At sea level, we bathe in air compressed by Earth's atmosphere, replete with oxygen molecules. At this low altitude, most of us can walk easily without huffing and puffing and we can exercise with some huffing and puffing because we're used to it. However, at 5050 m above sea level, the air is thin, the oxygen more scarce – we need to work harder just to supply energy for essential functions, let alone exercise.
The brain, arguably the most essential organ in the human body, is one of the greediest when it comes to oxygen consumption. It requires the steady delivery of a large supply of oxygen: merely being at high altitude causes blood flow to the brain to increase to meet oxygen demands. Of course, when people exercise at high altitude, they need even more oxygen – fuelling efficient metabolic reactions in many systems, in addition to the constant drain of the brain. The scarcity of oxygen at high elevations forces us to breathe harder, or slow down; and yet mountain climbers exist. Somehow, they manage to extract enough oxygen from the thin atmosphere to fuel their physical exertions and their greedy brains.
Kurt Smith, a graduate researcher at the University of British Columbia, Canada, and a team of international collaborators from the USA, Canada, New Zealand and UK, wanted to know how oxygen consumption and other metabolic supply chains to the brain are sustained during exercise at high altitude. To answer this question, the researchers took a group of plucky participants to perform a series of gruelling experiments both at sea level and at the base of Mount Everest, 5050 m above sea level. The team fitted the athletes with internal jugular vein and radial artery catheters, among other devices, so that they could measure a series of physiological and metabolic factors – cerebral blood flow, intracranial arterial blood velocity, extra-cranial blood flow, and substrate differences between arterial and venous blood to the brain – during rest, exercise and recovery periods.
At high altitude, the partial pressure of oxygen in the subjects' arterial blood during exercise was reduced from both rest and sea level values. Despite the drop in blood oxygen, cumulative oxygen delivery to the brain was maintained via increased cerebral blood flow during and after exercise. Furthermore, at rest at high altitude, the brain increased its uptake of lactate and glucose, suggesting that it may have switched some of its metabolism from an oxidative to a non-oxidative pathway.
The rate of oxygen metabolism in the brain only changed from the resting rate during maximal exercise at both sea level and high altitude – but the magnitude of this change in metabolic rate, and the way the change was effected, differed between the two conditions. At sea level, the brain supports its increased oxygen metabolism by increasing oxygen extraction from the blood. At high altitude, the brain's oxygen metabolic rate increases only half as much as it does at sea level, and this increase is facilitated by an increase in cerebral blood flow, not by increasing oxygen extraction.
Taken together, these findings suggest that in response to the multifaceted physiological challenges of high altitude, the body comes up with creative ways to maintain oxygen delivery to the brain. By increasing blood flow and using multiple metabolic pathways, the greedy brain can sate its hunger for oxygen, even atop mountains.