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In vertebrates, the circulatory system is a system of organs that includes the heart, blood vessels, and blood which is circulated throughout the body. [1] [2] It includes the cardiovascular system, or vascular system, that consists of the heart and blood vessels (from Greek kardia meaning heart, and Latin vascula meaning vessels).
Vertebrate red blood cell types, measurements in micrometers Frog red blood cells magnified 1000 times Turtle red blood cells magnified 1000 times Chicken red blood cells magnified 1000 times Human red blood cells magnified 1000 times. Human blood is typical of that of mammals, although the precise details concerning cell numbers, size, protein ...
For this reason, the blood flow velocity is the fastest in the middle of the vessel and slowest at the vessel wall. In most cases, the mean velocity is used. [18] There are many ways to measure blood flow velocity, like videocapillary microscoping with frame-to-frame analysis, or laser Doppler anemometry. [19]
Real-time MRI of the human heart The human heart is in the middle of the thorax, with its apex pointing to the left. [15] The human heart is situated in the mediastinum, at the level of thoracic vertebrae T5–T8. A double-membraned sac called the pericardium surrounds the heart and attaches to the mediastinum. [16]
Clinical death is the medical term for cessation of blood circulation and breathing, the two criteria necessary to sustain the lives of human beings and of many other organisms. [1] It occurs when the heart stops beating in a regular rhythm, a condition called cardiac arrest .
When matured, in a healthy individual these cells live in blood circulation for about 100 to 120 days (and 80 to 90 days in a full term infant). [62] At the end of their lifespan, they are removed from circulation. In many chronic diseases, the lifespan of the red blood cells is reduced.
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Greek anatomist 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. [53]