In cats anesthetized with pentobarbital, 188 direct microvascular pressure and diameter measurements were made of the tenuissimus muscle. The microvascular pressure in arterioles of 70 mum in diameter or larger was proportional to the systemic arterial pressure (PA). The arterioles with diameter-s ranging from 35 to 20 mum have been shown to be the principa-l source of arteriolar resistance regulating micropressures downstream. Across the capillary bed proper, the drop in pressures was about 15% of PA. Micropressures in the smallest venules (8-15 mum) averaged 24 mmHg and reflect closely capillary blood pressure. With a background of basic microvascular data, the vasodilatory mechanism of papaverine (P) and isoproterenol (IPR) in the skeletal muscle was analyzed. Administration of IPR decreases both arteriolar and venular pressure, while P infusion decreased the pressure in arterioles wider than 20 mum in diameter; however, in smaller arterioles there was a substantial elevation in micropressure. The data establish two basically different vasodilatory effects on the terminal vasculature: one with increased capillary pressure and fluid filtration (P), a second with a decrease in capillary pressure enhancing absorption (IPR).

The terminal lymphatics are a network of highly previous vessels that are distributed in a loose association with the blood capillary bed, in particular along with the collecting venules. The base-line pressure in these lymphatics is close to atmospheric, but after they converge to form valved collecting channels PL is increased with a pressure differential of 1-2 cmH20 built up across each valve. This increment in lymphatic pressure is clearly related to the presence of one-way valves, the contractile activity of the collecting channels, and the comparative impermeability of these channels. The pressure differential required to draw fluid from the interstitium into the lymphatics would appear to reside in the vasomotor activity of the collecting channels, although the data do not rule out changes in Pt coincident with net capillary filtration or absorption.

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