Local tissue oxygenation during constant red blood cell flux: a discrete source analysis of velocity and hematocrit changes.

Tissue oxygenation in the proximity of single capillaries was investigated mathematically, with the red blood cells (RBC) modeled as discrete oxygen sources separated by plasma gaps. The variables studied are the simultaneous change of capillary RBC flow velocity, and hematocrit (Hct), during constant flux (e.g., RBC velocity x Hct), with an oxygen tension-dependent tissue oxygen consumption rate. The model was used to analyze isovolemic hemodilution under the assumption that the decrease of capillary Hct is compensated by the increase of RBC velocity. The results show that during constant flux conditions, changes in the RBC velocity are directly related to the distance along the capillary to which oxygen is delivered and inversely related to the radial penetration of oxygen into the tissue. Therefore during isovolemic hemodilution, there is a redistribution of the volume of tissue oxygenated whereby a greater volume of tissue becomes oxygenated at an oxygen tension above the critical limit, but at a lower time averaged value. Hypothermia is stimulated and its added effect on constant flux assessed. Tissue oxygenation at low capillary hematocrit was found to be highly sensitive to capillary spacing, suggesting that one of the mechanisms for the maintenance of tissue oxygenation during hemodilution is the recruitment of capillaries.