Pathophysiology
Clinical meaning
The integumentary system -- the body's largest organ -- consists of the skin and its accessory structures (hair, nails, sweat glands, sebaceous glands) and performs vital protective, regulatory, and sensory functions. The skin has three layers: the epidermis (outermost layer composed of stratified squamous epithelium; contains keratinocytes producing keratin for waterproofing, melanocytes producing melanin for UV protection, Langerhans cells for immune surveillance, and Merkel cells for tactile sensation; renews approximately every 28 days in young adults, slowing with age), the dermis (beneath the epidermis; composed of connective tissue containing collagen and elastin fibers providing strength and elasticity, blood vessels, lymphatics, nerve endings, hair follicles, and sweat and sebaceous glands), and the hypodermis or subcutaneous layer (adipose tissue providing insulation, cushioning, and energy storage). The skin's functions include: barrier protection against pathogens, chemicals, and UV radiation; thermoregulation through vasodilation/vasoconstriction and sweat production; sensation (pain, pressure, temperature, touch); vitamin D synthesis (UV exposure converts 7-dehydrocholesterol to vitamin D3); fluid and electrolyte balance; and immune surveillance. Wound healing proceeds through four overlapping phases: hemostasis (platelet plug and fibrin clot formation, seconds to hours),...