POST-OCCLUSION HYPEREMIA - A STUDY ON A MODEL

The hydrodynamic properties of the capillaron, i.e. a compliant capsule which encloses one or more soft‐walled permeable capillaries together with extravascular fluid, can account for the known characteristics of reactive hyperemia. This has been demonstrated in a model of the capillaron. A constant arterial pressure head delivered fluid to the enclosed capillary and thence to a venous sink. The flow rate varied with the perfusion pressure and with the calibre of the soft‐walled vessel. Vascular calibre depended on the transmural capillary pressure. A positive pressure opens the capillary completely and the flow rate for the perfusion pressure is maximal. Capillary permeability results in ‘ultrafiltration’ which raises the extravascular pressure, thereby lowering the transmural pressure, until the capillary is partially collapsed. In the ‘resting’ or basal state, the flow rate through the capillaron is only a small percentage of the maximal flow through the system. Occlusion of the arterial supply is followed by drainage of extravascular fluid across the permeable membrane and tissue pressure falls progressively with the duration of the occlusion. Reopening of the artery raises the intravascular pressure and the positive transmural pressure opens the capillary, with the result that flow approaches maximal values. The flow during the hyperemic phase is affected by the duration of occlusion, the arteriovenous perfusion pressure, the permeability of the capillary, and the compliance of the capsule. The effects of these changes on transcapillary fluid exchanges are described. Several variations of the hyperemic manoeuvre, including the effects of arterial occlusion without hyperemia, hyperemia without occlusion, hypoemia, the inter‐ relationships between blood and ultrafiltrate flows, and the potential exchanges of diffusible and of filterable materials are demonstrated and shown to correspond to the findings in man and animals. © 1969 The Physiological Society