metabolism

Shivering is an involuntary muscle contraction that generates heat when your body temperature drops below its set point. Your hypothalamus acts as a thermostat, detecting temperature changes through cold receptors and triggering both shivering and non-shivering heat production. Brown fat provides extra warmth in infants, while shivering during fever occurs because your body's target temperature has been reset upward. Understanding this physiological response reveals how your nervous system maintains the narrow 37°C window needed for survival.

Alcohol is absorbed into the bloodstream within minutes of consumption, but the liver metabolizes it at a nearly constant rate of about one standard drink per hour, a rate that cannot be accelerated by food or hydration. The body treats ethanol as a toxin, converting it to acetaldehyde (more toxic than ethanol) and then to acetate through two enzyme-mediated reactions. Blood alcohol concentration (BAC) depends on body weight, sex, food intake, and genetics, making some individuals metabolically unable to process alcohol efficiently, as seen in alcohol flush syndrome. Once intoxicated, alcohol disrupts the balance of GABA and glutamate neurotransmitters in the brain, impairing judgment and coordination. Understanding alcohol metabolism reveals why hangovers occur, why some populations have lower tolerance, and why no trick can accelerate the clearance of alcohol from the body.

For decades, paleontologists sorted life into two metabolic boxes, warm-blooded mammals and cold-blooded reptiles, and argued over which box dinosaurs belonged in. Bone slices, isotope chemistry, and growth-rate analysis suggest most dinosaurs fit neither box cleanly, though the newest fossil-chemistry work pulls the answer back toward true endothermy.

A hibernating ground squirrel can drop its body temperature to -3°C, slow its heart to one beat per minute, and stop breathing for up to an hour at a time — then wake up three months later with no organ damage and no muscle atrophy. Understanding how they pull this off is one of the most medically important questions in biology.

The liver performs over 500 distinct biochemical functions simultaneously, filters roughly 1.4 liters of blood per minute, produces 800–1,000 mL of bile per day, and is the only internal organ that can regrow itself after losing up to 70% of its mass. Here is how it manages all of that.

Why do we shiver when cold? Inside the hypothalamus, muscle thermogenesis, brown fat (BAT), fever shivers, and what infants do instead.

How insulin controls blood glucose, why type 1 and type 2 diabetes differ, and how treatment moved from animal extracts to pumps, drugs, and stem cells.

How the Krebs cycle (citric acid cycle, TCA) powers cells: 8 steps, inputs, outputs (NADH, FADH2, ATP), and regulation explained simply.

What would actually happen if humans could photosynthesize like plants? The math is brutal: you'd need skin the area of a small room, you'd still need food, and you'd have to spend most of your life lying flat in direct sunlight.

The liver performs over 500 distinct functions and is the only internal organ that can fully regenerate itself from just 25% of its original tissue.

Bears enter hibernation — a state of profound metabolic depression where heart rate drops 90%, body temperature falls 10°C, and they survive months without eating. If humans evolved this ability, it would revolutionize medicine, space exploration, and civilization by allowing suspension of biological aging and safe exploration of extreme environments.

Intermittent fasting works for some people for some reasons. Here's the actual research on what it does, what it doesn't, and how it compares to ordinary calorie restriction.