physiology

A cubic centimeter of compact bone can withstand about 170 megapascals of compressive stress — comparable to reinforced concrete, at a fraction of the weight. But unlike concrete, bone is alive. It has its own blood supply, repairs cracks autonomously, completely replaces its mineral matrix every 10 years, and simultaneously serves as the body's primary calcium reservoir and the factory producing every blood cell in your body. Here is how it all works.

Pinch your nose while eating a strawberry and the flavor nearly disappears — what remains is just sweetness with a hint of acid. Flavor, it turns out, is about 80% smell. The tongue detects only five basic tastes; the nose handles everything else through 400 distinct olfactory receptors, making smell the most molecularly diverse sensory system in the human body. Here is how these two ancient senses combine to produce the richness of what we call taste.

Your body contains roughly 5 liters of blood, and every drop of it completes a full circuit through the heart and lungs in about 60 seconds. Blood is not a simple liquid — it is a suspension of at least four distinct components performing oxygen transport, immune defense, wound sealing, and chemical delivery simultaneously. Here is how it all works.

Every movement you make — from blinking to deadlifting — comes down to two proteins sliding past each other using energy from ATP. The mechanism is staggeringly elegant, and the reason muscles grow stronger is not what most people think.

Your kidneys filter your entire blood supply roughly 40 times every day — about 180 liters of fluid — and return 99% of it to the bloodstream while excreting only 1 to 2 liters as urine. The precision of that selective reclamation process is what keeps you alive.

The skin covers 1.5 to 2 square meters, makes up about 16% of adult body weight, replaces its waterproof outer layer roughly every four weeks, cools you with sweat, builds vitamin D from sunlight, and reads touch through specialized receptors. Here is how each part works.

A withdrawal reflex pulls your hand off a hot pan before you consciously feel pain. The brain alone holds about 86 billion neurons, and the fastest nerve fibres carry signals at up to 120 metres per second. Here is how it all works.

Aging is not a disease, a single clock winding down, or an inevitable consequence of time passing. It is a set of overlapping molecular failures that accumulate faster than cells can repair them. Researchers have now identified twelve distinct hallmarks that drive this process — and several of them can be slowed, at least in model organisms.

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.

Your lungs perform roughly 20,000 breath cycles per day, quietly swapping carbon dioxide for oxygen across a surface the size of a tennis court — all folded into a chest cavity the size of a shoebox. Here is the full mechanism, from the diaphragm's first pull to the moment oxygen binds hemoglobin.

The heart beats more than 100,000 times per day without a single conscious command, regulated by an electrical system so precise that it can be measured from electrodes on the skin. Here is how that system works, from the first spark in the SA node to the pressure wave that reaches your fingertips.

In 1921, Otto Loewi transferred fluid from one frog heart to another and proved that nerves communicate chemically. The substance he found became acetylcholine.