Capillary transit

Goresky CA, Rose CP. Blood-tissue exchange in liver and heart the influence of heterogeneity of capillary transit times. Fed Proc 1977 36 2629-34. 

The articles of L. S. Ornstein (1908-1909) are, first, very valuable contributions to the clarification of the foundations of Gibbs s treatment. In addition, they show (1) that operations with canonical ensembles occasionally furnish a more convenient computational scheme for the treatment of complicated problems of equilibrium (e.g., the equilibrium in capillary transition layers) than does Boltzmann s procedure (2) the reason why the two methods give the same result in a large group of equilibrium problems. 

On the basis of the above arguments, the energy balance for the full range (saturation, gravity/capillary transition and capillary) becomes... 

In addition to providing information on blood flow in large retinal vessels, LTA also permitted assessment of the microcirculation. This was based on the measurement of the capillary transit time. As expected, the capillary transit time changed as a function of blood pressure and, interestingly, showed a twofold variation within the cardiac cycle (3). 

The po2 values in maternal and fetal red blood cells are shown by the solid lines. Using the indicated values (see text) the po2 n maternal and fetal end-capillary vessels barely reaches equilibrium (<0.5 mm Hg). The insert shows the change in maternal and fetal oxyhemoglobin percent saturation during the capillary transit. When maternal and fetal end-capil-lary p0 values are essentially the same, the percent 02Hb of fetal blood is about twice that of maternal blood because of the higher 02 affinity of fetal blood. 

Figure 6. Effect of placental 02 consumption on maternal (solid lines) and fetal (dashed lines) p0 during the capillary transit. While calculated fetal placental end-capillary p02 is 31.8 mm Hg in the absence of 02 consumption, it is decreased about 14% (to 27.3 mm Hg) when placental 02 consumption is 8 ml/min.

Uneven Maternal and Fetal Capillary Transit Times. The standard values chosen for maternal and fetal blood flow rates are equal. In studying effects of variations of one of the flow rates (15), the maternal and fetal capillary transit times become unequal, and the time steps of the integration, At, do not correspond to the same distance along the capillary on each side. To prevent the integration from getting out of step (diffusion not occurring perpendicular to the membrane), Equation 5 must be modified (14) to ... 

Figure 7. The change in 02 partial pressure in maternal and fetal erythrocytes, during the course of a single capillary transit for various values of placental diffusing capacity, Dp. The mathematical model was used for the calculations.

Oxygen makes up 21% of air, with a partial pressure of 21 kPa (158 mm Hg) at sea level. The partial pressure drives the diffusion of oxygen thus, ascent to elevated altitude reduces the uptake and delivery of oxygen to the tissues. air is delivered to the distal airways and alveoli, the PO2 decreases by dilution with carbon dioxide and water vapor and by uptake into the blood. Under ideal conditions, when ventilation and perfusion are well matched the alveolar PO2 will be -14.6 kPa (110 mm Hg). The corresponding alveolar partial pressures of water and CO2 are 6.2 kPa (47 mm Hg) and 5.3 kPa (40 mm Hg), respectively. Under normal conditions, there is complete equilibration ( alveolar gas and capillary blood. In some diseases, the diffusion barrier for gas transport may be increased during exercise, when high cardiac output reduces capillary transit time, full equilibration may not occur, and the alveolar-end-capillary Po gradient may be increased. 

Inasmuch as the concentration of unbound hormone in plasma is too low to account for the amount of hormone that crosses the capillary bed, it has been postulated that a special mechanism must exist to promote release of hormone from the bound state, in particular from plasma albumin. On the other hand, it can be clearly shown that the dissociation rate from each of the transport proteins is fast enough to replenish the pool of unbound hormone during capillary transit. Furthermore, blood cells in the capillary lumen result in mixing of the contentsthus preventing laminar flow at the capillary wall and local depletion of the free hormone pool- Since the rate of dissociation from albumin is faster than from PA and, especially from TBG, it can be expected that much, if not most, of the and T3 that leaves the capillary is derived from the albumin-bound pool even though albumin binds only a small fraction of the total hormone. 

The G-L problem arises, for example, when the capillary transition zone in a thick gas-condensate reservoir is evaluated [42]. The physical interpretation of this problem is as follows We measure pressure and composition of the gas phase after the porous sample has been introduced into a gas vessel and would hke to determine the parameters related to the capillary condensate in the porous medium. The difference between normal and eapUlary equilibrium, related to the additional degree of freedom, is especially pronounced in this case. For ordinary equilibrium, the formulated problem would be overdefined, because it would include an additional condition of the equality of phase pressures. 

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