In anatomy, the circulatory system or cardiovascular system is the system that circulates blood around the body of humans and most other animals.
Human circulatory system
The main function of the circulatory system in humans is to deliver oxygen and nutrients to all parts of the body and to remove wastes. It also plays an important role in the immune system defending against infections, and transports hormones.
The human circulatory system consists of the heart which acts as a pump, and the blood vessels in which the blood flows. We distinguish between arteries in which blood flows away from the heart, veins in which blood flows back to the heart, and capillaries which are very thin vessels where the actual oxygen and nutrient exchange takes place and which form the interface between arteries and veins.
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The overall structure of the system, known as "double circulation", is as follows. The heart consists of two separated pumps, the right and the left side (seen from the perspective of the heart's owner). The right side pumps blood into the lungs, where it passes through a capillary network and is brought close to air-filled alveoli. This enables the release of carbon dioxide and the acquisition of oxygen from the air. The blood then moves back to the left side of the heart. The left side pumps it into all parts of the body. The progressively thinner arteries end in capillaries, where the blood flows very slowly and nutrients and oxygen are exchanged with the surrounding tissues. The capillaries turn into veins which return the blood to the right side of the heart.
There is one exception to this general picture: the capillaries in the intestines lead to the portal vein which, instead of going directly back to the heart, leads to another network of capillaries inside the liver. This allows the liver to take up the nutrients that were extracted by the intestines from food. The blood then moves from the liver back to the right side of the heart.
Some of the blood is pumped through the kidneys, which remove some wastes and water, to be excreted as urine.
In the capillaries, some of the blood plasma seeps into the tissues, turning into interstitial fluid[?]. This fluid is returned to the bloodstream via the lymphatic system, which is a system of vessels separate from the circulatory system.
The circulatory system of the fetus is different, as the fetus doesn't use its lungs yet and obtains oxygen and nutrients from the placenta through the umbilical cord. At or right after the moment of birth, the system undergoes a dramatic change. See fetus for the details.
Circulatory systems of other animals
The circulatory system of arthropods and most mollusks is open, meaning that there are no capillaries and veins: one or more hearts pump the blood (more properly called hemolymph in this case) through the arteries to spaces called sinuses which surround the organs, allowing the tissues to exchange materials with the hemolymph. The hemolymph is drawn back into the heart as the heart relaxes.
The circulatory systems of all vertebrates as well as of earthworms, squids and octopuses are closed, meaning that the blood never leaves the system of blood vessels consisting of arteries, capillaries and veins.
The systems of fish, amphibians, reptiles, birds and mammals show various stages of evolution. In fish, the system has only one circuit, with the blood being pumped through the capillaries of the gills and on to the capillaries of the body tissues. This is known as "single circulation". The heart of fish is therefore only a single pump (consisting of two chambers). In amphibians and reptiles "double circulation" (as described above for humans) is used, however the heart isn't always completely separated into two pumps. Amphibians have a three-chambered heart. Birds and mammals show complete separation of the heart into two pumps, for a total of four heart chambers.
All circulatory systems frequently employ countercurrent exchange systems to drive the diffusion of chemicals into or out of the bloodstream.
History
William Harvey announced the discovery of the human circulatory system as described above in 1616 and published an influential book about it in 1628. Harvey did not identify the capillary system connecting arteries and veins; these were later described by Marcello Malpighi.
Harvey's model clashed with the accepted model going back to Galen, which identified venous (dark red) and arterial (brighter and thinner) blood, each with distinct and separate functions. Growth and energy were derived from venous blood created in the liver from chyle, while arterial blood gave vitality by containing pneuma (air) and originated in the heart. Blood flowed from both creating organs to all parts of the body where it was consumed, no blood returned to the heart or liver. The heart did not pump blood around, the heart's motion sucked blood in during diastole and the blood moved by the pulsation of the arteries themselves.
Galen believed that the arterial blood was created by venous blood passing from the left ventricle to the right by passing through 'pores' in the interventricular septum, air passed from the lungs via the pulmonary artery to the left side of the heart. As the arterial blood was created 'sooty' vapors were created and passed to the lungs also via the pulmonary artery to be exhaled.
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