Three Essential Functions of Skin: Protection, Sensation, and Excretion

The skin is the body's largest organ, performing three critical physiologic roles that maintain homeostasis and interface with the environment.

1. Protection (Barrier Function)

• Physical: Stratum corneum (keratin + lipids) blocks pathogens, chemicals, mechanical stress.
• Immunologic: Langerhans cells, dermal dendritic cells, antimicrobial peptides (defensins).
• UV Defense: Melanin absorbs/scatters UVR; DNA repair enzymes.
• Thermoregulation: Vasodilation/constriction, sweating, piloerection.
• Hydration: Prevents excessive transepidermal water loss (TEWL).

2. Sensation (Neurosensory Interface)

• Mechanoreceptors: Merkel (sustained pressure), Meissner (light touch), Ruffini (stretch), Pacinian (vibration).
• Thermoreceptors: TRPV (heat), TRPM8 (cold).
• Nociceptors: Aδ (sharp, fast), C-fibers (dull, slow), polymodal.
• Proprioception: Cutaneous contribution to joint position sense.

3. Excretion & Secretion

• Eccrine Sweat: Thermoregulatory (water, NaCl, urea, lactate, antimicrobial peptides).
• Apocrine Sweat: Axillary/genital; odor precursors (bacterial metabolism).
• Sebum: Lipids, squalene, wax esters — lubrication, antimicrobial, pH buffer.
• Minor: Trace heavy metals, drugs, metabolites (forensic/toxicologic utility).

Integrated Homeostasis

Barrier disruption → inflammation → sensitization → altered excretion. Skin reflects systemic health (jaundice, cyanosis, vasculitis, endocrine stigmata).