Directory of health resources for healthcare professionals.

No one did more to advance the cause of scientific medicine than the English doctor William Harvey (1578-1657). He made history by showing that blood is constantly on the move through the body, so breaking with the accepted wisdom handed down by Galen, who believed that the heart acted as a low-temperature oven to keep the blood warm and that the movement of blood was not like water through a pipe, but was comparable to the ebb and flow of a tidal seepage.Harvey’s key contribution was his treatise on the movement of the heart and blood in animals, Exercitatio Anatomica de Motu Cordis et Sanguinis in Animalibus. In it he demonstrated with admirable scientific clarity that the heat beats by muscular contraction, squeezing the blood out of its interior into the arteries through the one-way valves, and returning it to the heart in the veins. He was able to show that what came back to the heart was indeed the same blood as that which had left it, not a newly manufactured quantity, as others had thought. In short, he concluded that blood was recycled.Although Harvey accurately described the phenomena of circulation and blood pressure, he did not have a microscope (the instrument had yet to be invented), and so could not identify the exact means by which blood pressure was maintained –the fine capillaries.The four to six litres of blood in our bodies accounts for one-fourteenth of our total body weight. Blood consists mostly of fluid. This fluid portion is plasma, a pale brown, sticky liquid containing, among other things, proteins, salts, cholesterol, glucose, lipids and hormones. All these need to be transported from one part of the body to another, by the blood.The plasma also carries the various blood cells. Red cells contain haemoglobin, which is a molecule capable of picking   up,   carrying   and   delivering oxygen. Haemoglobin is very similar in its molecular structure to chlorophyll in plants, which suggest an evolutionary link between the two. White cells, or leukocytes, are larger than red cells and are responsible for fighting infection. There are also special cells for causing the blood to clot: the platelets, or thrombocytes. In any cubic millimetre of blood there are approximately five million red cells, 10 000 white cells and around 250 000 platelets.*6/353/5*