Blood Flow in the Microcirculation

Segal S.S. Regulation of blood flow in the microcirculation. Microcirculation 12 33-45,2005. 

Piedimonte, G., Hoffman, J.I.E., Husseini, W.K. etal. (1992). Effect of neuropeptides released from sensory nerves on blood flow in the rat airway microcirculation. J. Appl. Physiol. 72, 1563-1570. 

Hepatic oxygenation, microcirculation, and function were studied by Jiao et al. [Ill] in a recent paper. The blood flow in the liver and its function were also studied directly in a cirrhotic animal model by the same authors [112], Peripheral blood clearance of indocyanine green was shown to be less accurate than direct NIR probes on the liver surface. 

Fig. 8. Interstitial velocity profiles. Representative regions in the microcirculation. Circles represent locations of fluorescence photobleaching experiments. The arrows inside the circles represent the direction of the interstitial fluid velocity at these locations. The nearby values show magnitudes of the velocity in fim/s. (a) An area where interstitial flow parallels blood flow in the vessels, (b) Interstitial flow is opposite prevailing blood flow, (c) Fluid is absorbed from the interstitium into a postcapillary venule. (From Chary and Jain, 1989, with permission.) The photobleaching technique has provided the first and to date the only measurements in the literature of interstitial convective velocities. We have now further improved this technique to permit measurements of binding parameters (Kaufman and Jain, 1990, 1991, 1992a, b) and of transport parameters in light-scattering media (Berk et al., 1993).

The geometrical parameters of the canine systemic and pulmonary circulations are summarized in Table 56.1. Vessel diameters vary from a maximum of 19 mm in the proximal aorta to 0.008 mm (8 m) in the capillaries. Because of the multiple branching, the total cross-sectional area increases from 2.8 cm in the proximal aorta to 1357 cm in the capillaries. Of the total blood volume, approximately 83% is in the systemic circulation, 12% is in the pulmonary circulation, and the remaining 5% is in the heart. Most of the systemic blood is in the venous circulation, where changes in compliance are used to control mean circulatory blood pressure. This chapter will be concerned with flow in the larger arteries, classes 1 to 5 in the systemic circulation and 1 to 3 in the pulmonary circulation in Table 56.1. Flow in the microcirculation is discussed in Chapter 59, and venous hemodynamics is covered in Chapter 60. 

The bile canaliculi form a network, which feed into ductules, which become bile ducts (Fig. 6.3). The structural and functional unit of the liver is the lobule, which is usually described in terms of the hepatic acinus (Fig. 6.5), based on the microcirculation in the lobule. When the lobule is considered in structural terms, it may be described as either a classical or a portal lobule (see "Glossary"). The acinus comprises a unit bounded by two portal tracts and terminal hepatic or central venules, where a portal tract is composed of a portal venule, bile ductile, and hepatic arteriole (Fig. 6.5). Blood flows from the portal tract toward the central... 

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). 

Within the vasculature of a tissue, blood flows in all directions, and the local direction of convection depends on the vascular morphology of the tissue. The situation is even more complex in tumors where the direction and magnitude of blood flow are not fixed. In tumors, blood flow is temporally and spatially inhomogeneous. Therefore, the local description of convective heat transfer term, Qb, in tissues would include a time-dependent velocity vector — a problem which is enormously complex and has thus far proven mathematically intractable. In order to circumvent a mathematical description of the details and complexities of the microcirculation in a capillary bed, primarily two approaches have been taken by investigators in this area of research (Charny, 1992). 

Pentoxifylline, a dimethylxanthine derivative, and its metabolites improve blood flow by decreasing blood viscosity. It produces dose-related hemorrheologic effects, lowering blood viscosity and improving erythrocyte flexibility. In patients with chronic peripheral arterial disease, this increases blood flow to the affected microcirculation and enhances tissue oxygenation. 

Qinkgo hiloha This herb does not seem to offer any benefits to healthy adults, but it may be helpful to people with various disorders, including dementia and depression, and seems to be somewhat effective in treating the sexual dysfunction that often appears as a side effect of antidepressants. Some doctors prescribe it along with an SSRl. It seems to work by enhancing microcirculation, improving blood flow to the tiny capillaries to the brain and elsewhere. 

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