On the oxygenation of hemoglobin in the human brain.

We test three hypotheses arising from a model of oxygen delivery to brain tissue. The hypotheses claim that mitochondrial oxygen is negligible in brain tissue such that oxygen consumption depends solely on the mean capillary oxygen tension for a given capillary density; that capillary density is adjusted to satisfy the average steady-state oxygen requirement; and that sudden changes of brain function are subserved by changes of blood flow which adjust the mean capillary oxygen tension in the required direction. The results of the tests did not reject the two former hypotheses and only partly rejected the latter: Sudden changes of blood flow are not always accompanied by increases of oxygen consumption. When they are, changes of apparent capillary density (physiological recruitment) can occur.