Physiology of skin — expanded notes
Barrier and innate defence
The skin barrier is formed by the stratum corneum, intercellular lipids, tight junctions, surface acidity, antimicrobial peptides and resident microbiota. Keratinocytes produce defensins, cathelicidins and cytokines. Barrier disruption permits increased transepidermal water loss and facilitates allergen and microbial penetration.
Thermoregulation
The hypothalamus integrates core and peripheral temperature signals. In heat, cutaneous vasodilatation and eccrine sweating increase heat transfer and evaporative cooling. In cold, vasoconstriction reduces heat loss. Sweating becomes less effective in high humidity because evaporation is reduced. Dehydration and electrolyte loss may occur with prolonged heavy sweating.
Gland physiology
Eccrine glands: widely distributed; ducts open directly onto the skin surface. Secretion is initially isotonic and becomes hypotonic as sodium and chloride are reabsorbed in the duct.
Apocrine glands: concentrated in axillae, areolae and anogenital regions; become active at puberty and discharge into hair follicles.
Sebaceous glands: holocrine glands associated with follicles; sebum lubricates hair and skin and contributes to the surface lipid film.
Immune and microbiome function
Langerhans cells and dermal dendritic cells present antigen to T cells. Resident memory T cells provide local immune surveillance. Mast cells participate in immediate hypersensitivity and inflammation. Commensal organisms compete with pathogens and help shape immune responses; dysbiosis may contribute to inflammatory disease.
Metabolic and endocrine roles
UVB converts 7-dehydrocholesterol to previtamin D3 in the epidermis. The liver and kidney perform subsequent hydroxylation. Skin also metabolises hormones and produces local signalling mediators. It is a sensory organ and participates in wound repair, social signalling and protection from environmental stress.
Environmental adaptation
Heat acclimatisation improves sweating efficiency and sodium conservation. Cold exposure causes vasoconstriction and may precipitate chilblains or frostbite. Low humidity worsens xerosis; occlusion and moisture promote maceration and intertrigo. UV exposure drives tanning, photoageing, immunosuppression and photocarcinogenesis.