The control of cerebral blood flow in
hypoxia, anaemia and
hypocapnia is reviewed with an emphasis on the links between cerebral blood flow and possible stimuli. A mathematical model is developed to examine the changes in the partial pressure of
oxygen in brain tissue associated with changes in cerebral blood flow regulation produced by
carbon dioxide, anaemia and
hypoxia. The model demonstrates that
hypoxia, anaemia and
hypocapnia, alone or in combination, produce varying degrees of
cerebral hypoxia, an effect exacerbated when blood flow regulation is impaired. The suitability of
brain hypoxia as a common regulator of cerebral blood flow in
hypoxia and anaemia was explored, although we failed to find support for this hypothesis. Rather, cerebral blood flow appears to be related to arterial
oxygen concentration in both anaemia and
hypoxia.
ABSTRACT: A mathematical model is developed to examine the changes in the partial pressure of
oxygen in brain tissue associated with changes in cerebral blood flow regulation produced by
carbon dioxide, anaemia and
hypoxia. The model simulation assesses the physiological plausibility of some currently hypothesized cerebral blood flow control mechanisms in
hypoxia and anaemia, and also examines the impact of anaemia and
hypoxia on
brain hypoxia. In addition,
carbon dioxide is examined for its impact on
brain hypoxia in the context of concomitant changes associated with anaemia and
hypoxia. The model calculations are based on a single compartment of brain tissue with constant metabolism and perfusion pressure, as well as previously developed equations describing
oxygen and
carbon dioxide carriage in blood. Experimental data are used to develop the control equations for cerebral blood flow regulation. The interactive model illustrates that there are clear interactions of anaemia,
hypoxia and
carbon dioxide in the determination of cerebral blood flow and brain tissue
oxygen tension. In both anaemia and
hypoxia, cerebral blood flow increases to maintain
oxygen delivery, with
brain hypoxia increasing when cerebral blood flow control mechanisms are impaired.
Hypocapnia superimposes its effects, increasing
brain hypoxia.
Hypoxia, anaemia and
hypocapnia, alone or in combination, produce varying degrees of
cerebral hypoxia, and this effect is exacerbated when blood flow regulation is degraded by conditions that negatively impact cerebrovascular control. Differences in
brain hypoxia in anaemia and
hypoxia suggest that brain
oxygen tension is not a plausible sensor for cerebral blood flow control.