The systemic vasculature can be divided function-ally into arteries, arterioles, capillaries, and veins. Arteries are the high-pressure conduits that supply the various organs. Arterioles are the small vessels that directly feed and control blood flow through each capillary bed. Capillaries are thin-walled ves-sels that allow the exchange of nutrients between blood and tissues. Veins return blood from capillary beds to the heart.
The distribution of blood between the various components of the circulatory system is shown in Table 20–5. Note that most of the blood volumeis in the systemic circulation—specifically, within systemic veins. Changes in systemic venous tone allow these vessels to function as a reservoir for blood. Following significant blood or fluid losses, a sympathetically mediated increase in venous tone reduces the caliber of these vessels and shifts blood into other parts of the vascular system. Conversely, venodilation allows these vessels to accommodate increases in blood volume. Sympathetic control of venous tone is an important determinant of venous
return to the heart. Reduced venous tone following induction of anesthesia frequently results in venous pooling of blood and contributes to hypotension.
A multiplicity of factors influences blood flow in the vascular tree. These include mechanisms of local and metabolic control, endothelium-derived factors, the autonomic nervous system, and circulat-ing hormones.
Most tissue beds regulate their own blood flow (autoregulation). Arterioles generally dilate in response to reduced perfusion pressure or increased tissue demand. Conversely, arterioles constrict in response to increased pressure or reduced tissue demand. These phenomena are likely due to both an intrinsic response of vascular smooth muscle to stretch and the accumulation of vasodilatory met-abolic by-products. The latter may include K+, H+, CO2, adenosine, and lactate.
The vascular endothelium is metabolically active in elaborating or modifying substances that directly or indirectly play a major role in controlling blood pressure and flow. These include vasodilators (eg, nitric oxide, prostacyclin [PGI2]), vasoconstrictors (eg, endothelins, thromboxane A2), anticoagulants (eg, thrombomodulin, protein C), fibrinolytics (eg, tissue plasminogen activator), and factors that inhibit platelet aggregation (eg, nitric oxide and PGI2). Nitric oxide is synthesized from arginine by nitric oxide synthetase. This substance has a number of func-tions In the circulation, it is a potent vasodilator. It binds guanylate cyclase, increasing cGMP levels and producing vasodilation. Endothelially derived vaso-constrictors (endothelins) are released in response to thrombin and epinephrine.