Chapter 7

Chapter 7 — Skin and Body Membranes

Section: Body Membranes

(Whole → Tissue → Cellular → Mechanism → Pathophysiology connections)

Before learning the specific membranes, reconnect to something you already studied.

Earlier we learned:

cells
↓
tissues
↓
organs
↓
organ systems

Membranes are important because they form protective interfaces between body structures.

They help:

• protect organs

• reduce friction

• regulate fluid movement

• allow communication between tissues

So membranes act as protective and functional barriers throughout the body.

Big Picture: What Body Membranes Are

Body membranes are thin layers of tissue that cover surfaces, line cavities, and protect organs.

They act as boundaries and protective linings.

Think of membranes as biological coverings that separate environments.

Example:

external environment
↓
skin membrane
↓
internal body tissues

or

body cavity
↓
serous membrane
↓
organ surface

Membranes are classified into two major categories based on their tissue composition.

Classification of Body Membranes

There are two major groups:

This classification connects directly to the tissue types we studied earlier.

1. Epithelial Membranes

Epithelial membranes contain:

epithelium
+
connective tissue

Remember when we learned epithelial tissue functions:

• protection

• absorption

• secretion

• filtration

That explains why epithelial membranes appear in areas that interact with external or internal environments.

There are three major epithelial membranes.

Cutaneous Membrane (Skin)

The cutaneous membrane is simply the skin.

Structure:

epidermis (epithelial tissue)
↓
dermis (connective tissue)

The epidermis forms the protective outer layer.

The dermis provides:

• strength

• blood supply

• sensory receptors

Functions of Skin

Skin performs several critical functions.

Skin is actually the largest organ in the body.

It protects the body from:

• pathogens

• dehydration

• physical injury

Serous Membranes

Serous membranes line internal body cavities that do not open to the outside.

These membranes are made of:

simple squamous epithelium
+
connective tissue

This epithelium is extremely thin, allowing smooth movement of organs.

Serous membranes produce serous fluid, which acts as a lubricant.

Two Layers of Serous Membranes

Serous membranes always occur in two layers.

Between these layers is serous fluid.

This fluid reduces friction when organs move.

Example:

lung expansion
↓
lungs slide smoothly against chest wall

Without this fluid, breathing would be painful.

Major Serous Membranes

Three important serous membranes exist.

Pleura

The pleura surrounds the lungs.

Structure:

parietal pleura
↓
pleural cavity (fluid)
↓
visceral pleura
↓
lung

Function:

Allows lungs to expand and contract smoothly during breathing.

Peritoneum

The peritoneum surrounds abdominal organs.

Structure:

parietal peritoneum
↓
peritoneal cavity
↓
visceral peritoneum
↓
abdominal organs

This membrane protects organs such as:

• stomach

• intestines

• liver

It also allows them to move during digestion without friction.

Pericardium

The pericardium surrounds the heart.

Structure:

parietal pericardium
↓
pericardial cavity
↓
visceral pericardium
↓
heart

This membrane prevents friction as the heart beats.

Serous Membrane Diseases

When serous membranes become inflamed, friction increases.

Pleurisy

Pleurisy is inflammation of the pleural membranes.

Mechanism:

pleural inflammation
↓
fluid imbalance
↓
membranes rub during breathing
↓
sharp chest pain

This pain worsens with breathing.

Peritonitis

Peritonitis is inflammation of the peritoneum.

Causes include:

• infection

• ruptured appendix

• abdominal injury

Mechanism:

infection enters abdominal cavity
↓
inflammation spreads
↓
fluid accumulates
↓
abdominal pain and rigidity

This condition can become life-threatening.

Mucous Membranes

Mucous membranes line body surfaces that open to the outside environment.

Examples include:

• digestive tract

• respiratory tract

• urinary tract

• reproductive tract

These membranes secrete mucus.

