An analysis on the exchange of indicators in single capillaries

Diffusion, pericapillary distribution and clearance of Na-fluorescein in the human nailfold

An intravital fluorescence videomicroscopy technique is described in an attempt to develop non-invasive methods for the study of transcapillary exchange in human physiology and clinical medicine. The apparatus used consists of a Ploemopak incident light fluorescence microscope and a low light level television camera with linear output. After intravenous bolus injection of Na-fluorescein in 12 healthy subjects the movement of the dye particles in the nailfold was observed and stored on video-tape. Immediately after arrival the dye leaks into a halo-like section and later into the more remote parts of the pericapillary space. By moving a densitometric window on an axis transversal to the capillary loop (single frames of the tape), characteristic patterns of fluorescent light intensity distribution were obtained at different time intervals. At the edge to the halo the high pericapillary light intensity decreases abruptly moving further away from the capillary (mean: 10.0 micron) for 20 min and more in each individual indicating the presence of a diffusion barrier at this location between halo and more remote areas. Clearance of the dye is much slower than transcapillary diffusion and lasts approximately 2 h.

Dispersion of indicator measured from microvessels of cat mesentery

The indicator-dilution method was used to investigate blood flow in the microvascular network of the mesenteric membrane of the cat by replacing the normal blood flow into the mesenteric artery with a dextran-saline solution for a finite period of time. The dilution of the dextran-saline bolus as it flowed downstream was measured in a selected arteriole and its adjacent venule with a microphotometric system. Based on an in vitro calibration study done on glass capillary tubes, the measured optical density was converted to the hematocrit. The dilution curve for a finite injection and for a step injection, the mean transit time (MTT), and the appearance time for the arteriole and the venule were computed. The dispersion found in the arterial system, a network of diverging branches, was reasonably well simulated by the dispersion in a parabolic flow system. However, the dispersion found in the venules, where the irregular capillary blood flow converges, was considerably skewed from that in a parabolic system. A wide distribution of MTT was found for the arterioles and the venules, and there was considerable overlap in the MTT distribution for these two groups. This finding cannot be simulated by a network with a parallel arrangement. The difference in MTT for pairs of arterioles and venules was distributed over a narrow range, probably indicating shunt flow in the mesenteric microvascular network.

A concurrent flow model for extraction during transcapillary passage

A model for capillary-tissue exchange in a uniformly perfused organ with uniform capillary transit times and no diffusional capillary interactions was designed to permit the exploration of the influences of various parameters on the interpretation of indicator-dilution curves obtained at the venous outflow following the simultaneous injection of tracers into the arterial inflow. These parameters include tissue geometric factors, longitudinal diffusion and volumes of distribution of tracers in blood and tissue, hematocrit, volumes of nonexchanging vessels and the sampling system, capillary permeability, P. capillary surface area, S , and flow of blood- or solute-containing fluid, Fs ′. An assumption of instantaneous radial diffusion in the extravascular region is appropriate when intercapillary distances are small, as they are in the heart, or permeabilities are low, as they are for lipophobic solutes. Numerical solutions were obtained for dispersed input functions similar to normal intravascular dye-dilution curves. Axial extravascular diffusion showed a negligible influence at low permeabilities. The “instantaneous extraction” of a permeating solute can provide an estimate of PS /Fs ′, the ratio of the capillary permeability–surface area product to the flow, when PS /Fs ′ lies between approximately 0.05 and 3.0; the limits of the range depend on the extravascular volume of distribution and the influences of intravascular dispersion. The most accurate estimates were obtained when experiments were designed so that PS /Fs ′ was between 0.2 and 1.0 or peak extractions were between 0.1 and 0.6.