The height of the systemic vascular resistance is largely dependent on the total cross-sectional area of the arterioles in the peripheral vasculature. The smooth muscle tone of these vessels is influenced by neural, humoral, and local environmental factors. In addition, changes in the caliber of these vessels may result from structural alterations, including the accumulation of water and electrolytes in the walls. The tone of these vessels also may be altered by so-called autoregulatory adjustments which tend, through several possible mechanisms, to keep flow relatively constant despite changes in perfusion pressure. The systemic vascular resistance cannot be measured directly but rather is calculated as the ratio of mean arterial pressure to cardiac output. Resistance calculated in this way is a mean value which disregards the pulsatile nature of flow and also disregards the marked regional differences in resistance which may exist in the various parallel circuits that make up the total vascular resistance.