Autoregulatory escape and post-escape hyperaemia in a capillaron model.

Abstract

Autoregulatory escape and post-escape hyperaemia were studied in a passive hydrodynamic model of the capillaron (ie, one or more soft-walled permeable capillaries enclosed together with extracapillary filtrate in a compliant capsule). Constriction was simulated in the model by reduction of the calibre of an orifice in the arteriole through which flow entered the ‘vascular bed’ of the capillaron. Constriction lowered the intracapillary pressure, and the capillary collapsed partially, with a resultant decrease in flow rate. The rate of filtration across the capillary wall into the extracapillary space of the capsule also declined. The falling extracapillary pressure increased the transmural pressure and reopened the capillary, returning flow to preconstriction values (autoregulatory escape). Reopening of the arterial constrition increased intravascular pressure, distending the capillary, with a transient increase in flow above the control value (post-escape hyperaemia). The time rate of constriction of the arterioles determined the magnitude of the fall in flow. The rate of removal of the arterial constriction determined the magnitude of the post-escape hyperaemia. These results show that the phenomena of autoregulatory escape and of post-escape hyperaemia resemble effects generated by passive changes in transmural capillary pressure. Such passive changes are affected by the rates of change of arteriolar constriction or relaxation. The effects of downstream capillary permeability and of capsular compliance on these phenomena were also examined.