Major Organs

★The heart starts beating just 4 weeks after conception and doesn't stop until death — roughly 3 billion beats in a lifetime. It can even continue beating outside the body if supplied with oxygen.

★The brain is about 73% water and uses 20% of the body's energy despite being only 2% of body weight. It generates about 23 watts of power — enough to power a dim light bulb.

★The right lung is slightly larger than the left, which makes room for the heart. If your lung's air sacs (alveoli) were unfolded and laid flat, they'd cover roughly half a tennis court.

★The liver performs over 500 distinct functions — the most of any organ. It's also the only internal organ that can fully regenerate: you can remove up to 75% of it and it will grow back.

★Each kidney contains about 1 million nephrons (filtering units). Your kidneys filter your entire blood volume 40 times a day — roughly 200 liters — producing only 1–2 liters of urine.

★Stomach acid (hydrochloric acid) is strong enough to dissolve metal — pH 1.5–3.5. The stomach lining replaces itself every 3–4 days to avoid digesting itself. An empty stomach is roughly the size of your fist.

★The small intestine is about 20 feet long — six times your height. Its inner wall is covered in millions of tiny finger-like projections (villi) that, if unfolded, would cover a tennis court.

★Your gut contains about 38 trillion bacteria — more than the total number of human cells in your body. This microbiome weighs up to 2 kg and influences digestion, immunity, and even mood via the gut-brain axis.

★The pancreas does two entirely separate jobs using different cell types. Its 'exocrine' cells make digestive enzymes; its 'endocrine' cells (islets of Langerhans) make hormones. Damage to the latter causes diabetes.

★Skin is the body's largest organ — about 2 square meters and 15% of body weight. You shed 30,000–40,000 dead skin cells every hour, generating roughly 1.5 grams of dust per day.

★You can live without a spleen — but you become permanently more vulnerable to certain bacterial infections. The spleen can also act as a blood reservoir, releasing extra red blood cells during exercise or blood loss.

★The thyroid controls how fast or slow virtually every cell in your body works. Too much thyroid hormone and your metabolism races — rapid heartbeat, weight loss, anxiety. Too little and everything slows — fatigue, weight gain, depression.