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Biomedical subjects

J A Sirs

Publications and source records attributed to J A Sirs.

13 recordsLinked to original sources
PubMed1991

The flow of human blood through capillary tubes.

1. The current interpretation of in vivo blood flow is mainly based on the Hagen-Poiseuille equation, although blood is not a Newtonian fluid. In this paper, experimental pressure-flow curves of blood are explained on the basis that the viscosity of the blood is the sum of two components, a Newtonian viscosity term, N, and an anomalous viscosity term equal to A/(B + D), where A and B are constants, and D the shear rate. 2. To a first approximation, blood flow in capillary tubes, comparable to that in vivo, can be deduced if the applied pressure in Poiseuille's equation is reduced by an effective back-pressure, p, equal to 8Al/3R, where l is the length of the capillary tube, and R its radius. 3. The theory explains the progressive change, from a parabolic velocity profile in large vessels, to a flattened profile in small vessels, as observed in vivo. 4. Experimental evidence is given that p is proportional to the length, and increases with decrease of R. The effect of the anomalous viscosity coefficient A was studied by varying the haematocrit, fibrinogen level, erythrocyte flexibility and temperature. 5. As the tube bore is decreased, the Fahraeus-Lindqvist effect decreases N, but this is offset by an increase of the anomalous component, A. This results, at lower pressures, in an increase of the effective blood viscosity in small vessels and of the peripheral resistance, and, at higher pressures, in a decrease of the effective blood viscosity. 5. Blood flow is proportional to the radius to the power n, where n is a variable that increases with increase of A and decrease of the applied pressure.

Blood Circulation↗
PubMed1977

The relationship of plasma fibrinogen, erythrocyte flexibility and blood viscosity.

Measurements have been made of plasma fibrinogen concentration, erythrocyte flexibility and blood viscosity at shear rates from 5.75 to 230 sec-1 during and following surgery. In the post-operative period the plasma fibrinogen level in the patient rose to over 1,000 mg/dl and because there were subsequent complications, only returned to normal after 4 weeks. There was an associated change of erythrocyte flexibility, with a correlation coefficient of 0.98. The blood viscosity also varied with the plasma fibrinogen level, the effect being more pronounced at low shear rates. The internal viscosity of the red blood cell, calculated from the plasma viscosity and whole blood viscosity at 230 sec-1, decreases with increasing plasma fibrinogen concentration, in agreement with the direct measurements made of erythrocyte flexibility. It is proposed that at high shear rates an increase in plasma viscosity due to an elevation of fibrinogen concentration, is offset by a decrease in the rigidity of the erythrocytes, and these 2 effects counter-balance.

Blood Viscosity↗
PubMed1977

Indicator dilution measurements of flow parameters in curved tubes and branching networks.

An experimental investigation has been made of the application of the indicator-dilution bolus-injection technique to measure flow rate, mean transit time and vessel volume with steady flow in curved tubes and branching networks. Measurements were made with 131I in 2N KCl and 131I-human serum albumin in isotonic saline, plasma and whole blood under laminar flow conditions up to a Reynolds' number of 460. The flow rate estimations are more reliable than those obtained for flow through straight tubes, due to secondary flows tending to disperse the indicator uniformly over the vessel cross-section. Even so, estimations of mean transit time and vessel volumes, using accepted theoretical formulae, may be as much as 200% in error. Similar measurements have been made of the flow parameters with bifurcations and branching networks. The indicator is not partitioned between the branches in proportion to the flow rate, as is assumed in the original indicator dilution theory, except when there is complete symmetry of the flow. An alternative method of estimating the vessel volume was used and shown to give an accurate estimate of the true volume in these circumstances.

Blood↗
PubMed1976

The effect of temperature on the reaction of carbon monoxide with oxygenated haemoglobin.

1. The rate at which carbon monoxide displaces oxygen from its combination with haemoglobin in solution, has been measured spectrophotometrically, using a rapid-mixing stopped-flow technique. 2. In the presence of carbon dioxide, the reaction proceeds by a unimolecular dissociation, with a rate constant r. 3. The relationship of the reciprocal of r to the ratio PO2/PCO is non-linear, and a different curve is obtained at each carbon monoxide concentration. 4. From measurements of the rate constant at temperatures between 5 and 35 degrees C, it is concluded that the non-linearity is due to the formation at a finite rate of an intermediate complex, CO2Hb4O6, by the reaction of carbon dioxide with Hb4O6.

Carbon Monoxide↗
PubMed1976

The Bohr effect on the reaction of carbon monoxide with fully oxygenated haemoglobin.

1. The rate at which CO displaces O2 from its combination with haemoglobin in solution, has been measured spectrophotometrically, using a rapid-mixing stopped-flow technique. 2. In the presence of CO2, the reaction proceeds by a unimolecular dissociation, with a rate constant r. 3. The relationship of the reciprocal of r to the ratio PO2/PCO is nonlinear, and a different curve is obtained at each CO concentration. 4. Measurements were made of the rate of the reaction when the pH was varied, with constant or varying PCO2. In both situations the value of r was found to have a miximum, for a given PO2/PCO ratio and CO cencentration, at pH 7.2. 5. An analysis of these results suggest that the Bohr effect, of pH and PCO2, is a dynamic equilibrium between four stable tertiary states of each of the alpha and beta chains. Each intermediatory complex has different rate constants for CO and O2.

Carbon Dioxide↗
PubMed1975

The variation of indicator dilution curves with velocity profile.

A theoretical analysis has been made of the effect of different velocity profiles on the dispersion of a non-diffusable solute in fluid flow down a straight tube. An experimental investigation of this type of dispersion was made by monitoring the transport of radioactively labelled red blood cells in whole blood at flow rates within the non-Newtonian viscosity range. The dispersion curves obtained are consistent with a progressive flattening of the velocity profile as the flow rate is reduced, though a more rapid clearance than that predicted theoretically occurs in the tail region of the curves. The accepted indicator dilution techniques of estimating the flow rate and vessel volumes are considerably in error. An alternative method of accurately estimating the vessel volume is suggested.

Blood Circulation↗