Mechanisms in man for rapid refill of the circulatory system in hypovolaemia

Abstract

Compensatory, net fluid transfer across the capillaries was studied in the arm of man with plethysmographic technique during experimental hypovolaemia induced by lower body negative pressure (LBNP). Thirty, 60, and 110 cmH2O LBNP evoked rapid transfer of fluid from tissue to blood at average rates of 0.053, 0.088 and 0.147 ml min-1 100 ml-1 soft tissue, i.e. graded responses typical for a true homeostatic regulation. Other experiments demonstrated a net fluid absorption not only from the arm but also from a wide range of skeletal muscle and skin regions in the body during experimental hypovolaemia, i.e. the more or less generalized response required if the absorption process is to contribute importantly to plasma volume regulation. In a third series of experiments it was shown that gradually applied LBNP was a much less efficient stimulus for fluid gain into the circulation than rapidly instituted LBNP, tentatively explaining the fairly slow plasma volume refill in main in previous literature after experimental, true and necessarily slow blood loss. Taken together, the findings described warrant the conclusion that the described process of fluid gain into the circulation may be a very important component in the overall homeostatic circulatory regulation in states of hypovolaemia. The data in fact suggest that the process might be capable of increasing plasma volume by as much as 600 ml within only 10 min, suggesting that such plasma volume control might be much more potent than previously believed.

Lower capacitance response and capillary fluid absorption in women to defend central blood volume in response to acute hypovolemic circulatory stress

“…*Sex differences (P Ͻ 0.05). which it was initiated seems to be of importance for the cardiovascular response, since a more rapidly induced hypovolemic stimulus elicits a greater circulatory response (28). No sex differences in Cap 50 were found, however.…”

“…The combined effect of the capacitance response and transcapillary fluid absorption during the limited period of 8 min of hypovolemic circulatory stress, corresponding to LBNP of 44 mmHg, adds another 900 ml to the effective circulating blood volume. A well-maintained capacitance response and net capillary fluid absorption seem to constitute very important compensatory responses for the restitution of plasma volume during acute hemorrhage and hypovolemia (28). The inability for women compared with men to defend central blood volume with time might thus be due to the reduced capacitance response with time and decreased net capillary absorption, leading to a decrease in venous return to the heart and thus a more prominent reduction in stroke volume.…”

Cardiovascular regulation in women with vasovagal syncope : With special reference to the venous system

“…Furthermore, studies in healthy subjects without a history of VVS have suggested that not just the pooled volume, but also the rate by which the hypovolemic stimulus is instituted can affect the responses to orthostatic stress (27,77). Despite a similar capacitance response, Lindenberger and Länne (77) found a much slower lower limb blood pooling in otherwise healthy women experiencing vasovagal reactions during moderate levels of LBNP compared to hemodynamic stable women.…”

“…Fluid absorption from tissue to blood serves as a powerful mechanism to increase plasma volume during hypovolemic stress (27)(28)(29)(30) and an effective fluid absorption is dependent on both a high hydrodynamic conductivity (i.e., CFC), and a reflex decline in capillary pressure due to α-and β-adrenergic adjustment of pre-and postcapillary resistance (25,26). The present finding of a similar calf CFC between VVS and controls makes capillary fluid permeability characteristics unlikely as an explanation of the differences in net fluid absorption.…”

“…Active venoconstriction within the splanchnic circulation may also help to counteract the loss of central blood volume (11). Further, the slower but continuous net capillary absorption of extra-vascular fluid is also dependent on a reflex decline in capillary pressure due to adjustment of pre-and postcapillary resistance (25,26) and act as a powerful mechanism to increase plasma volume (27)(28)(29)(30). Thus, the combined effect of mobilizing venous capacitance blood and net capillary absorption serve as important factors for preserving venous return and act momentarily to maintain cardiovascular homeostasis (24,28,29,31).…”