Integumentary System — revision sheet
The integumentary system — the skin together with hair, nails and cutaneous glands — is the largest organ system, forming a self-renewing barrier against microbes, ultraviolet radiation, water loss and mechanical injury. It also regulates body temperature, senses the environment, synthesises vitamin D and broadcasts signs of internal disease through its colour and texture.
Surface area (adult): ≈1.5–2 m²Mass: ≈16% of body weight (heaviest organ)Thickness: ≈0.5 mm (eyelid) to >4 mm (upper back)Epidermal renewal: ≈4 weeks (slower with age)Eccrine sweat glands: ≈2–4 millionMaximal sweat rate: ≈1–2 L/h (higher when heat-acclimatised)Skin blood flow: ≈0.3 L/min at rest → 6–8 L/min in heat stressScalp hairs: ≈100 000; 50–100 shed per dayScalp hair growth: ≈1 cm/month (≈0.35 mm/day)Nail growth: Fingernail ≈3 mm/month; toenail ≈1–1.5 mm/monthEpidermal melanin unit: 1 melanocyte : ≈36 keratinocytesSkin surface pH: ≈4.5–5.5 ("acid mantle")
Skin — organ & layers
- Skin Organ
- The skin is the body's largest organ: a two-layered sheet of epidermis (epithelium) and dermis (connective tissue) that covers the whole external surface and protects, senses, thermoregulates and synthesises vitamin D.
- Epidermis Tissue
- The epidermis is the avascular, keratinised stratified squamous epithelium forming the outer layer of the skin; it renews itself about every four weeks and provides the body's main barrier.
- Dermoepidermal junction Membrane
- The dermoepidermal junction is the undulating basement membrane zone that binds the epidermis to the dermis through hemidesmosomes, laminin, collagen IV and anchoring fibrils.
- Dermis Tissue
- The dermis is the connective-tissue layer of the skin beneath the epidermis, providing tensile strength and elasticity and housing blood vessels, lymphatics, nerves, sensory receptors and skin appendages.
- Epidermal (friction) ridges Organ part
- Epidermal (friction) ridges are the parallel surface ridges of thick skin on the palms, soles and digits, formed over dermal ridges in fetal life; they create unique, permanent fingerprints and improve grip and touch.
- Hypodermis Tissue
- The hypodermis (subcutaneous tissue) is the layer of loose connective and adipose tissue beneath the dermis that anchors the skin, stores energy, insulates and cushions underlying structures.
Epidermal cells
- Keratinocyte Cell
- Keratinocytes are the dominant (≈90%) cells of the epidermis; they proliferate in the basal layer, fill with keratin as they move outward and die to form the protective stratum corneum.
- Melanocyte Cell
- Melanocytes are neural crest-derived dendritic cells in the stratum basale that synthesise melanin in melanosomes and transfer it to surrounding keratinocytes, determining skin and hair colour and protecting against UV.
- Langerhans cell Cell
- Langerhans cells are antigen-presenting dendritic cells of the epidermis that capture antigens and migrate to regional lymph nodes to activate T cells; they contain characteristic Birbeck granules.
Cutaneous receptors
- Merkel cell Receptor
- Merkel cells are neuroendocrine cells in the stratum basale that, with the disc-shaped nerve terminal they contact (the Merkel disc), form slowly adapting type I mechanoreceptors for sustained pressure, edges and texture.
- Tactile (Meissner) corpuscle Receptor
- Tactile (Meissner) corpuscles are small encapsulated, rapidly adapting mechanoreceptors in the dermal papillae of glabrous skin that detect light touch, flutter and slip.
- Lamellar (Pacinian) corpuscle Receptor
- Lamellar (Pacinian) corpuscles are large, onion-like encapsulated mechanoreceptors in the deep dermis and hypodermis that adapt very rapidly and detect high-frequency vibration and deep pressure changes.
- Ruffini ending Receptor
- Ruffini endings are spindle-shaped, slowly adapting encapsulated mechanoreceptors in the deep dermis and joint capsules that respond to skin stretch and sustained deformation.
- Free nerve endings Receptor
- Free nerve endings are unencapsulated terminals of thin Aδ and C fibres that branch through the dermis and into the epidermis to detect pain, temperature, itch and crude touch.
Hair & nails
- Hair Organ part
- A hair is a flexible keratinised filament produced by a hair follicle, consisting of a medulla, cortex and cuticle; hairs cover almost the entire skin except palms, soles, lips and parts of the genitalia.
- Hair follicle Organ part
- The hair follicle is a tubular invagination of epidermis into the dermis that produces a hair from its bulb, houses stem cells in its bulge and, with its sebaceous gland and arrector pili, forms the pilosebaceous unit.
