In zootomy, the circulatory system or cardiovascular system is the organ system which circulates blood around the body of most 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.

The double circulation system in humans. In the first circuit, the blood is pumped to the lungs, where it acquires oxygen. It then returns to the heart and enters the second circuit, going to the rest of the body, eventually returning to the heart. Blood circulates through the body two to three times every minute. In one day, the blood travels a total of 19,000 km (12,000 miles)--that's four times the distance across the US from coast to coast.

Double circulatory system

The "double circulation" system of blood flow in the human body is divided into the pulmonary circulation and the systemic circulation.

The adult human heart consists of two separated pumps, the right side with the right atrium and ventricle, and the left side with the left atrium and ventricle (seen from the perspective of the heart's owner).

Pulmonary circulation

The right ventricle pumps deoxygenated blood into the pulmonary arteries. These arteries bring 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 acquisition of oxygen from the air. The now oxygenated blood returns to the left atrium of the heart via the pulmonary veins.

Systemic circulation

From the left atrium the blood moves to the left ventricle, which pumps it into the aorta, distributing the blood to all parts of the body. The progressively thinner arteries end in capillary beds, where nutrients and oxygen are exchanged with the surrounding tissues. The capillaries turn into veins which return the deoxygenated blood to the right atrium of the heart through the superior and inferior vena cavae. From there the blood moves into the right ventricle and pulmonary circulation continues.

Portal circulation

There is one exception to this general picture: 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 which goes to the right side of the heart.

Lymphatic system

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, a system of vessels separate from the circulatory system and without a central pump.

Fetal circulation

The circulatory system of the fetus is different, as the fetus does not 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 is not 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; it is thought that the four-chambered heart of birds evolved independently of that of mammals.

All circulatory systems frequently employ countercurrent exchange systems to drive the diffusion of chemicals into or out of the bloodstream.

History of discovery

The valves of the heart were discovered by a physician of the Hippocratean school around the 4th century BC. However their function was not properly understood then. Because blood pools in the veins after death, arteries look empty. Ancient anatomists assumed they were filled with air and that they were for transport of air.

Herophilus distinguished veins from arteries but thought that pulse is a property of arteries themselves. Erasistratus observed that arteries that were cut during life bleed. He ascribed the fact to the phenomenon that air escaping from an artery is replaced with blood that entered by very small vessels between veins and arteries. Thus he apparently postulated capillaries but with reversed flow of blood.

Galen in the 2nd century AD knew that blood vessels carry blood and 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.

Ibn Nafis in 1268 was the first person to accurately describe the process of blood circulation in the human body. Contemporary drawings of this process have survived. In 1552 Servetus described the same and Realdo Colombo proved the concept. All these results were not widely accepted however.

Finally William Harvey, a pupil of Hieronymus Fabricius (who had earlier described the valves of the veins without recognizing their function), performed a sequence of experiments and announced in 1628 the discovery of the human circulatory system as his own and published an influential book about it. This work with its essentially correct exposition slowly convinced the medical world. Harvey was not able to identify the capillary system connecting arteries and veins; these were later described by Marcello Malpighi.

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