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Circulatory system

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This is an article about circulation in animals. For transport in plants, see Vascular tissue. For the band, see Circulatory System.
File:3DScience cardiovascular system.jpg
Diagram of the human circulatory system. Arteries are shown red, veins are shown blue.

The circulatory system (scientifically known as the 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). There are three types of circulatory systems (from simplest to most complex): no circulatory system, open circulatory system, and closed circulatory system.

Open circulatory system

An open circulatory system is an arrangement of internal transport present in some invertebrates like simple molluscs and arthropods in which circulatory fluid in a cavity called the hemocoel (also spelled haemocoel) bathes the organs directly and there is no distinction between blood and interstitial fluid; this combined fluid is called hemolymph / haemolymph. Muscular movements by the animal during locomotion can facilitate hemolymph movement, but diverting flow from one area to another is limited. When the heart relaxes, blood is drawn back toward the heart through open-ended pores.

Hemolymph fills all of the interior hemocoel of the body and surrounds all cells. Hemolymph is composed of water, inorganic salts (mostly Na+, Cl-, K+, Mg2+, and Ca2+), and organic compounds (mostly carbohydrates, proteins, and lipids). The primary oxygen transporter molecule is hemocyanin.

There are free-floating cells then, the hemocytes, within the hemolymph. They play a role in the arthropod immune system.

Closed circulatory system

The main components of the circulatory system are the heart, the blood, and the blood vessels.

The circulatory systems of all vertebrates, as well as of annelids (for example, earthworms) and cephalopods (squid and octopus) are closed, meaning that the blood never leaves the system of blood vessels consisting of arteries, capillaries and veins.

Arteries bring oxygenated blood to the tissues (except pulmonary arteries), and veins bring deoxygenated blood back to the heart (except pulmonary veins). Blood passes from arteries to veins through capillaries, which are the thinnest and most numerous of the blood vessels and these capillaries helps to join tissue with arterioles for transportation of nutrition to the cells.

The systems of fish, amphibians, reptiles, and birds show various stages of the evolution of the circulatory system.

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 most reptiles, a double circulatory system is used, but 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.

Mammalian circulation

Having circulated through the body, all the relatively de-oxygenated blood collects in the venous system which coalesces into two major veins: the superior vena cava (roughly speaking from areas above the heart) and the inferior vena cava (roughly speaking from areas below the heart). These two great vessels empty into the right atrium of the heart. The coronary sinus empties the heart's veins themselves into the right atrium. The right atrium is the larger of the two atria, although both receive the same amount of blood. The blood is then pumped through the tricuspid valve, or right atrioventricular valve, into the right ventricle. From the right ventricle, blood is pumped through the pulmonary semi-lunar valve into the pulmonary artery. This blood enters the two pulmonary arteries (one for each lung) and travels through the lungs, where it is oxygenated and then flows into the pulmonary veins. This oxygenated blood then enters the left atrium, which pumps it through the bicuspid valve, also called the mitral or left atrioventricular valve, into the left ventricle. The left ventricle is thicker and more muscular than the right ventricle because it pumps blood at a higher pressure.

From the left ventricle, blood is pumped through the aortic semi-lunar valve into the aorta, a massive and thick-walled artery. The aorta arches and gives off major arteries to the upper body before piercing the diaphragm in order to supply the lower parts of the body with its various branches. Once the blood enters the peripheral tissues oxygen and nutrients are extracted from it and carbon dioxide and wastes added, and it will again be collected in the veins and the process will be repeated. Peripheral tissues do not fully deoxygenate the blood, so venous blood does have oxygen, but in a lower concentration than in arterial blood.

The release of oxygen from red blood cells or erythrocytes is regulated. It increases with an increase of carbon dioxide in tissues, an increase in temperature, or a decrease in pH. Such characteristics are exhibited by tissues undergoing high metabolism, as they require increased levels of oxygen. This is for mammals only.

Flow of blood in the human heart

First the blood enters through the right atrium of the heart. It flows into the right ventricle and into the pulmonary arteries. The oxygen-poor blood flows through to the lungs. The pulmonary veins take the oxygen-rich blood to the left atrium and into the left ventricle. From the left ventricle the oxygen-rich blood flows into the aorta. The aorta pumps the blood to the rest of the body.

No circulatory system

Circulatory systems are absent in some animals. An example is flatworms (phylum Platyhelminthes). Their body cavity has no lining or fluid. They have a muscular pharynx leading to a digestive system. Digested materials can be diffused to all the cells of the flat worm due to an extensively branched digestive system and being flattened dorso-ventrally. Oxygen can diffuse from water into the cells of the flatworm. Consequently every cell is able to obtain nutrients, water and oxygen without the need of a transport system.

Measurement techniques

Health and disease

Main article: Cardiovascular disease
Main article: Congenital heart defect

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 the pulse was 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.

The 2nd century AD Greek physician, Galen knew that blood vessels carried 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 and there was no return of blood 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.

In 1242 the Arab scholar Ibn Nafis became the first person to accurately describe the process of blood circulation in the human body. He said, "The nourishment of the heart is from the blood that goes through the vessels that permeate the body of the heart." Contemporary drawings of this process have survived. In 1552, Michael Servetus described the same, and Realdo Colombo proved the concept, but it remained largely unknown in Europe.

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.

On March 28, 2007, Stephen Colbert discovered that the circulatory system actually serves to deliver prayers from the brain to the hands. From their, prayers shoot out the hands somewhat like lasers. However, this discovery is troubling to those concerned with the animal kingdom. For example, if bears were taught to pray, they would essentially have lasers for arms. To date, no government agency has admitted to training eagles to pray in order to combat other enemies within the animal kingdom.

See also

References

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