Integumentary System

Main Functions of Skin

The skin is the largest organ of the body and serves multiple essential functions for protection and homeostasis.

• Protection: Acts as a barrier against mechanical injury, pathogens, and harmful substances.

• Sensory: Contains receptors for touch, pain, temperature, and pressure.

• Vitamin D Synthesis: Initiates production of vitamin D when exposed to UV light.

• Thermoregulation: Regulates body temperature through sweat and blood flow.

• Excretion: Removes waste products via sweat.

• Water Balance: Prevents excessive water loss and maintains hydration.

Major Layers of the Skin

The skin consists of two primary layers, each with distinct tissue types and functions.

• Epidermis: Composed of epithelial tissue (keratinized stratified squamous epithelium).

• Dermis: Made of connective tissue (loose and dense irregular connective tissue).

• Hypodermis: Subcutaneous layer containing adipose tissue and loose connective tissue (not always considered part of the skin).

Tissue Types in Skin Layers

Each layer of the skin contains specific tissue types that contribute to its function.

• Epidermis: Keratinized stratified squamous epithelium.

• Dermis: Loose connective tissue (papillary layer) and dense irregular connective tissue (reticular layer).

• Hypodermis: Adipose tissue and loose connective tissue.

Layers of the Epidermis

The epidermis is organized into five distinct layers, each with unique characteristics.

1. Stratum corneum: Outermost layer; dead, flat cells; barrier function.

2. Stratum granulosum: Keratin begins forming; cells are dying.

3. Stratum lucidum: Only present in thick skin (palms/soles); clear layer.

4. Stratum spinosum: Spiny cells; strength; some mitosis.

5. Stratum basale: Deepest layer; mitotic cells; produces new skin cells; contains melanocytes.

Cell Types in the Epidermis

Several specialized cell types are found in the epidermis, each with specific roles.

• Keratinoctyes: Found in all layers; produce keratin for strength and waterproofing.

• Melanocytes: Located in the stratum basale; produce melanin for UV protection.

• Dendritic (Langerhans) cells: Found in stratum spinosum; immune defense.

• Tactile (Merkel) cells: Located in stratum basale; function as touch receptors.

Regeneration of Skin Cells

The stratum basale is responsible for the continuous regeneration of skin cells.

• Stratum basale: Constantly divides to replace lost skin cells; dead cells form new layers.

Dermis Structure

The dermis is divided into two layers, each with distinct connective tissue types and functions.

• Papillary layer: Thin, superficial, loose connective tissue; contains dermal papillae (fingerprints).

• Reticular layer: Thicker, deeper, dense irregular connective tissue.

Factors Affecting Skin Color

Skin color is determined by genetic, environmental, and physiological factors.

• Melanin: Main pigment; protects against UV radiation.

• Genetic factors: Albinism (all white), hemoglobin, erythema, pallor.

• Environmental factors: Freckles, moles, carotene.

Markings of the Skin

Skin markings include epidermal ridges, cleavage lines, and flexure lines, each with clinical significance.

• Epidermal ridges: Fingerprints; improve grip.

• Cleavage lines: Direction of collagen fibers; incisions along these lines heal faster and with less scarring.

• Flexure lines: Permanent folds/creases at joints; help skin bend without tearing (e.g., ankles, wrists).

Glands of the Skin

The skin contains several types of glands, each with specific secretions and locations.

• Sudoriferous (sweat) glands:

  • Eccrine glands: Secrete sweat for cooling and waste removal; found all over skin, more in palms, soles, forehead.

  • Apocrine glands: Secrete more sweat, active at puberty (armpits/genitals); found in dermis.

  • Ceruminous glands: Secrete earwax (cerumen) in external acoustic canal.

  • Mammary glands: Secrete milk.

• Sebaceous (oil) glands: Secrete sebum to lubricate skin and hair; attached to hair follicles.

Sensory Receptors in the Dermis

The dermis contains specialized receptors for detecting various stimuli.

• Free nerve endings: Detect pain and temperature.

• Meissner corpuscles: Detect light touch.

• Pacinian corpuscles: Detect deep pressure and vibration.

Homeostatic Imbalances of the Skin

Several conditions can affect skin homeostasis, leading to visible changes.

• Striae: Stretch marks.

• Jaundice: Yellow skin from liver issues.

• Cyanosis: Blue skin from low oxygen.

• Tanning: Darkening from UV exposure.

• Acne: Inflammation of sebaceous glands.

Types of Skin Cancer

Skin cancer arises from different cell types and varies in severity and prognosis.

Burns: Causes and Degrees

Burns are injuries to the skin caused by heat, chemicals, electricity, or UV radiation. They are classified by depth and severity.

• First-degree: Affects epidermis only; red and painful.

• Second-degree: Affects epidermis and part of dermis; blisters (fluid-filled bubbles).

• Third-degree: Destroys epidermis and dermis; skin may be white or brown; numbness due to nerve destruction.

Example: Sunburn is typically a first-degree burn, while severe burns from fire or chemicals may be second or third degree.