What is the largest organ of the human body?

Skin is the largest organ, with surface area ~1.5-2 square meters and accounting for ~15% of body weight. Skin (integumentary system) functions: (1) Barrier — protects against infection, dehydration, UV radiation, mechanical injury; (2) Thermoregulation — sweating cools body; vasodilation/constriction adjusts heat loss; (3) Sensation — touch, pressure, temperature, pain receptors; (4) Vitamin D synthesis — UV light converts cholesterol precursor to vitamin D; (5) Excretion — small amounts of waste through sweat. Skin layers: (1) Epidermis — outermost, no blood vessels, contains keratinocytes (produce keratin), melanocytes (produce melanin/pigment), Langerhans cells (immune); (2) Dermis — beneath epidermis; contains blood vessels, nerves, sweat glands, sebaceous (oil) glands, hair follicles; (3) Hypodermis (subcutaneous) — fat, connective tissue, larger vessels. Largest internal organ: liver (~1.5 kg, ~3% body weight). Other big organs: lungs, brain (1.4 kg), heart, kidneys. The brain is highly metabolically active despite only 2% of body weight — uses 20% of body's energy.