The cardiovascular system, also known as the circulatory system, includes the heart, arteries, veins, capillaries and blood. The heart functions as the pump that moves blood through the body. The arterial circulation delivers blood from the heart to the body, and the venous circulation carries it back to the heart. Capillaries are tiny blood vessels at the interface of the arterial and venous circulation where exchange of substances between the blood and body tissues occurs. The cardiovascular system serves several major functions that are necessary for life.
Oxygen and Carbon Dioxide Transport
Supplying oxygen to the body is the most essential function of the cardiovascular system. Although all cells require oxygen, brain cells are the most sensitive and begin to die in as little as 3 minutes if deprived of oxygen. During inhalation, air enters the lungs and oxygen is absorbed through the air sacs into the bloodstream. This oxygen-rich blood is pumped through the heart into the arterial circulation. In the capillaries, oxygen diffuses out of the blood and into the cells of the body's organs and tissues. At the same time, carbon dioxide -- a waste product produced by cells -- is absorbed into the blood and transported to the lungs through the venous circulation. When this oxygen-poor blood reaches the lungs, carbon dioxide diffuses through the air sacs and is then exhaled. This cycle occurs with every breath.
Nutrient and Waste Product Transport
Delivering nutrients to the body is another critical function of the cardiovascular system. After food is digested in the stomach and intestines, its component nutrients are absorbed into the bloodstream. These nutrients include sugars, fats, vitamins, minerals and protein building blocks called amino acids. Each of these nutrients is vital to healthy body function. For example, the sugar glucose is the body's primary fuel to generate energy, and amino acids enable the body to manufacture new cells. Like oxygen, nutrients diffuse from the bloodstream into body cells via the capillaries. In addition to carbon dioxide, the circulatory system picks up metabolic waste products and toxins and transports them to the liver, kidneys and lungs for eventual elimination from the body.
Disease Protection and Healing
The circulatory system serves as the highway for disease-fighting cells and proteins, and messengers of the immune system. Immune system cells called white blood cells patrol the body in search of invading germs. If an infection occurs, these cells send chemical alarm signals that travel through the bloodstream, which subsequently transports infection-fighting cells to the site of the infection. The circulatory system also carries chemical messengers that attract cells to heal tissues that have been damaged due to injury or disease.
Hormones are chemical messengers produced by endocrine glands that affect distant organs. The cardiovascular system serves as the transportation connection between the endocrine glands and the organs or tissues they control via hormones. For example, hormones produced in the pituitary gland in the brain control other endocrine glands -- such as the thyroid, ovaries and testes -- as well as directing childhood growth. Similarly, the blood-sugar-lowering hormone insulin produced in the pancreas affects the uptake and use of blood sugar throughout the body. And thyroid hormones affect the metabolic rate of virtually every body organ and tissue, thanks to their body-wide delivery via the circulatory system.
Body Temperature Regulation
Body temperature regulation is an often overlooked but important function of the cardiovascular system. Optimal function of the human body occurs within a relatively narrow temperature range, which is tightly regulated. If body temperature begins to rise, blood vessels close to the body surface dilate, increasing in size. This allows the body to rid itself excess heat through the skin. Conversely, if body temperature drops, surface blood vessels constrict to conserve body heat. The cardiovascular system works in concert with the body's sweating mechanism as the primary regulators of body temperature.
Reviewed by: Tina M. St. John, M.D.