Figure 7.06. Veins. Schematic diagram of the regulation of the intrathoracic blood volume (right heart, pulmonary vascular bed and left heart) by the three components of the systemic venous system (splanchnic venous bed muscle and cutaneous veins). These three components adjust the intrathoracic volume and hence the filling pressure of the heart by active expulsion of blood due to contraction of the venous smooth muscle and by passive expulsion due to a decrease in venous distending pressure resulting from constriction of the precapillary resistance vessels. The latter will occur in the systemic venous bed with any stimulus that causes an increase in sympathetic outflow to the systemic resistance vessel or an increase in circulating catechalamines. In the skeletal muscles the compression of the veins by the contracting muscles is an additional but important means for their passive emptying. While the active expulsion of blood from the splanchnic venous bed is controlled by mechanorefelxes and chemoreflexes from the carotid sinus region, the aortic arch, the heart and lungs and the skeletal muscles, that from the cutaneous veins is governed mainly by local and central changes in temperature, by respiratory reflexes and by emotion. The veins of the skeletal muscles have little or no sympathetic innervation, but like the splanchnic venous bed and the cutaneous veins they can be constricted by circulating catecholamines.
Click to Return