- Arrector pili muscle Muscle
- The arrector pili is a small bundle of smooth muscle running from the papillary dermis to the bulge of a hair follicle; its sympathetic contraction erects the hair ("goose bumps") and squeezes sebum from the sebaceous gland.O Papillary layer of the dermis (just beneath the epidermis).I Connective tissue sheath of the hair follicle at the bulge, below the sebaceous gland.A Pulls the hair erect (piloerection), dimples the skin and compresses the sebaceous gland.N Sympathetic postganglionic adrenergic fibres (noradrenaline on α1-adrenoceptors).
- Nails Organ part
- Nails are plates of hard keratin produced by the nail matrix on the dorsal surface of the distal phalanges; they protect the fingertips, aid fine manipulation and are a classic window on systemic disease.
Cutaneous glands
- Sweat glands Gland
- Sweat glands are coiled tubular exocrine glands of the skin of two main types: widespread eccrine glands that produce watery sweat for cooling, and apocrine glands of the axillae and anogenital region that produce a protein-rich secretion.
- Eccrine sweat gland Gland
- Eccrine sweat glands are simple coiled tubular merocrine glands, distributed over almost the whole skin, that secrete hypotonic watery sweat onto the surface for evaporative cooling.
- Apocrine sweat gland Gland
- Apocrine sweat glands are large coiled tubular glands of the axillae, areolae and anogenital skin that empty into hair follicles and, from puberty, secrete a viscous, protein-rich fluid that bacteria convert into body odour.
- Sebaceous glands Gland
- Sebaceous glands are holocrine, branched acinar glands that usually open into hair follicles and secrete sebum, an oily mixture that lubricates hair and skin and contributes to the barrier.
- Ceruminous glands Gland
- Ceruminous glands are modified apocrine sweat glands in the skin of the outer (cartilaginous) external acoustic meatus whose secretion, mixed with sebum and shed keratinocytes, forms cerumen (earwax).
Blood supply & physiology
- Cutaneous vascular plexuses Vessel
- The skin is supplied by two horizontal networks of vessels — a deep plexus at the dermis–hypodermis junction and a superficial subpapillary plexus — whose flow far exceeds the skin's metabolic needs because it serves thermoregulation.
- Skin colour and pigmentation Physiological process
- Skin colour results from three pigments — melanin produced by melanocytes, dietary carotene, and haemoglobin in dermal blood — modified by skin thickness and blood flow.
- Skin barrier and protection Physiological process
- The skin's protective barrier combines a physical layer (stratum corneum lipids and tight junctions), a chemical layer (acid pH, antimicrobial peptides, sebum), a microbial layer (commensals) and an immune layer (Langerhans cells, mast cells, T cells).
- Thermoregulation by the skin Physiological process
- The skin is the main effector organ of temperature regulation: under hypothalamic control it varies blood flow and sweating to lose or conserve heat, keeping core temperature near 37 °C.
- Cutaneous vitamin D synthesis Physiological process
- In the epidermis, ultraviolet B light converts 7-dehydrocholesterol into vitamin D3 (cholecalciferol), which the liver and kidney then hydroxylate to the hormone calcitriol that controls calcium and phosphate absorption.
- Epidermal renewal and keratinisation Cell process
- The epidermis replaces itself roughly every four weeks as basal stem-cell progeny differentiate, keratinise and die to form the stratum corneum, which is then shed.
- Cutaneous wound healing Physiological process
- Skin wounds heal through four overlapping phases — haemostasis, inflammation, proliferation and remodelling — ending in a collagen scar that never regains the full strength of normal skin.
Mnemonics
- Epidermal strata, superficial → deep
- "Come, Let's Get Sun Burned" — stratum Corneum, Lucidum (thick skin only), Granulosum, Spinosum, Basale.
- Melanoma warning signs
- ABCDE — Asymmetry, Border irregularity, Colour variation, Diameter >6 mm, Evolving (changing size, shape, colour, or new symptoms such as bleeding or itch).
- Rule of nines (adult)
- Head 9, each arm 9, front of trunk 18 (2 × 9), back of trunk 18, each leg 18, perineum 1 = 100%.
- Cutaneous mechanoreceptors
- "Meissner = Moving touch, Merkel = Maintained pressure, Pacinian = Pulsing vibration, Ruffini = stRetch." Meissner and Pacinian are rapidly adapting; Merkel and Ruffini are slowly adapting.
- Hair growth cycle
- Anagen = Active growth (years), Catagen = Collapse/regression (weeks), Telogen = Time-out/resting (months), then eXogen = eXit (shedding).
- Sebaceous vs sweat secretion
- Sebaceous = Self-destructive (holocrine — whole cell becomes secretion); eccrine = Exocytosis (merocrine).
From the Corpus anatomy atlas — for education only.