Integumentary System Overview
The skin — the body's largest organ and first defense.
The integumentary system is the skin plus its appendages (hair, nails, and glands). The skin is the body's largest organ and its first line of defense against the outside world.
Functions of the Skin
- Protection
- Barrier against microbes, chemicals, UV light, and water loss
- Temperature regulation
- Sweating and adjusting blood flow
- Sensation
- Nerve receptors detect touch, pressure, pain, and temperature
- Vitamin D synthesis
- UV light starts vitamin D production
- Excretion
- Small amounts of water and salts via sweat
The Three Layers
- Epidermisouter
- Thin, avascular barrier; keratin & melanin
- Dermismiddle
- Blood vessels, nerves, hair follicles, glands, collagen
- Subcutaneoushypodermis
- Fat for insulation, cushioning, energy storage
Intact skin is a powerful barrier against infection — so any break (a wound, IV site, surgical incision, or pressure injury) is a potential entry point for bacteria. Understanding the skin's layers and how it heals is the basis of wound care, pressure-injury prevention, and burn management.
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The Epidermis
The thin outer barrier.
The epidermis is the outermost layer. It is avascular (has no blood vessels) and gets its nutrients from the dermis below. It is constantly shedding dead cells from the top while new cells are made at the bottom.
- Keratinocytes — make keratin, a tough protein that waterproofs and protects.
- Melanocytes — make melanin, the pigment that colors skin and shields against UV damage.
- The tough outer surface (stratum corneum) is the actual barrier; the deepest layer (stratum basale) constantly produces new cells.
Because the epidermis has no blood supply of its own, a shallow scrape that stays in the epidermis bleeds little and heals quickly, while deeper wounds that reach the vascular dermis bleed more and need more support to heal. Skin color changes (pallor, cyanosis, jaundice) are easiest to assess where skin is thin.
The Dermis
The thick, living middle layer.
The dermis is the thick layer beneath the epidermis. It is made of connective tissue with collagen (strength) and elastin (stretch), and it contains most of the skin's working parts:
- Blood vessels — nourish the skin and help control body temperature.
- Nerve endings — sense touch, pressure, pain, and temperature.
- Hair follicles and glands (oil and sweat).
Because the dermis holds the blood vessels and nerves, wounds that reach it bleed and are painful — and partial-thickness burns (into the dermis) are extremely painful and may blister. With age, collagen and elastin decline, so older adults' skin becomes thin, fragile, and slow to heal.
The Subcutaneous Layer (Hypodermis)
Insulation, cushioning, and energy storage.
The subcutaneous layer (also called the hypodermis) lies beneath the dermis. It is mostly fat (adipose tissue) and connective tissue. Its jobs:
- Insulates the body and helps conserve heat
- Cushions and protects deeper structures from injury
- Stores energy as fat
- Anchors the skin to the muscle and bone beneath
This layer is the target for subcutaneous (sub-Q) injections like insulin and heparin, because it has fewer blood vessels and absorbs medication slowly and steadily. Deeper intramuscular injections go below it, into muscle, for faster absorption.
Hair, Nails & Glands
The skin's appendages.
- Hair
- Protection, sensation, and helps with temperature regulation
- Nails
- Protect fingertips/toes; nail bed color reflects circulation/oxygenation
- Sebaceous (oil) glands
- Secrete sebum to lubricate skin and hair and slow water loss
- Eccrine sweat glands
- Widespread; produce watery sweat for cooling
- Apocrine sweat glands
- In armpits/groin; activate at puberty (body odor)
Nail beds are an easy place to check circulation: press the nail and watch the color return (capillary refill — normal is under 2 seconds). Bluish nail beds suggest poor oxygenation, and clubbing can indicate chronic low oxygen.
Functions of the Skin in Depth
How the skin protects and regulates the body.
Temperature Regulation
- Too hot: dermal blood vessels dilate (release heat) and sweat glands produce sweat that cools as it evaporates.
- Too cold: vessels constrict (conserve heat) and tiny muscles raise the hairs (goosebumps).
Barrier, Sensation & Vitamin D
- Barrier: intact skin keeps microbes out and water in.
- Sensation: nerve receptors warn of pain, pressure, and temperature.
- Vitamin D: UV light starts vitamin D production, later activated by the liver and kidneys to help absorb calcium.
Burns and large wounds destroy the skin barrier, leading to fluid loss, heat loss, and a high infection risk. That's why major burns are managed with aggressive fluids, warmth, and strict infection control — it follows directly from what the skin normally does.
Wounds & Pressure Injuries
How skin breaks down and heals.
Phases of Wound Healing
Healing is supported by good blood flow, protein, vitamin C, and zinc, and slowed by poor circulation, diabetes, smoking, infection, and poor nutrition.
Pressure Injury Stages
- Stage 1
- Intact skin, non-blanchable redness
- Stage 2
- Partial-thickness loss; shallow open ulcer or blister
- Stage 3
- Full-thickness loss into subcutaneous fat
- Stage 4
- Full-thickness loss exposing muscle, tendon, or bone
Pressure injuries form over bony areas (sacrum, heels, hips) when pressure cuts off blood flow. The nursing priority is prevention: reposition at least every 2 hours, keep skin clean and dry, relieve pressure, and support nutrition and hydration.
? A Stage 2 pressure injury has partial-thickness skin loss. Which two layers of the skin could be involved?
Clinical Applications
Connecting the anatomy and physiology to the bedside.
A&P at the Bedside
- Skin assessment — inspect color, temperature, moisture, turgor, and integrity; check pressure points.
- Capillary refill — press a nail bed; normal color return is under 2 seconds.
- Injection layers — subcutaneous (insulin, heparin) vs intramuscular (faster absorption).
- Burn severity — estimated by depth (superficial, partial-, full-thickness) and the percent of body surface burned (the "rule of nines").
What to Watch For
- Pallor (pale) — poor perfusion or anemia
- Cyanosis (bluish) — low oxygen
- Jaundice (yellow) — liver or bile problems
- Poor turgor (tenting) — dehydration
You don't need to memorize every skin disorder in A&P — you need to know the skin's layers and its jobs (barrier, temperature, sensation, vitamin D). Once you know that intact skin keeps microbes out and that the dermis holds the blood and nerves, wound care, pressure-injury staging, and burn management all make sense.
Frequently Asked Questions
Common student questions on this topic.
What are the three layers of the skin?
What does the epidermis do?
How does skin help make vitamin D?
How does the skin help regulate body temperature?
What are the stages of a pressure injury?
What are the phases of wound healing?
Quick-Reference Cheatsheet
Every key point from this guide on a single printable page.
Integumentary System Cheatsheet
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