Comparative studies of the venous resistance and capacitance functions and their sympathetic regulation in skeletal muscle

The relation between the capacitance response (regional blood volume mobilization) and the venous resistance response, i.e. the two main functions of the venous system, was investigated during graded sympathetic nerve excitation in cat gastrocnemius muscle under well defined experimental conditions. The neural capacitance response was also compared with the total regional vascular resistance response and its precapillary resistance component. A reliable distinction between precapillary and postcapillary resistance reactions was made possible by a whole‐organ technique permitting continuous recordings of hydrostatic capillary pressure. In the control state, in which a transcapillary fluid equilibrium prevailed, total regional vascular resistance response, precapillary resistance component and venous resistance response averaged 16.6, 14.5 and 2.1 PRU. respectively and total regional blood volume was calculated to comprise 1.0 ml 100 g tissue‐. Graded sympathetic activation, causing graded increments in total regional vascular resistance response, precapillary resistance component and venous resistance response by maximum values of 96, 88 and 8 PRU respectively, were associated with graded decrease in total regional blood volume, at maximum by an average value of 0.57 ml 100 g tissue‐1. The relations between regional blood volume mobilization on the one hand, and total regional vascular resistance response, precapillary resistance component and venous resistance response on the other, were all non‐linear, implying that the sympathetic volume decrease per unit resistance increase was greater for small than large resistance increases. The experimentally established relation between regional blood volume mobilization and total regional vascular resistance response agreed quite well with theoretical data calculated from the simple assumption that both capacitance and resistance responses are the consequences of changes of vascular radius, the capacitance response, though, only to the second, and the resistance response to the fourth, power of the radius change. The distribution of the total adrenergic capacitance response between the pre‐ and postcapillary compartments was further estimated by a theoretical analysis. The relative contribution of the sympathetic capacitance response and the sympathetic transcapillary fluid absorption response to functional circulatory filling was also defined, providing some aspects of short‐ and long‐term volume control and its importance for arterial blood pressure regulation.