Circulatory system

This is an article about circulation in animals. For transport in plants, see Vascular tissue. For the band, see Circulatory System.

A circulatory system (cardiovascular system in biology classes) 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.

No circulatory system

An example of an animal with no circulatory system is the flatworm (phylum Platyhelminthes). Their body cavity has no lining or fluid. They have a mouth leading into a digestive system. The digestive system is very branched, and because the worm is so flat, digested materials can be diffused to all the cells of the flat worm. Oxygen can diffuse from water into the cells of the flatworm. Ergo every cell is able to obtain nutrients, water and oxygen without the need of a transport system.

Open circulatory system

An open circulatory system is an arrangement of internal transport present in some invertebrates like mollusks 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 (also spelled 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, 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 vessles.

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, 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

Deoxygenated blood (containing little or no oxygen) collects in two major veins: the superior vena cava and the inferior vena cava. The superior and inferior vena cava empty into the right atrium. The coronary sinus which brings blood back from the heart itself also empties into the right atrium. The right atrium is the larger of the two atriums because it needs to be able to hold the larger amount of blood coming from the body (as opposed to the amount coming from the lungs). The blood is then pumped through the tricuspid atrioventricular valve into the right ventricle. From the right ventricle, blood is pumped through the pulmonary semi-lunar valve into the pulmonary trunk. The deoxygenated blood leaves the heart by the pulmonary arteries and travels through the lungs (where it is oxygenated) and into the pulmonary veins. The oxygenated blood then enters the left atrium. The blood then travels through the bicuspid valve, also called mitral valve, into the left ventricle. The left ventricle is thicker and more muscular than the right ventricle because it pumps blood throughout the body, systemic circulation. From the left ventricle, blood is pumped through the semi-lunar valve into the aorta. Once the blood goes through systemic circulation, deoxygenated blood will again collect inside the vena cava and the process will continue.

Measurement techniques

Health and disease

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.

Ibn Nafis in 1242 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, Michael 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.

External links

Cardiovascular Topics
Studies on Hemodynamics and Coronary Circulation
The Circulatory System, a comprehensive overview
The InVision Guide to a Healthy Heart An interactive website

References

Iskandar, Albert Z. "Comprehensive Book on the Art of Medicine by Ibn al-Nafis". Retrieved May 2 2005.

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