Pressure within the microvascular bed of the frog mesentery has been recorded continuously with micropressure transducers which have tip diameters ranging between 0.5–5 µ. The mean values of pressure are in the range of those previously reported. The waveform of the pressure pulses in arterioles and metarterioles closely resembles that in the larger arteries. This observation signifies that the vascular walls are considerably stiffer than is generally believed. The stiffness is probably attributable to the constricted vascular smooth muscle.
Fluid balance at the capillary level has been simulated with an analogue computer program, based on experimental data on regional differences in capillary permeability, surface areas, and hydrostatic pressures. The program takes into account fluid and protein fluxes into and out of the interstitial space. Solutions are obtained for tissue hydrostatic pressure, tissue fluid osmotic pressure, interstitial space volume, and lymph flow. Simulation of a variety of physiological experiments and clinical disease states has yielded reasonable agreement between experimental data and data obtained by computer analysis. Dilution of the interstitial plasma protein pool with a consequent reduction of its oncotic pressure appears to be a major factor, which prevents edema unless plasma oncotic pressures are reduced by 10-15 mm Hg or, alternatively, venous pressures are elevated by a similar amount. The computer analysis in all instances yields positive values for tissue pressure, in agreement with experimental data obtained by needle puncture. The negative tissue pressures observed in subcutaneous capsules can be reproduced in the computer program, if the interface between the capsule and the surrounding interstitial space is assumed to have the properties of a semipermeable membrane.Partition of body fluids between the circulation, on the one hand, and the interstitial and intracellular spaces, on the other, is normally maintained within narrow limits. The mechanism maintaining this precise partition is still not well understood, but a balance of capillary hydrostatic and colloid osmotic pressures is generally acknowledged to play an important role, as originally pointed out by Starling (1). Current views on capillary water balance are summarized in Fig. 1, which is based on experimental data of Starling (1) and Landis (2). The graphs illustrate the relationship between hydrostatic pressures along the length of a capillary and the colloid osmotic pressure of plasma proteins.The oncotic pressure of plasma (7r,,) averages 25 mm Hg (3), corresponding to a plasma protein concentration of 7%. The hdyrostatic capillary pressures are averages based on extensive series of pressure recordings in skin capillaries of man by Landis (2). In arterial capillaries ( Fig. 1 A, left), the pressures averaged 32 mm Hg (Pa). The hydrostatic pressures in the 29 The Journal of General Physiology TRANSCAPILLARY EXCHANGES venous capillaries (Fig. 1 A, right) averaged 15 mm Hg (Pv). The transcapillary flux of fluid is governed by the difference in the hydrostatic and colloid osmotic pressure of the plasma; thus fluid is filtered from the capillary into the tissues in the arterial end of the capillary, and it is reabsorbed from the tissues into the capillary at the venous end. The hydrostatic pressures in the arterial and venous ends of the capillary bracket the colloid osmotic pressure, and the amount of fluid filtered at the arterial end is similar to the volume reabsorbed at the venous end. 20-In. This equilibrium of filtration and reabso...
Microvascular dimension and flow responses to stepwise changes in arterial and venous pressures, ranging from zero to +100 mmHg and zero to -75 mmHg have been recorded. Observations were made in arterioles, terminal arterioles, and precapillary sphincters in the wing web of intact, unanesthetized bats. The results show for all categories of vessels that with reduced transmural pressures there is a progressive increase in mean diameter and a decrease in rhythmic vasomotion rate. Flow changes are variable. For elevated transmural pressures there is a vasoconstriction with drastic flow reduction that is inconsistent with metabolic control. However, after prolonged elevation of pressure there is a progressive increase in flow, suggesting a "metabolic escape". Computed wall tension remains reasonably constant for a wide range of transmural pressures, suggesting that wall tension may be the controlled variable. These findings support the hypothesis of a myogenic reaction as a mechanism for maintenance of basal vascular tone in the intact unanesthetized bat.
A system which permits continuous recording of dimensions of microscopic blood vessels is described. The system utilizes information contained in the video signal of a television microscope to develop an analog voltage proportional to the time required for the electron beam to sweep across the image of the blood vessel. This time interval is also proportional to the dimension of the vessel. Calibration of the system yielded a standard error of estimate of ±3.7 μ on a series of glass capillaries, ranging in size from 15 to 150 μ. The rise time of the system was in the order of 40 msec. Long- and short-term drift was less than 3 μ/hr. The system is used in an experimental study of viscoelastic properties of small arteries and arterioles. microcirculation; viscoelastic properties; frog mesentery Submitted on April 17, 1963
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