Function of Mucus

Mucus protects tissues by:

• trapping microbes

• trapping particles

• keeping surfaces moist

Example in the respiratory tract:

dust enters airway
↓
mucus traps particles
↓
cilia move mucus upward
↓
particles expelled

This mechanism protects the lungs.

2. Connective Tissue Membranes

Unlike epithelial membranes, these membranes contain only connective tissue.

They occur primarily in joint structures.

Synovial Membranes

Synovial membranes line the cavities of movable joints.

They produce synovial fluid.

Function of Synovial Fluid

Synovial fluid acts as a lubricant.

Mechanism:

joint movement
↓
synovial fluid reduces friction
↓
cartilage protected

This allows smooth movement of joints.

Synovial Structures

Synovial membranes also line bursal sacs.

Bursae are small fluid-filled sacs that reduce friction between:

• tendons

• muscles

• bones

Pathophysiology Connection

Inflammation of synovial membranes leads to joint disorders.

Example:

Arthritis.

Mechanism:

synovial inflammation
↓
excess fluid production
↓
joint swelling
↓
pain and stiffness

Big Picture Connections

Membranes are crucial because they:

They act as interfaces between tissues and environments.

Chapter 7 — Skin Structure

(Whole → Tissue → Cellular → Molecular → Pathophysiology connections)

Before diving into the layers, reconnect to what you already learned.

Earlier you studied:

• epithelial tissue → protection

• connective tissue → support

• inflammation → defense

• homeostasis → maintaining internal balance

The skin integrates all of those mechanisms.

The skin is not just a covering. It is an organ system interface that:

• protects the body

• regulates temperature

• prevents infection

• allows sensation

• maintains fluid balance

Think of the skin as the body’s first line of defense.

Big Structure of Skin

The skin has two main layers.

Epidermis (outer protective barrier)
↓
Dermis (supportive, functional layer)

Below the skin lies another structure that supports it.

Subcutaneous layer (hypodermis)

Important point:

The hypodermis is not technically part of the skin, but it plays a major supportive role.

1. Epidermis

Whole Concept

The epidermis is the outermost protective layer of skin.

It is composed of stratified squamous epithelium, which you learned earlier is specialized for protection against abrasion and pathogens.

Structure:

multiple layers of epithelial cells
↓
outer layers keratinized
↓
protective barrier forms

Epidermal Cell Production

At the base of the epidermis is a layer called the stratum germinativum.

This is where new skin cells are produced.

Mechanism:

basal cells divide by mitosis
↓
new cells pushed upward
↓
cells fill with keratin
↓
cells die and flatten
↓
dead cells form surface barrier

This process takes about 3–4 weeks.

Keratin

Keratin is a tough waterproof protein.

Remember when we studied proteins earlier?

Proteins determine cell structure and function.

Keratin makes the skin:

• strong

• waterproof

• resistant to pathogens

The outermost layer of keratinized cells is called the stratum corneum.

This layer constantly flakes off and is replaced.

Skin Pigmentation

Skin color depends largely on melanin.

Melanin is produced by specialized cells called melanocytes.

Location:

melanocytes
↓
basal layer of epidermis

These cells produce melanin and distribute it to nearby skin cells.

Melanin serves an important function:

protecting DNA from ultraviolet radiation.

Skin Color Changes

Skin color can change due to several physiological factors.

Increased Blood Flow

When blood flow increases, skin appears pink or flushed.

Example:

exercise or inflammation.

Cyanosis

Cyanosis is a bluish discoloration of the skin.

Mechanism:

low oxygen in blood
↓
hemoglobin becomes darker
↓
skin appears blue

This is a clinical sign of hypoxia.

Vitiligo

Vitiligo occurs when melanocytes are destroyed.

Mechanism:

melanocytes lost
↓
melanin not produced
↓
patches of pale skin appear

Vitiligo is often related to autoimmune mechanisms.

Dermal-Epidermal Junction

Between the epidermis and dermis lies a junction layer.

Think of it as the glue that holds the layers together.

The dermis forms small projections called dermal papillae that interlock with the epidermis.

