Single Perfused Microvessels
Eugene Landis first reported measures of Lp of individual microvessels in the early 20th century . By occluding capillaries of the frog mesentery, he described that on occasions erythrocytes moved toward the micro-occluder and on other occasions they moved away from the occlusion site. Cells moving toward the occluder were interpreted as representing fluid filtration, whereas those moving away denoted reabsorption. He calculated the rate of filtration or reabsorption per unit surface area (J/S) based on the erythrocyte velocity. By correlating JJS to capillary pressure (PC) measured with a micropipette, he determined a linear relation between PC and JJS. The slope of that relation is the Lp, and the x pressure axis intercept in Landis' experiments approximated the oncotic pressure of frog plasma. Those experiments provided evidence in vivo of Starling's hypothesis for fluid filtration. The Landis technique to assess Lp, modified by Michel and colleagues , is illustrated in Figure 2A. A microvessel is perfused with a micropipette connected to a water manometer, to measure microvascular hydrostatic pressure, containing marker erythrocytes in a physiological salt solution of predetermined composition. Downstream from the site of cannulation, a micro-occluder is used to stop flow intermittently in the vessel. Assuming cylindrical geometry, and negligible interstitial hydrostatic pressure (P), Lp is the slope of the measures of fluid filtration at the set of hydrostatic pressures (see Figure 2B). This technique allows for paired measures of Lp in individual microvessels under control conditions and following exposure to a variety of experimental conditions in the luminal, abluminal, or both surfaces of the microvessel. Further, given that the oncotic pressure is known, the intercept on the pressure axis can be used to determine s for the specific vessel and colloid (Figure 2B). Much of our current understanding of physiologic and pathologic regulation of hydraulic permeability is based on experiments using the Landis-Michel technique to measure Lp.
Experimental evidence of fluid filtration consistent with Starling's hypothesis was described in whole organ preparations in the mid-20th century  when hind limbs from cats and dogs were isolated, perfused, and the weight of the hind limb recorded continuously. Using this technique, fluid filtration was determined from the rate of gain of limb weight, and fluid reabsorption from the rate of loss of limb weight. The arterial and venous hydrostatic pressures were adjusted experimentally, and values that resulted in neither filtration nor reabsorption were established, resulting in an "isogravi-metric state." The hydrostatic pressures required for the iso-gravimetric state varied according to the oncotic pressure of the perfusate solution, as predicted by the Starling equation. The relationship between filtration rate and "capillary" pressure (calculated from the arterial and venous hydrostatic pressures) was linear, with the slope equal to the filtration coefficient (Kf). Kf is the product of Lp and surface area, the latter coefficient being unknown in whole-organ experiments but measurable in single-perfused microvessels (assuming cylindrical geometry).
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