glucose homeostasis
Glucose homeostasis is the body's balancing act of keeping blood sugar within a narrow lane — high enough to feed the brain, low enough to avoid damage — despite meals, fasting, exercise, and stress. Think of a thermostat that holds room temperature steady by switching heating and cooling on and off; here insulin cools blood sugar and glucagon heats it.
The system relies on opposing hormones and a few key organs. After eating, insulin from beta cells drives glucose into muscle and fat and stores it in the liver. During fasting, falling insulin and rising glucagon make the liver release and manufacture glucose. Other counter-regulatory hormones, such as cortisol, growth hormone, and epinephrine, add longer-range backup.
The brain is the central stakeholder because it depends almost entirely on a steady glucose supply and cannot store much fuel. This is why defenses against low blood sugar are layered and powerful, and why diabetes — a failure of this homeostasis — is dangerous in both directions, from hyperglycemia over years to acute hypoglycemia within minutes.
Despite all this regulation, the brain consumes glucose at a fairly constant rate; the famous dictum that the brain runs only on glucose is an oversimplification, because during prolonged starvation it adapts to use ketone bodies for much of its energy.