11.5 Skin System: Structure, Thermoregulation & Protection

The Largest Organ

The integumentary system — skin plus hair, nails and glands — covers about $1.8\text{ m}^2$ in an adult and is the interface at which homeostasis meets the outside world. The AAMC lists it as a full organ system alongside the circulatory and digestive systems, with subtopics spanning structure, homeostasis and osmoregulation, thermoregulation, physical protection and hormonal control.

Structure

Epidermis

Stratified squamous keratinized epithelium (see 8.3), avascular and nourished by diffusion from the dermis. Cells are produced at the bottom and die on the way up.

Four resident cell types:

• Keratinocytes (~90%) — produce keratin, the tough intermediate filament (see 8.2).
• Melanocytes — in the stratum basale; synthesize melanin from tyrosine and transfer it in melanosomes to keratinocytes, where it caps nuclei and absorbs UV. Skin color reflects melanin activity and distribution, not melanocyte number.
• Langerhans cells — dendritic antigen-presenting cells; the immune sentinel of the epidermis (see 10.5).
• Merkel cells — associated with sensory nerve endings for light touch.

Full epidermal turnover takes roughly four weeks; accelerated turnover with retained nuclei produces the scaling plaques of psoriasis.

Dermis

Connective tissue beneath the basement membrane, in two layers:

• Papillary layer — loose areolar tissue thrown into dermal papillae that interlock with the epidermis (forming fingerprint ridges) and carry capillary loops, Meissner's corpuscles (light touch) and free nerve endings (pain, temperature).
• Reticular layer — dense irregular connective tissue of interwoven type I collagen and elastic fibers giving strength and elasticity, plus Pacinian corpuscles (deep pressure, vibration), Ruffini endings (stretch), hair follicles, glands and blood vessels.

Hypodermis (Subcutaneous Layer)

Not technically part of the skin: adipose and areolar tissue anchoring skin to underlying fascia and serving as thermal insulation, shock absorption and energy store (see 5.4).

Appendages

• Eccrine (merocrine) sweat glands — distributed over nearly the whole body; secrete a dilute watery solution of $\text{NaCl}$, urea and lactate directly onto the surface; the primary thermoregulatory effector.
• Apocrine sweat glands — axillae and groin; secrete a protein- and lipid-rich fluid into hair follicles beginning at puberty; odor arises from bacterial degradation.
• Sebaceous glands — secrete oily sebum into hair follicles, lubricating hair and lowering surface pH.
• Hair follicles with arrector pili smooth muscle; nails of hard keratin.

Why Skin Is Waterproof

The barrier is not the corneocytes alone. In the stratum granulosum, lamellar bodies extrude a hydrophobic mortar of ceramides, cholesterol and free fatty acids into the space between cells. The result is often described as a brick-and-mortar arrangement: keratin-packed corneocyte bricks in lipid mortar. This is what limits transepidermal water loss and makes the skin the first line of osmoregulation — and why extensive burns cause catastrophic fluid loss and hypovolemic shock.

Thermoregulation

Core temperature is defended near $37,^\circ\text{C}$ by a classic negative-feedback loop. Peripheral thermoreceptors in the skin and central receptors in the hypothalamic preoptic area report temperature; the hypothalamus compares this to a set point and drives effectors through the autonomic and somatic systems.

Heat Loss Responses (core temperature rising)

• Cutaneous vasodilation. Sympathetic tone to dermal arterioles falls, blood is shunted into the superficial plexus, and heat is lost by radiation, conduction and convection. This is the same capillary-bed heat-exchange function introduced in 10.2.
• Sweating. Eccrine glands secrete onto the surface, and evaporation removes heat as latent heat of vaporization — about $2400\text{ J}$ per gram of water. Efficiency collapses at high ambient humidity because the vapor-pressure gradient disappears.
• Behavioral responses and reduced muscle tone.

Autonomic exception worth memorizing. Eccrine sweat glands are innervated by sympathetic fibers that are cholinergic, releasing acetylcholine onto muscarinic receptors rather than norepinephrine. This is one of the few sympathetic postganglionic cholinergic synapses in the body and a standard passage discriminator (see 9.2).

Heat Retention and Production (core temperature falling)

• Cutaneous vasoconstriction — sympathetic $\alpha_1$-mediated; shunts blood to the core and can make skin appear pale. Countercurrent heat exchange between deep arteries and veins conserves core heat in the limbs.
• Piloerection — arrector pili contract, raising hairs to trap an insulating air layer. Vestigial in humans (goosebumps) but effective in furred mammals.
• Shivering thermogenesis — somatic motor drive produces rapid involuntary skeletal muscle contraction whose inefficiency releases heat (see 11.3).
• Non-shivering thermogenesis — brown adipose tissue uncoupling protein 1 (thermogenin) dissipates the proton gradient as heat (see 4.3); important in neonates.
• Insulating subcutaneous fat reduces conductive loss.
• Endocrine support: thyroid hormone raises basal metabolic rate over days to weeks, and epinephrine raises it acutely.

Hormonal Control of Cutaneous Function

Beyond autonomic control, aldosterone acts on the eccrine duct to reabsorb $\text{Na}^+$ from forming sweat — which is why heat-acclimatized individuals produce more dilute sweat and lose less salt. Antidiuretic hormone defends plasma volume against sweat loss (see 10.6), and androgens drive sebaceous gland activity at puberty.

Protection, Sensation and Synthesis

Physical protection. Keratinized stratified squamous epithelium resists abrasion; calluses are adaptive local thickening of the stratum corneum in response to repeated friction; nails protect digit tips and improve fine manipulation; hair shields the scalp from UV and traps particulates at body orifices.

Chemical and biological barrier. Sebum and sweat produce an acid mantle near pH 5, inhibiting bacterial colonization; defensins and lysozyme are secreted onto the surface; Langerhans cells initiate adaptive responses; melanin absorbs UV and prevents thymine-dimer formation, linking directly to the nucleotide excision repair pathway and xeroderma pigmentosum of Section 6.2.

Sensation. The receptors listed above make skin the largest sensory organ, feeding the somatic afferent pathways of Section 9.2.

Vitamin D synthesis. UVB photons convert epidermal 7-dehydrocholesterol to cholecalciferol (vitamin $\text{D}_3$). The liver adds a 25-hydroxyl group and the kidney adds a 1-hydroxyl under PTH stimulation to yield active calcitriol, which raises intestinal calcium absorption (see 11.4). The skin is therefore an endocrine organ, not merely a barrier — and this three-organ pathway is a favorite integrated-passage subject.