Human Anatomy
The human body contains 37 trillion cells, 206 bones, and 78 organs working in constant coordination.
🫀 Cardiovascular System
The heart pumps about 5 liters of blood per minute at rest, up to 25 liters during exercise. The circulatory system spans about 100,000 km of blood vessels. Red blood cells carry oxygen via hemoglobin and live for about 120 days.
🧠 Nervous System
The brain contains about 86 billion neurons, each forming up to 10,000 synaptic connections. Nerve signals travel at up to 120 m/s. The central nervous system (brain and spinal cord) processes information; the peripheral nervous system connects it to the rest of the body.
🫁 Respiratory System
The lungs have a surface area of about 70 m2 (roughly half a tennis court) for gas exchange. We breathe about 15 times per minute, taking in ~500ml per breath. Oxygen crosses the alveoli membrane into capillaries by diffusion, while CO2 moves the opposite way.
🦴 Skeletal System
206 bones provide structure, protect organs, and produce blood cells in bone marrow. Bone is a living tissue constantly being remodeled. The femur is the strongest bone; the stapes (in the ear) is the smallest. Bone density decreases with age and radiation exposure.
☢️ Radiation and the Body
Ionizing radiation damages DNA in cells. The most vulnerable tissues are rapidly dividing ones: bone marrow (blood cell production), intestinal lining, and skin. Acute radiation syndrome (ARS) occurs above ~1 Gy. The thyroid readily absorbs radioactive iodine-131, which is why potassium iodide tablets are distributed after nuclear accidents.
🧬 Endocrine System
Glands produce hormones that regulate metabolism, growth, reproduction, and mood. The thyroid controls metabolism. The pancreas regulates blood sugar with insulin and glucagon. The adrenal glands produce cortisol and adrenaline. Hormones travel via the bloodstream to their target cells.