Purpose:

• stabilize the skin layers

• increase nutrient exchange

Blisters

Blisters form when the connection between these layers breaks.

Mechanism:

friction or burn
↓
epidermis separates from dermis
↓
fluid fills space
↓
blister forms

2. Dermis

The dermis is the deeper and thicker skin layer.

Unlike the epidermis, which is epithelial tissue, the dermis is primarily connective tissue.

Functions include:

• strength

• elasticity

• sensory detection

• blood supply

Two Layers of Dermis

Papillary Layer

The upper portion of the dermis.

Contains dermal papillae, which form patterns that create fingerprints.

Functions:

• improve grip

• increase surface contact

• enhance sensory perception

Reticular Layer

The deeper dermal layer.

Contains a network of strong collagen fibers and elastic fibers.

Remember earlier when we studied connective tissue proteins?

• collagen → strength

• elastin → flexibility

These fibers allow skin to stretch and return to its original shape.

Aging and Wrinkles

With age, elastic fibers decrease.

Mechanism:

elastic fibers decline
↓
skin loses elasticity
↓
wrinkles develop

Stretch Marks (Striae)

Stretch marks occur when skin stretches faster than connective tissue can adapt.

Mechanism:

rapid stretching
↓
collagen fibers tear
↓
elongated marks appear

Common causes include:

• pregnancy

• rapid weight gain

• growth spurts

Structures Found in the Dermis

The dermis contains many functional structures:

• nerve endings

• blood vessels

• hair follicles

• sweat glands

• sebaceous glands

These structures allow skin to act as a sensory and regulatory organ.

Subcutaneous Layer (Hypodermis)

Below the dermis lies the hypodermis.

Composition:

loose connective tissue
+
adipose tissue

Functions include:

• cushioning organs

• storing fat

• insulating the body

• allowing skin to move over muscles

This layer also provides energy storage.

Hair

Hair develops from hair follicles located in the epidermis and dermis.

Hair formation occurs at the hair papilla.

Mechanism:

hair cells divide
↓
cells keratinize
↓
hair shaft forms

Hair has two major parts:

Hair Loss

Hair loss is called alopecia.

Causes may include:

• aging

• hormonal changes

• disease

• chemotherapy

Arrector Pili Muscle

Each hair follicle connects to a tiny muscle called the arrector pili.

When this muscle contracts:

muscle contracts
↓
hair stands upright
↓
"goosebumps" appear

This response is part of thermoregulation.

Nails

Nails are produced by epidermal cells.

They are made of hard keratin.

Main structures include:

Nail Changes and Health

Nail color can indicate health status.

Examples:

Skin Receptors

The skin contains sensory receptors.

These receptors allow the skin to function as a sensory organ.

Examples include:

These receptors connect to the nervous system.

Skin Glands

Two major types of glands exist in the skin.

Sweat Glands

Sweat glands are also called sudoriferous glands.

They help regulate body temperature.

Eccrine Sweat Glands

These are the most numerous.

Mechanism:

body temperature rises
↓
sweat glands release sweat
↓
evaporation cools skin

This process helps maintain thermal homeostasis.

Apocrine Sweat Glands

These glands are located primarily in:

• armpits

• genital region

Their secretion is thicker.

When bacteria break down the secretion, body odor develops.

Sebaceous Glands

Sebaceous glands produce sebum, an oily secretion.

Functions of sebum:

• lubricates skin

• prevents drying

• protects hair

Acne

Acne occurs when sebaceous ducts become blocked.

Mechanism:

excess sebum production
↓
duct blockage
↓
bacteria multiply
↓
inflammation develops

This leads to acne vulgaris.

Big Mechanism Connections

The skin integrates multiple systems:

This is why the skin is considered a multifunctional organ.

Chapter 7 — Functions of the Skin

(Whole → Tissue → Cellular → Mechanism → Clinical reasoning)

Before learning the functions, reconnect this to something important you already studied.

