Transcapillary passage of albumin,effects of tissue cooling and of increases in filtration and plasma colloid osmotic pressure

‘Initial’ clearance of radiolabeled serum albumin was measured in the perfused, maximally vasodilated muscle vascular bed of rat hindquarters during tissue cooling, during increases in filtration and during changes in serum colloid osmotic pressure. Albumin clearance during ordinary serum perfusion at iso-gravimetry amounted to 0.03 ml/min x 100 g, increasing linearly with filtration rate to some 0.07 ml/minx 100 g at 0.5 ml/minx 100 g of filtration. During cooling from 36°C to 14°C both CFC and initial albumin clearance at isogravimetry decreased some 40%, in due proportion to the increased viscosity of the fluid. Increases of the colloid osmotic pressure of the perfusate correspondingly increased both the isogravimetric capillary pressure and ‘initial’ albumin clearance during isogravimetry. – It is concluded that even during isogravimetry the transmicrovascular albumin passage is to about 70 per cent due to filtration, and only some 30 per cent of transport at ordinary serum colloid osmotic pressures takes place by diffusion, both events presumably via ‘large pores’. There was no evidence that transendothelial vesicular transport should to any significant extent contribute to the passage of albumin from vessels to tissue.