An in vitro capillary system for studies on microcirculatory O2 transport

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Abstract
An in vitro artificial capillary system has ben developed for use in examining the O2 transport properties of free hemoglobin and erythrocytes. The artificial capillary was constructed by casting a thin film of transparent silicone rubber around a strand of tungsten wire that was 24 .mu.m in diameter. After the rubber had polymerized, the wire was removed. Typical dimensions of the silicone rubber film were 170 .mu.m thick, 1 cm wide, 5 mm long in the direction of flow, and a 27-.mu.m lumen diameter. The artificial capillary bed was mounted on a microscope and perfused by either hemoglobin solutions or cell suspensions. Fractional saturation was measured as a function of axial position by a dual-wave-length microspectrophotometer, and the flow rate was regulated precisely by a syringe pump. O2 release experiments were carried out by suffusing the gas space surrounding the artificial capillary film with 100% N2 and perfusing with an oxygenated sample. O2 uptake experiments were carried out by suffusing the gas space with O2-N2 mixtures and perfusing with deoxygenated samples. The axial velocities were varied from 3 to 15 mm/s. The residence time (the time a particular red cell or hemoglobin molecule has spent in the capillary) for 50% oxygenation of a 4 mM (heme) deoxyhemoglobin solution was .apprx. 0.05 s at 37.degree. C when the gas space surrounding the capillary contained air. The corresponding time for 50% oxygenation of an equivalent red cell suspension was .apprx. 0.25 s. The residence time for 50% deoxygenation of a 4 mM (heme) oxyhemoglobin preequilibrated with 100% O2 was .apprx. 0.5 s at 37.degree. C, and this value increased approximately twofold when a red cell suspension was used. Thus the hemoglobin solution took up and released O2 two to five times as rapidly as an equivalent red cell suspension. The slower rates observed for intact erythrocytes demonstrate qualitatively that the solution flowing through the lumen of the capillary exerts a substantial portion of the total resistance to O2 transport and that there is substantial resistance to O2 transport associated with encapsulation of hemoglobin into cells.