The primary function of the arterial system is to distribute oxygenated blood from the heart to all the organs of the body. Nutrients (energy; fuel) are made available to living tissues primarily via the flowing blood. The arterial system is miraculously complex. It is a system of tough, regenerable, flexible, tapering pipes, created and forced by evolutionary pressures to perform this function optimally in some sense, constantly and under a daunting variety of conditions. The pipes taper from about 2 cm at the aorta to 0.0010 cm at the capillaries, which are of almost identical size and structure at the periphery of all organ systems. In a resting adult, the cardiac output of some tens of milliliters per beat is distributed differentially and according to need to all the various organs . The total blood volume of about 5 liters is pumped by the left ventricle through the high-pressure systemic circuit and by the right through the low-pressure pulmonary circuit, achieving a complete turnover once each minute. In exercise, the demands of skeletal muscle for blood may increase sevenfold; the arterial system dynamically adapts to the dramatically changing distribution requirements for fuel and nutrients.
Some have portrayed the major conducting arteries in the tree, and in each organ's subtree, as having a distribution function, moving large amounts of blood rapidly over relatively large distances, while the smaller vessels (<2mm diameter) through which blood moves over perhaps the final several centimeters to the capillaries have been called the delivery vessels . It may be that the mophometry, the diameters and branching characteristics, of distributing vessels is optimized in a different sense than is that of the delivery vessels. It will be seen at least qualitatively that the appearance of the branching patterns of the large, conduit vessels is quite different from species to species and from organ to organ, whereas the characteristics of the smaller (arteries and arterioles of less than about 1 mm diameter), precapillary subtrees toward the periphery of all organs are quite similar across species and tissue types.
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