiac axis), superior mesenteric artery, and inferior mesenteric artery.

Portal circulation
There are two exceptions to the system of double circulation.
The deoxygenated blood from the capillaries of the gastrointestinal tract drains into the portal vein which, instead of going directly back to the heart, leads to the liver. This allows the liver to take up the nutrients that were extracted by the intestines from food. The liver also neutralizes some toxins taken up by the intestines. Blood from the liver drains via the hepatic veins into the inferior vena cava and then the right side of the heart. There is also a small portal flow from the hypothalamus to the anterior pituitary gland.

Functions of the circulatory system

The circulatory system performs many vital functions. It plays an important role in respiration. The circulatory system is also important in nutrition, in the removal of wastes and poisons, and in several other body processes.

In respiration. The circulatory system plays a part in respiration by delivering oxygen to the cells and removing carbon dioxide from them. During this process, the blood follows two routes called the systemic circulation and the pulmonary circulation.

From the left side of the heart, blood full of oxygen is pumped into the systemic circulation. This blood leaves the heart through the aorta, the main artery of the body. The blood leaving the aorta is full of oxygen. This is important for the cells in the brain and the body to do their work. A number of major arteries branch off the aorta. These arteries, in turn, branch into smaller and smaller vessels, finally emptying into the tiny capillaries. The oxygen rich blood travels throughout the body in its system of arteries into the smallest arterioles. There, oxygen leaves the blood and enters the tissues through the thin capillary walls. In a similar way, carbon dioxide leaves the tissues and enters the blood. The blood, now carrying carbon dioxide, leaves the capillaries and flows through larger and larger veins. Eventually, the blood enters the right side of the heart through two large veins--the superior vena cava, which carries blood from the head and arms, and the inferior vena cava, which carries blood from the trunk and legs.

From the right side of the heart, the blood is pumped into the pulmonary circulation. Pulmonary arteries carry the blood that contains carbon dioxide to capillaries in the lungs. As it reaches the lungs, the carbon dioxide (a waste product) is removed from the blood and replaced with fresh oxygen that we have inhaled through the lungs. The carbon dioxide passes through the capillary walls into the lungs and is then exhaled. Oxygen that has been inhaled passes from the lungs into the blood in a similar way. The blood returns through the pulmonary veins to the left side of the heart and begins its journey again.

In nutrition. The circulatory system carries digested food substances to the cells of the body. These nutrients 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.

The liver removes certain nutrients from the blood and stores them. It later releases the nutrients into the blood when the body needs them. The liver also changes some nutrients into substances needed by the body. Blood leaving the liver contains nutrients that the cells use in the production of energy, enzymes, and new building materials for the body.

In removal of wastes and poisons. The circulatory system helps dispose of waste products and poisons that would harm the body if they accumulated. These substances include carbon dioxide, salts, and ammonia, a by-product of the cell's use of protein.

The liver removes ammonia and other wastes, and various poisons that enter the body through the digestive system. The liver converts the wastes and poisons into water-soluble substances, which are carried by the blood to the kidneys. The kidneys filter out these and other water-soluble wastes and expel them from the body in urine.

Other functions. The circulatory system helps protect the body from disease. White blood cells engulf and destroy bacteria, viruses, and other harmful invaders. As the blood circulates, it also helps keep the body temperature stable by absorbing heat from the cells' production of energy.

If the temperature of the body begins to rise, the flow of blood into vessels in the skin increases. The heat from cells deep in the body is thus carried to the skin, from where it passes out of the body. If the temperature of the body begins to drop, the flow of blood to the skin is restricted. This action enables the body to retain as much heat as possible.

The circulatory system also carries hormones. These chemical substances affect or control the activities of various organs and tissues. Hormones are produced by the endocrine glands, including the thyroid, pituitary, adrenal, and sex glands. These glands release their hormones into the bloodstream.

Disorders of the circulatory system

The circulatory system can be damaged by disease or injury. One of the most common diseases of the system is arteriosclerosis, which results from the accumulation of fatty deposits in the arteries. Such deposits stiffen and thicken the walls of the arteries. As a result, the flow of blood is restricted. In some cases, blood clots develop in vessels affected by arteriosclerosis. Such clots may lead to a heart attack or a stroke, a condition in which the brain does not receive enough blood.

Another disease, hypertension, commonly called high blood pressure, is often associated with arteriosclerosis. Hypertension makes the heart work harder and may lead to such complications as a heart attack, a stroke, or kidney failure.

Treatment for arteriosclerosis and hypertension includes rest, exercise, and changes in the diet. Doctors may prescribe various drugs to lower the blood pressure, strengthen the heart, or prevent infection and blood clots. In extreme cases, a surgeon may remove clots or replace one or more diseased blood vessels.

Other disorders of the circulatory system result from damage or defects in the heart or blood vessels. For example, rheumatic fever 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 defects called congenital heart disorders. Some cases of damage or defects can be corrected by surgery.