Skin: protection, sensation and temperature regulation

The epidermis and dermis perform different jobs

Skin consists mainly of epidermis and dermis, with subcutaneous tissue beneath. The outer epidermal stratum corneum contains mature cells that have lost their nuclei, together with surrounding lipids. This organisation limits water loss and entry of external substances. Deeper cells proliferate and differentiate, move outward and eventually shed. Renewal occurs continuously rather than replacing the entire surface at one moment. OpenStax A&P: layers of the skin

The epidermis has no blood vessels of its own. Its living cells receive nutrients mainly by diffusion from underlying tissues. The dermis contains collagen, elastic fibres, vessels, nerves and many accessory structures, providing support and elasticity. Subcutaneous fat contributes cushioning, insulation and energy storage. These connected layers are not interchangeable. A superficial abrasion and an injury entering the dermis can differ in bleeding, sensation and repair. Structure also varies by location, so the palm, eyelid and back should not be interpreted using identical proportions. OpenStax A&P: layers of the skin

Barrier function depends on cells, lipids and local conditions

The stratum corneum is sometimes compared with a brick wall: cells provide bricks and intercellular lipids help seal the spaces. The real barrier also includes cellular junctions, chemical conditions and immune surveillance. It allows selected substances through under particular conditions rather than acting as completely impermeable plastic. Sweat and sebaceous openings have specific roles; pores are not adjustable valves for removing every bodily waste. OpenStax A&P: layers of the skin; OpenStax A&P: accessory structures of the skin

Sebum lubricates skin and hair, while sweat mainly supports cooling. Surface microbes interact with the host environment, so they are neither all contaminants nor better simply when more numerous. Barrier disruption can increase water loss and access of irritants to sensitive tissues, linking dryness, stinging and inflammation. A clean-looking surface therefore does not establish a well-functioning barrier. Excessive removal of surface components can change protective conditions, while the cause of discomfort still depends on exposures, location and duration. OpenStax A&P: accessory structures of the skin; OpenStax A&P: functions of the integumentary system

Sensation combines receptors with neural processing

Cutaneous receptors respond to deformation, temperature and potentially damaging events. Receptor types differ in receptive fields and adaptation: some emphasise changing stimuli, while others provide more sustained information. Fine discrimination at the fingertips compared with broader localisation on the back involves receptor distribution and central processing, not simply skin thickness. Converting a physical event into neural activity is sensory transduction. OpenStax A&P: layers of the skin; OpenStax A&P: functions of the integumentary system

The skin does not deliver an unprocessed photograph to the brain. Signals from different places are integrated, and attention, experience and context influence the resulting experience. Pain combines sensory and emotional dimensions and cannot be explained by injured surface area alone. Damaged nerves may actually reduce sensation, so greater pain does not necessarily indicate deeper injury. A similar touch can feel different when someone expects safety versus when they are highly alert. That difference can reflect processing rather than pretence. OpenStax A&P: functions of the integumentary system; IASP: Pain terminology

Sweat cools effectively when it evaporates

When internal heat production or environmental heat increases, regulation can increase skin blood flow, moving heat toward the surface, while promoting sweating. Radiation, conduction, convection and evaporation exchange heat according to temperature, airflow and other conditions. When the environment is hotter than skin, some pathways bring heat into the body. Vasodilation therefore does not guarantee successful cooling in every setting. OpenStax A&P: functions of the integumentary system; OpenStax A&P: energy and heat balance

Evaporation requires energy and is the important cooling step for sweat. High humidity restricts evaporation, so liquid dripping off the body does not represent an equivalent amount of useful heat loss. Sweating also removes water and electrolytes while circulation must support skin and other organs. Two equally sweaty people can have different heat balances if one is in dry moving air and the other in humid still air. Compare heat production, environmental gain and actual loss rather than inferring core temperature or heat tolerance from wetness. OpenStax A&P: functions of the integumentary system; OpenStax A&P: energy and heat balance

Pigmentation, defence and repair have limits

Melanocytes produce melanin and transfer it to surrounding keratinocytes, helping absorb some ultraviolet radiation and protect cellular structures. Skin-colour differences involve pigment amount, distribution and type, rather than absence of melanocytes in particular groups. Protection is incomplete, and ultraviolet exposure can still damage DNA. Skin participation in vitamin D synthesis does not make unlimited sunlight exposure a health goal. OpenStax A&P: layers of the skin; OpenStax A&P: functions of the integumentary system

After injury, haemostasis, inflammation, proliferation and remodelling interact. Deeper injuries may form scars whose organisation differs from the original tissue. Healing depends on blood supply, nutrition, cellular capacity and local conditions. Surface closure does not establish complete restoration of tissue strength. Inflammation contributes to repair, but persistent dysregulation can impede it. For a skin change, record colour, integrity, temperature, sensation and timing separately. Specific observations are more informative than labelling the person a fast healer or assuming weak immunity. OpenStax A&P: diseases disorders and injuries of the integumentary system

Connect a surface observation with the full mechanism

Consider three cases: dry skin stings after a cleanser, skin becomes flushed and sweaty during exercise, and a deeper injured region has reduced sensation. The first directs attention to barrier disruption and irritation, the second to circulation and heat transfer, and the third to sensory pathways. All occur in skin, but one vague cause cannot explain them. They are mechanistic hypotheses requiring further evidence. OpenStax A&P: layers of the skin; OpenStax A&P: functions of the integumentary system; OpenStax A&P: diseases disorders and injuries of the integumentary system

Now evaluate two claims: no bleeding means no skin damage, and heavy sweating means body temperature must be falling. The first overlooks the avascular epidermis; the second overlooks evaporation conditions and continuing heat production. Use structure and energy accounting to identify the missing steps and needed observations. Epidermal barrier function, dermal circulation and sensation each provide distinct information. A normal finding in one does not replace assessment of the others, showing why skin is a physiological organ rather than merely an appearance concern. OpenStax A&P: layers of the skin; OpenStax A&P: energy and heat balance

Bilingual terms

表皮 · Epidermis

The outer epithelial layer of skin.

真皮 · Dermis

The skin layer containing connective tissue, vessels and nerves.

角质层 · Stratum corneum

The outer epidermal layer contributing to barrier function.

感觉转导 · Sensory transduction

Conversion of physical or chemical stimuli into neural signals.

蒸发 · Evaporation

Conversion of liquid to vapour requiring energy.

Sources and further reading

• OpenStax A&P: layers of the skin
• OpenStax A&P: accessory structures of the skin
• OpenStax A&P: functions of the integumentary system
• OpenStax A&P: diseases disorders and injuries of the integumentary system
• IASP: Pain terminology
• OpenStax A&P: energy and heat balance

Original course source-check record: 9 September 2026. Full Chinese and English sentence-by-sentence language review: 14 September 2026. AI editing and language review are not human clinical review. Linked institutions have not participated in or endorsed this course.