Increasing evidence indicates that capillary recruitment plays a significant role in regulating the oxygenation of intestinal tissue. Measurements of permeability-surface area product (PS) and capillary filtration coefficients (Kf) in isolated perfused gut loops indicate that changes in capillary density modulate oxygen extraction in a variety of experimental circumstances. Moreover, the intestinal microvasculature seems capable of independently regulating resistance and capillary exchange. Although "precapillary sphincters" have been identified in the intestine, the capillary density changes have not yet been confirmed by intravital microscopy, as they have been in skeletal muscle. Nevertheless, these changes in capillary density have quantitatively significant effects on oxygen extraction. For example, sympathetic stimulation depresses oxygen uptake in gut loops perfused at constant blood flow, presumably by reducing capillary density to such an extent that oxygen extraction becomes diffusion limited. The microvascular elements that control intestinal capillary density (presumably precapillary sphincters) are apparently under the control of neurogenic, myogenic, and local metabolic mechanisms, but the interaction among these mechanisms is poorly understood. In addition, the PS and Kf data, although well documented, could result from a redistribution of blood flow or an alteration in capillary permeability rather than a change in capillary density. Thus, the physiological mechanisms regulating capillary permeability and the intramural distribution of intestinal blood flow will have to be better understood before the role of capillary recruitment in regulating intestinal oxygenation will be firmly established.
Cerebral extraction of N-13 ammonia: its dependence on cerebral blood flow and capillary permeability, surface area product. [Dogs; monkeys]