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Circulation of the Blood

Blood is a life - giving source on which all cells depend . It is pumped around the body through a complex system of vessels by the heart muscle . But although blood is such an obvious commodity in the body , its function and means of passage were not fully understood until the end of the Renaissance .




By the 18th century scientists were faced with the challenge of reassessing blood production and blood flow in the body . They set out to disprove the established idea that large amounts of blood were constantly produced in the body in order to put forward the theory that we all possess a fixed quantity of blood , which is pumped around the body by the heart .


The pumping heart


Once scientists had ascertained that blood circulated around the body , their next challenge was to determine how this circulation took place . The ancient Greeks had thought that the heart worked by suction and that the pulse was due to arteries pumping blood . But it wasn't until the mid 16th century that Colombo suggested that the heart itself was the pump . In 17th - century England the physician William Harvey ( see left ) had a special interest in blood and was a profound believer in the importance of experiments in science . In 1628 he established once and for all that blood flow was entirely circulatory . He experimented on animals to quantify the daily volume of blood pumped by the heart . He realized that such a great amount could not be continually made , Renaissance scholars , such as French anatomist Jean Riolan the Younger , insisted on adhering to older Galenic ideas , by 1700 Harvey's doctrine was established science In the late 17th century Danish polymath and physician Nicolas Steno found that the heart wall was composed of muscle ; he was also the first to show that the heart consists of two relatively independent pumps . Around the same time , French anatomist Raymond Vieussens examined the left ventricle in detail , noting its extra thick muscle , necessary to pump blood all around the body . went as far as suggesting that blood carried oxygen from the lungs . However , it was many years before scientists could prove that it was hemoglobin ( a protein respiratory pigment ) in the red blood cells that carried oxygen from the lungs , where blood is oxygenated , to the body tissues .




Capillaries fill a gap


There remained one part of the story of circulation to complete : how blood passed from arteries to veins . Harvey predicted that blood passed through minute vessels too small to see and , in 1661 , Italian doctor Marcello Malpighi discovered these microscopic tubes , called capillaries , in a frog's lung . Capillaries permeate all body tissues , bringing food and oxygen to respiring cells and removing waste carbon dioxide .


How the heart beats


The cardiac cycle is the series of heart movements associated with a single heart beat . Contraction of heart muscle is called systole , relaxation is called diastole . Blood always flows from high to low pressure , with valves preventing backflow .

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