Dynamics of intestinal oxygenation: interactions between oxygen supply and uptake.

The oxygenation of intestinal tissue is dependent on the interaction of three distinctive transport processes: 1) convection (i.e., blood flow), 2) diffusion, and 3) chemical reaction. Oxygen is transported to the capillary level by convection. Local adjustments of blood flow in accordance with tissue oxygen demand serve to stabilize capillary oxygen tension. After reaching the capillary, oxygen is transferred into the parenchymal cells by diffusion. Intrinsic modulation of the number of perfused capillaries allows vascular control of exchange surface area and effective capillary-to-cell diffusion distance. On entering the cell, molecular oxygen acts as the electron acceptor responsible for maintaining continuous mitochondrial respiration and the interlinked oxidative formation of ATP. The basic interaction of blood flow, diffusion, and mitochondrial respiration under a variety of conditions are illustrated utilizing a simplified graphic approach. The analysis demonstrates the ability of the flow and exchange controllers, operating in unison, to provide a wide margin of safety against development of cell hypoxia under a variety of stresses. Finally, the role of oxygen, adenosine, and prostaglandins as possible mediators of intrinsic vasoregulation in intestine is discussed.