Circulatory Basis of Fluid Volume Control

Abstract

The status of the high-pressure or arterial system is primarily determined by the regulation of cardiac output and the tone of the resistance vessels. It is proposed that corresponding parameters for the low pressure system are the blood volume and the tone of the capacitance vessels, which together determine the "fullness of the blood stream". The fullness of the blood stream is an important determinant of cardiac performance. The concept is also advanced that volume regulation is an integral part of over-all cardiovascular regulation, which latter has hitherto been primarily considered with regard to adjustments of pressor or depressor activity affecting the heart and vascular wall tension. The control of the 2 systems seems to depend on the total sensory input from receptors in both low- and high-pressure regions. Evidence is presented which suggests that this input may be integrated into information describing the performance of the heart in relation to the load imposed on it. In the case of the low-pressure system the adjustment of the contained volume is of greater importance for homeostasis than the adjustment of capacity. The latter appears to have an emergency function. Volume regulatory activity has been judged from changes of mineral and water excretion by the kidney. It is concluded that the controls of water and mineral excretion are relatively independent, and that the system(s) responsible for the regulation of water excretion are more sensitive and react faster than do those concerned with minerals. The equally important parameters of regulation of plasma proteins and blood cell volume have not been considered.