Earlier you learned:

• epithelial tissue → protection

• connective tissue → structural support

• nervous tissue → sensation

• circulatory system → temperature regulation

The skin actually combines all of those mechanisms.

That is why the skin is considered a multifunctional organ system interface.

Big Picture: What the Skin Does

The skin performs five major functions:

Each function depends on specific skin structures and physiological mechanisms.

1. Protection — The First Line of Defense

The most important function of skin is protection.

Remember when we studied the immune system and infection?

Before the immune system even responds, the skin acts as a physical barrier.

external environment
↓
skin barrier
↓
internal body protected

Protection Against Microbes

The epidermis prevents microorganisms from entering the body.

Mechanism:

keratinized epidermis
↓
tightly packed cells
↓
microbes cannot penetrate

This is why cuts and wounds increase infection risk.

Once the barrier is broken, microbes can enter tissues.

Protection Against UV Radiation

The skin protects deeper tissues from ultraviolet radiation.

This protection depends on melanin.

Remember earlier:

melanocytes produce melanin
↓
melanin absorbs UV radiation
↓
DNA damage reduced

Without melanin protection, UV radiation can cause mutations that lead to skin cancer.

Protection Against Chemicals

The keratinized epidermis also protects against chemical exposure.

Keratin forms a waterproof barrier that limits penetration of many substances.

However, some chemicals can still penetrate the skin, which is why protective gloves are often used in healthcare and laboratories.

Protection Against Physical Injury

The skin protects the body from:

• cuts

• abrasions

• tearing

This protection depends on the dermis, which contains strong connective tissue fibers.

Remember the proteins we discussed earlier:

• collagen → strength

• elastin → flexibility

These proteins give the skin mechanical resistance.

Bruising

Bruising occurs when small blood vessels break under the skin.

Mechanism:

trauma damages capillaries
↓
blood leaks into surrounding tissue
↓
hemoglobin breaks down
↓
skin discoloration appears

As hemoglobin breaks down, the bruise changes color.

Skin Grafts

Severe burns or injuries may destroy large areas of skin.

Because the skin is essential for protection and fluid balance, damaged skin may need to be replaced.

This is done with a skin graft.

A skin graft involves:

• transplanting healthy skin to damaged areas

Skin grafts help restore:

• protective barrier

• fluid regulation

• infection resistance

2. Temperature Regulation

The body must maintain a stable internal temperature.

This is part of homeostasis.

The skin plays a major role in regulating temperature.

The skin can release up to 3,000 calories of heat per day.

Two main mechanisms control temperature.

Sweat Production

Sweat glands release water onto the skin surface.

Mechanism:

body temperature rises
↓
hypothalamus activates sweat glands
↓
sweat released onto skin
↓
evaporation removes heat

Evaporation cools the body.

This is one of the most effective cooling mechanisms.

Blood Flow Regulation

The skin contains many blood vessels.

These vessels can widen or narrow to regulate heat loss.

Vasodilation

When body temperature rises:

blood vessels dilate
↓
more blood flows near skin surface
↓
heat released to environment

This causes flushed skin.

Vasoconstriction

When body temperature drops:

blood vessels constrict
↓
less blood flows to skin
↓
heat conserved in body core

This helps maintain body temperature during cold exposure.

3. Sensation

The skin contains many sensory receptors connected to the nervous system.

These receptors allow the body to detect environmental changes.

Examples of sensations detected include:

• touch

• pressure

• pain

• temperature

Mechanism:

stimulus touches skin
↓
receptor activated
↓
nerve impulse generated
↓
signal travels to brain

This allows the brain to interpret sensory information.

Example:

Touching a hot surface triggers rapid withdrawal due to pain receptors.

4. Excretion

The skin also removes small amounts of waste through sweat.

Sweat contains:

• water

• salts

• small amounts of waste products

Examples include:

• urea

• ammonia

• uric acid

Although the kidneys perform most waste removal, the skin contributes slightly.

