Sir Frederick Smith Memorial Lecture. A superb transport system--the circulation

This paper discusses certain aspects of circulatory physiology, including foetal circulation and the role of the thorax, and gives parameters for total blood volume, cardiac output, stroke volume and the distribution of blood between the pulmonary and peripheral circulation within the horse. The reader is taken on a journey around the circulatory system, beginning in the left atrium, into which richly oxygenated blood runs from the pulmonary veins. From here the blood is squeezed via the mitral valve into the left ventricular pump, where it is forced under high pressure through the peripheral circulation, the arterioles and capillaries, to collect, deoxygenated and under much lower pressure, into the venules and veins. The pumping action of the muscles and the negative intrathoracic pressure help to draw the blood back up to the right atrium, itself exerting a suction effect on the great veins. The blood is then expelled from the right ventricle into the low resistance, high capacitance pulmonary circuit, where it picks up oxygen, releases carbon dioxide and returns to the left atrium.