Thermodynamic limitations on brain oxygen metabolism: physiological implications.

Recent thermodynamic modelling indicates that maintaining the brain tissue ratio of O 2 to CO 2 (abbreviated tissue O 2 /CO 2 ) is critical for preserving the entropy increase available from oxidative metabolism of glucose, with a fall of that available entropy leading to a reduction of the phosphorylation potential and impairment of brain energy metabolism. This provides a novel perspective for understanding physiological responses under different conditions in terms of preserving tissue O 2 /CO 2 . To enable estimation of tissue O 2 /CO 2 in the human brain, a detailed mathematical model of O 2 and CO 2 transport was developed, and applied to reported physiological responses to different challenges, asking: how well is tissue O 2 /CO 2 preserved? Reported experimental results for increased neural activity, hypercapnia and hypoxia due to high altitude are consistent with preserving tissue O 2 /CO 2 . The results highlight two physiological mechanisms that control tissue O 2 /CO 2 : cerebral blood flow, which modulates tissue O 2 ; and ventilation rate, which modulates tissue CO 2 . The hypoxia modelling focused on humans at high altitude, including acclimatized lowlanders and Tibetan and Andean adapted populations, with a primary finding that decreasing CO 2 by increasing ventilation rate is more effective for preserving tissue O 2 /CO 2 than increasing blood haemoglobin content to maintain O 2 delivery to tissue. This work focused on the function served by particular physiological responses, and the underlying mechanisms require further investigation. The modelling provides a new framework and perspective for understanding how blood flow and other physiological factors support energy metabolism in the brain under a wide range of conditions. KEY POINTS: Thermodynamic modelling indicates that preserving the O 2 /CO 2 ratio in brain tissue is critical for preserving the entropy change available from oxidative metabolism of glucose and the phosphorylation potential underlying energy metabolism. A detailed model of O 2 and CO 2 transport was developed to allow estimation of the tissue O 2 /CO 2 ratio in the human brain in different physiological states. Reported experimental results during hypoxia, hypercapnia and increased oxygen metabolic rate in response to increased neural activity are consistent with maintaining brain tissue O 2 /CO 2 ratio. The hypoxia modelling of high-altitude acclimatization and adaptation in humans demonstrates the critical role of reducing CO 2 with increased ventilation for preserving tissue O 2 /CO 2 . Preservation of tissue O 2 /CO 2 provides a novel perspective for understanding the function of observed physiological responses under different conditions in terms of preserving brain energy metabolism, although the mechanisms underlying these functions are not well understood.