Skin and Body Membranes

Types of Body Membranes

Body membranes are thin layers of tissue that cover surfaces, line body cavities, and form protective sheets around organs. There are four main types:

• Cutaneous Membrane: Commonly known as the skin, this is the only dry membrane. It is exposed to air and functions as a protective barrier.

• Mucous Membranes: Line body cavities that open to the exterior (e.g., respiratory, digestive, urinary tracts). They are moist membranes adapted for absorption or secretion.

• Serous Membranes: Line body cavities that are closed to the exterior (e.g., thoracic and abdominal cavities). Each serous membrane has two layers:

  • Parietal Layer: Lines the cavity wall.

  • Visceral Layer: Covers the outside of the organs within the cavity.

  Serous fluid between these layers reduces friction.

• Synovial Membranes: Line the cavities of freely movable joints (e.g., knee, elbow). They secrete synovial fluid for lubrication.

Example: The pleura (lungs), pericardium (heart), and peritoneum (abdominal organs) are all serous membranes.

Vital Functions of the Skin

The skin, or cutaneous membrane, is the body's largest organ and serves several essential functions:

• Protection against mechanical, chemical, and biological damage.

• Prevents water loss and regulates body temperature.

• Synthesizes vitamin D when exposed to sunlight.

• Excretes wastes through sweat.

• Sensory reception for touch, pressure, pain, and temperature.

Structure of the Skin

Cells of the Epidermis

The epidermis is the outermost layer of the skin, composed of stratified squamous epithelium. It contains four main cell types:

• Keratinocytes: Produce keratin, a tough, protective protein that makes the skin waterproof.

• Melanocytes: Produce melanin, the pigment responsible for skin color and protection against UV radiation.

• Dendritic (Langerhans) Cells: Act as immune cells, detecting and fighting pathogens.

• Tactile (Merkel) Cells: Function as touch receptors, associated with sensory nerve endings.

Layers of the Skin

• Epidermis: Outermost layer, composed of stratified squamous epithelium. It is avascular (contains no blood vessels).

• Dermis: Deeper layer, made of connective tissue. It is vascular and provides strength and elasticity.

Layers of the Dermis

• Papillary Layer: Upper dermal region; contains dermal papillae (projections) that form fingerprints and contain capillaries and sensory receptors.

• Reticular Layer: Deeper dermal region; contains blood vessels, sweat and oil glands, and deep pressure receptors.

Five Layers (Strata) of the Epidermis

The epidermis is composed of five distinct layers (from deepest to most superficial):

1. Stratum Basale: Deepest layer; site of mitosis (cell division) and new cell production.

2. Stratum Spinosum: Cells begin to produce keratin and become spiny in appearance.

3. Stratum Granulosum: Cells flatten and accumulate keratin granules; organelles begin to disintegrate.

4. Stratum Lucidum: Present only in thick skin (palms, soles); provides extra protection.

5. Stratum Corneum: Outermost layer; consists of dead, fully keratinized cells that are continuously shed.

Example: The stratum basale is where new skin cells are generated, while the stratum corneum is the protective, outermost barrier.

Accessory Structures of the Skin

Hair Structure

• Shaft: The part of the hair that projects above the skin's surface.

• Root: The part embedded in the skin.

• Three Layers of Hair:

  • Medulla: Central core.

  • Cortex: Surrounds the medulla; provides strength and color.

  • Cuticle: Outermost layer; provides protection.

• Arrector Pili Muscles: Small muscles attached to hair follicles; contraction causes "goosebumps."

Nail Structure

• Nail Bed: Skin beneath the nail plate.

• Lunule: Crescent-shaped white area at the base of the nail.

• Cuticle: Fold of skin at the base of the nail.

• Matrix: Growth area of the nail; cells divide to produce new nail.

• Composition: Nails are composed of hard keratin.

Glands of the Skin

• Sudoriferous (Sweat) Glands:

  • Eccrine Glands: Most numerous; secrete watery sweat for temperature regulation.

  • Apocrine Glands: Found in axillary and genital areas; secrete a thicker, milky sweat that can produce body odor.

• Sebaceous (Oil) Glands: Secrete sebum (oil) into hair follicles; lubricates skin and hair, and has antibacterial properties.

Homeostatic Imbalances of the Skin

Infections and Allergies

• Bacterial Infections: e.g., impetigo, cellulitis.

• Viral Infections: e.g., cold sores, warts.

• Fungal Infections: e.g., athlete's foot.

• Allergic Reactions: e.g., contact dermatitis, eczema.

Skin Cancer

• Primary Cause: Overexposure to ultraviolet (UV) radiation from the sun or tanning beds.

• Three Types of Skin Cancer:

  • Basal Cell Carcinoma: Most common, least malignant; arises from stratum basale.

  • Squamous Cell Carcinoma: Arises from stratum spinosum; can metastasize if not removed.

  • Malignant Melanoma: Most dangerous; arises from melanocytes; high risk of metastasis.

• ABCDE Rule for recognizing melanoma:

  • Asymmetry: Two sides of the mole do not match.

  • Border irregularity: Edges are ragged or blurred.

  • Color: Pigmentation is not uniform.

  • Diameter: Greater than 6 mm (about the size of a pencil eraser).

  • Evolving: Changes in size, shape, or color.

• Metastasized: Refers to cancer that has spread to other parts of the body.

Burns

Immediate Threats from Burns

• Dehydration: Loss of body fluids can lead to shock.

• Infection: Loss of protective barrier increases risk of bacterial invasion.

Types of Burns

• First-Degree Burns: Affect only the epidermis; cause redness and pain.

• Second-Degree Burns: Affect the epidermis and upper dermis; cause blisters and more severe pain.

• Third-Degree Burns: Destroy the entire thickness of the skin; area appears gray-white or blackened; usually not painful initially due to nerve damage.

Rule of Nines

The Rule of Nines is a method used to estimate the percentage of body surface area affected by burns. The body is divided into regions, each representing approximately 9% (or multiples thereof) of the total body surface area.

Example: If both legs are burned, approximately 36% of the body surface area is affected.

Additional info: The Rule of Nines helps guide fluid resuscitation and treatment decisions in burn patients.