From the discovery of the circulation of the blood to the first steps in hemorheology: part 1.

In this article (the first of two on the subject) a brief historical review is presented of the prevailing ideas on the nature of the blood and its circulation from antiquity to the 16th century, when the problem was solved by William Harvey. On the basis of vivisection of various types of animals, Harvey constructed a general and logical model for the whole systemic circulation, which contradicted previous concepts, mainly those that had been put forward by Galen fourteen centuries before. The influence that Galen still exercised on virtually all areas of medicine justified Harvey's hesitations and scruples, forcing him to delay publishing his conclusions for thirteen years. It also explains the controversy with fellow physicians on the subject, which continued until his death. However, through careful observation and painstaking investigation, Harvey demonstrated clearly that the heart was the central organ of the circulatory system, on which depended the propulsion of the blood to the arteries and its subsequent return by different vessels, the veins, to its starting point. The blood coming from the heart was different from that which returned to the organ, the difference (in color and fluidity) being attributed to the presence of constituents which nourished the organism it irrigated. Harvey characterized blood pulsation as the result of the arteries filling with arterial blood during each heart contraction. He demonstrated that the arterial blood left the heart by contraction of the left ventricle, which happened simultaneously with contraction of the right ventricle and, in both, after the contraction of the atria. He confirmed that blood passed through the lung circulation from the right ventricle to the left atrium and from there to the left ventricle. By calculating the volume of blood pumped daily by the heart, Harvey reasoned that the blood could not be consumed by the body and would have to circulate continually through the heart and vascular network. Although Harvey did not confirm the continuity of the circulatory network, he went so far as to hypothesize the existence of minuscule imperceptible passages between arteries and veins, which was later confirmed by Marcello Malpighi, in the form of networks of capillaries. The one-way direction of blood flow was ensured by valves in the heart and veins. The model established by Harvey for blood circulation in animals and extrapolated to humans was confirmed in the following centuries. Malpighi and van Leeuwenhoek, in particular, helped clarify the composition and characteristics of blood and their importance for its perfusion of the different vessels of the circulatory network.