Harvey Circulation Model

William Harvey discovered the circulation of the blood and published his treatise on the motion of the heart and blood in 1628. Harvey did not trace the connections between the arteries and the veins, nor did he follow blood around the circuit. Harvey never saw how arteries and veins connected, but his intuition told him they did. Malpighi finally observed capillaries in 1661.

Harvey was a systems modeler. He demonstrated that circulating blood had to be a necessary logical consequence of his observations. Harvey's reasoning is important as it demonstrates how model makers think. Harvey's first question was: Does the blood move the heart or does the heart move the blood? Because with each beat Harvey had seen the heart grow hard enough to resemble a contracting muscle as blood left it during the squeeze phase, called systole, and he had then seen that the heart then softened like a relaxed muscle after blood left it, Harvey reasoned that the heart had to be the prime mover in the system.

Harvey studied next the anatomy of the valves of the heart and showed that the valves would permit blood to flow in just one direction. In the living body then Harvey ligatured arteries and veins and saw blood accumulating on one side of the tie and draining away from the other side. Harvey measured the differences in volume between the dilated heart and the contracted heart and calculated that within an hour the human heart must pump some five hundred pounds of blood into the arteries, a volume that exceeded the weight of the whole body. Obviously, the only hypothesis that made sense of Harvey's observations was that the blood pumped into the arteries had to return to the heart and that this could only be happening through the veins. It, therefore, followed that arteries and veins had to connect somewhere, and that blood circulated through the body.

The major transport functions of mammalian blood are to move molecules of digested food from the gut to the rest of the body, to transport wastes from the rest of the body to the kidneys, to transport respiratory gasses between the lungs and the cells of the body, and to transport hormones from where they are made to where they have their actions. Blood, of course, also serves in hemostasis, combating infection, heat distribution and a host of other functions. As we shall see shortly, insect blood does not transport respiratory gases.

0 0

Post a comment