Volume of the capillary exchange vessels measured with a gravimetric method in rat hindquarters

Abstract

To measure the volume of the true exchange vessels, i.e., that of the “functional” capillaries, in rat skeletal muscle, two parallel-coupled hindquarter vascular beds were perfused at a constant flow with oxygenated plasma substitute. Continuous recordings were performed of changes in the total weight of the two preparations, caused by sudden arterial occlusion in one, thus doubling the flow to the other while venous drainage was left open in both. Venous outflow pressure was kept constant, usually at a level where the veins remained well distended. After a rapid and small change in total weight, reflecting the balance between vascular distension and recoil in the two circuits, the subsequent weight change showed a continuous increase, but after a distinct delay. From this delay, caused by transient absorption into the semistagnant capillary contents of the arterially occluded vascular bed, the absorbed fluid amount, Q, was calculated. Functional capillary volume, V c, was then calculated from the following equation (see text for derivation): V c= Q ln [ π c π c−P c+P v ] where π c, P c, and P v represent effective plasma colloid osmotic pressure, equivalent capillary pressure, and venous outflow pressure during isogravimetric conditions. This functional capillary volume, which thus includes permeable venular sections, amounted to 0.55–0.60% of the tissue weight, varying slightly with the prevailing P c, and corresponding to 10–15% of the total volume of the regional vascular bed. Capillary density in skeletal muscle was also deduced from these data and from known microvascular diameters, giving a value of 450–500/mm 2, which is in fair agreement with morphological data. The physiological significance of measuring the functional capillary volume by this method is discussed.