blood circulatory system
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blood circulatory systemthe mechanism by which blood is moved around the body. All vertebrates and a few invertebrates such as ANNELIDS have ‘closed’ blood circulatory systems which involve muscular tubes (BLOOD VESSELS) connecting the heart to all areas of the body. Other invertebrates (notably the ARTHROPODS) have an ‘open’ system in which a pool of blood is circulated by action of the heart.
The various vertebrate classes display a fascinating range of circulatory structures which is thought to relate to their evolutionary history (see HAEKEL'S LAW). The circulatory system of fish is regarded as primitive because blood from the heart goes straight from the respiratory organs (gills) to the body tissues of the systemic circulation. Such a single circulation through two CAPILLARY BEDS ‘in series’ produces a relatively sluggish flow of blood around the system, which is inefficient. In amphibia, evolution has progressed so that the circulation to the lungs is ‘in parallel’ with the systemic system, and the heart is partially divided (two atria, one ventricle) to produce a double route. The two separate circuits enable a higher blood pressure to be maintained and thus a more efficient flow of blood. However, a true ‘double’ circulation system did not occur until the birds and mammals evolved from the reptiles. In the mammal the heart is divided into two halves, the right half pumping blood to the lungs in the pulmonary circulation, the left half pumping blood to the body tissues in the systemic circulation.
Although the circulatory system of the mammal is complex, it is possible to recognise several important features:
- in the systemic circulation, arteries carry oxygenated blood from the left ventricle of the heart, starting at the AORTA, while veins carry deoxygenated blood, collecting together to form the VENAE CAVAE which return blood to the right atrium of the heart.
- the pulmonary circulation is unusual in that the pulmonary artery carries deoxygenated blood from the right ventricle to the lungs, while the pulmonary vein carries oxygenated blood back to the left atrium of the heart.
- while all blood vessels are basically similar there are quite large differences between the various types. See ARTERY, VEIN and CAPILLARY.
- the blood supply from the gut does not feed back to the venae cavae directly, but via the liver in the HEPATIC PORTAL SYSTEM.