5. Vitamin D Synthesis

The skin is responsible for producing vitamin D.

Vitamin D is essential for:

• calcium absorption

• bone health

Mechanism:

UV radiation from sunlight
↓
skin converts precursor molecules
↓
vitamin D formed
↓
vitamin D activated in liver and kidneys

Without adequate vitamin D, calcium absorption decreases.

This can lead to bone disorders such as rickets or osteoporosis.

Big Concept Connections

The skin connects to several organ systems.

This is why the skin is considered an integrated organ system interface.

Chapter 7 — Conditions of the Skin

(Whole → Tissue → Cellular → Mechanism → Clinical reasoning)

Before breaking down specific conditions, connect this section to what you already learned.

Earlier we studied:

• epithelial tissue → protection

• connective tissue → structural support

• inflammation → immune response

• infection → pathogen invasion

Skin conditions usually develop when one of these mechanisms fails.

Most skin disorders follow this pattern:

skin barrier disrupted
↓
inflammation or infection begins
↓
tissue damage develops
↓
visible lesion forms

Understanding lesions and burns helps clinicians recognize disease mechanisms early.

1. Skin Lesions

A skin lesion is any visible change from normal skin structure.

Lesions are important because they help identify:

• infections

• inflammatory disorders

• cancers

• trauma

Lesions are classified based on shape and appearance.

Elevated Lesions

These lesions rise above the skin surface and cast a shadow.

They usually occur when:

• inflammation causes swelling

• fluid accumulates

• cells proliferate

Papule

A papule is a small raised lesion.

Example:

• acne

• insect bites

Mechanism:

localized inflammation
↓
cells and fluid accumulate
↓
small raised bump forms

Plaque

A plaque is a larger raised lesion.

Example:

psoriasis plaques.

These occur when skin cells grow rapidly and accumulate on the surface.

Vesicle

A vesicle is a small blister filled with clear fluid.

Mechanism:

epidermal cells damaged
↓
fluid accumulates between layers
↓
blister forms

Examples include:

• burns

• herpes infections

Pustule

A pustule contains pus.

Pus contains:

• white blood cells

• bacteria

• dead tissue

Example:

infected acne lesions.

Crust (Scab)

A crust forms when:

wound occurs
↓
blood and fluid dry
↓
protective scab forms

The scab protects the wound during healing.

Wheal (Hive)

A wheal is a raised swelling with a pale center.

Mechanism:

allergic reaction
↓
histamine release
↓
blood vessels leak fluid
↓
localized swelling forms

Hives are common in allergic reactions.

Flat Lesions

Flat lesions do not rise above the skin.

Macule

A macule is a flat discoloration of the skin.

Example:

freckles.

These lesions occur due to pigment changes, not swelling.

Depressed Lesions

These lesions sink below the surrounding skin.

They occur when tissue has been lost or destroyed.

Excoriation

Excoriation is loss of the epidermis due to scratching.

Mechanism:

mechanical injury
↓
epidermis removed
↓
raw skin exposed

Ulcer

An ulcer is a deep crater-like lesion.

Mechanism:

tissue destruction
↓
epidermis and dermis lost
↓
deep wound forms

Examples include:

• pressure ulcers

• diabetic ulcers

Fissure

A fissure is a deep crack in the skin.

These often occur in dry or inflamed skin.

Example:

cracked heels.

Skin Repair

The skin can repair itself through cell regeneration.

Mechanism:

injury occurs
↓
basal epidermal cells divide
↓
new cells migrate to surface
↓
wound heals

This process connects to what you learned earlier about cell mitosis.

2. Burns

Burns damage skin tissue and disrupt homeostasis.

The severity of burns depends on:

• depth of tissue injury

• body surface area involved

• risk of infection

• fluid loss

Partial-Thickness Burns

These burns affect only part of the skin.

First-Degree Burns

Damage only the epidermis.

Example:

sunburn.

