Skin: Structure, Functions & Why It's the Largest Organ

The skin (integumentary system) is the body's largest organ—about 2 m² (22 ft²) in adults, weighing 3.5-5 kg (8-11 lbs). It comprises three layers:

Layers

• Epidermis: Outer keratinized stratified squamous epithelium; barrier, waterproofing, UV protection (melanin). Constantly renews (28-day cycle).
• Dermis: Dense connective tissue; collagen/elastin for strength/elasticity. Contains blood vessels, nerves, hair follicles, sweat/sebaceous glands.
• Hypodermis (subcutis): Loose connective tissue + adipose; insulation, energy storage, shock absorption, anchors skin to fascia.

Key Functions

1. Protection: Physical, chemical, microbial barrier; UV shielding.
2. Thermoregulation: Sweating (evaporative cooling), vasodilation/vasoconstriction, piloerection, insulation.
3. Sensation: Mechanoreceptors (touch, pressure), nociceptors (pain), thermoreceptors.
4. Excretion: Sweat eliminates water, electrolytes, urea, trace metals.
5. Vitamin D synthesis: UVB converts 7-dehydrocholesterol → previtamin D3.
6. Immunological: Langerhans cells, dermal dendritic cells, antimicrobial peptides.
7. Psychosocial: Appearance, identity, tactile communication.

Clinical Relevance

Skin reflects systemic health (jaundice, cyanosis, vasculitis, endocrine signs). Disorders range from acne and eczema to melanoma. Sun protection, hydration, and regular self-exams are foundational for skin health.