Rob Kurzban at The Nature-Nurture-Nietzsche Newsletter:
1. Breathing isn’t only about oxygen.
If you ever find yourself in a room of pure nitrogen—which you should try very hard not to—here is what will happen. You will steadily have less oxygen in your blood, but your carbon dioxide levels won’t change much: Your cells produce the gas at the usual rate, and you exhale it at the usual rate. Because the urge to breathe depends largely on the level of carbon dioxide, not oxygen, you won’t feel the same urge you would if, for instance, you were underwater, where carbon dioxide builds up as you hold your breath. You’ll feel little or nothing before you pass out.
Measures: partial pressure of carbon dioxide in the blood. Motivates: inhaling.
2. Fatigue is more like a stop signal than a fuel constraint.
As you exercise, the risk of a catastrophic failure such as heat stroke goes up. Your body has a control system designed to persuade you to stop challenging your heart and other systems before that happens. Fatigue is the feeling that takes those measurements and estimates the chance of failure. You can overcome it—you’ll keep running even when you’re tired if you’re being chased by a bear—but the feeling is telling you to stop unless something important is at stake.
Measures: the probability of physical system failure. Motivates: reduce effort.
More here.