Symptoms:

• redness

• mild pain

• no blistering

Mechanism:

heat damages epidermal cells
↓
inflammation develops
↓
redness and pain occur

Second-Degree Burns

Damage the epidermis and upper dermis.

Symptoms:

• blistering

• severe pain

• swelling

Blisters form because fluid accumulates between skin layers.

Full-Thickness Burns

These burns destroy the entire skin layer.

Third-Degree Burns

Damage both epidermis and dermis completely.

Important feature:

The burned area may initially feel no pain.

Why?

Because nerve endings are destroyed.

However, surrounding tissues still experience pain.

Major risks include:

• infection

• fluid loss

• shock

Fourth-Degree Burns

These burns extend deeper into:

• muscle

• bone

These are life-threatening injuries.

Rule of Nines

Doctors estimate burn severity using the rule of nines.

The body is divided into sections representing 9% of body surface area.

This helps determine:

• treatment strategy

• fluid replacement needs

3. Skin Infections

Skin infections occur when pathogens penetrate the skin barrier.

Impetigo

Impetigo is a bacterial infection caused by:

• Staphylococcus

• Streptococcus

It is highly contagious and common in children.

Symptoms include:

• crusty lesions

• redness

• itching

Tinea (Fungal Infection)

Tinea is a fungal infection of the skin.

Examples include:

• ringworm

• athlete’s foot

Fungi grow best in warm, moist environments.

Warts

Warts are benign growths caused by human papillomavirus (HPV).

The virus stimulates abnormal epithelial cell growth.

Boils (Furuncles)

Boils occur when hair follicles become infected with bacteria.

Mechanism:

bacteria infect follicle
↓
inflammation develops
↓
pus-filled lesion forms

Scabies

Scabies is a parasitic infection caused by mites.

The mites burrow into the skin, causing intense itching.

4. Vascular and Inflammatory Skin Disorders

Some skin diseases occur due to circulatory problems or chronic inflammation.

Decubitus Ulcers (Bedsores)

These ulcers develop when prolonged pressure reduces blood flow to skin.

Mechanism:

pressure compresses blood vessels
↓
oxygen supply decreases
↓
tissue ischemia occurs
↓
skin tissue dies

Bedsores commonly occur in patients who remain immobile for long periods.

Urticaria (Hives)

Hives result from allergic reactions.

Mechanism:

allergen exposure
↓
histamine release
↓
blood vessels leak fluid
↓
raised itchy lesions appear

Scleroderma

Scleroderma involves hardening of skin and connective tissue.

This occurs due to abnormal collagen production.

Psoriasis

Psoriasis is a chronic inflammatory skin disease.

Mechanism:

immune system overactivation
↓
skin cells divide too rapidly
↓
thick scaly plaques form

Eczema

Eczema is an inflammatory skin reaction, often linked to allergies.

Symptoms include:

• redness

• itching

• blisters

• crusting

5. Skin Cancer

Skin cancer occurs when DNA damage causes uncontrolled cell growth.

The most important risk factor is ultraviolet radiation from sunlight.

Types of Skin Cancer

Basal Cell Carcinoma

The most common type.

Originates in basal epidermal cells.

Characteristics:

• slow growing

• rarely spreads

Squamous Cell Carcinoma

Develops from squamous epithelial cells.

More aggressive than basal cell carcinoma.

Often appears as raised hardened tumors.

Melanoma

Melanoma is the most dangerous form of skin cancer.

It develops from melanocytes.

Melanoma spreads rapidly through the bloodstream and lymphatic system.

Early detection is critical.

Kaposi Sarcoma

Kaposi sarcoma is associated with viral infection and immune suppression.

It produces purple lesions on the skin.

It is commonly seen in patients with AIDS.

Big Mechanism Connections

Skin diseases usually involve one or more mechanisms:

infection
↓
inflammation
↓
cell injury
↓
tissue damage

or

DNA mutation
↓
abnormal cell growth
↓
tumor formation

Understanding these mechanisms helps clinicians identify the cause of skin conditions and choose appropriate treatment.