🫁Honors Anatomy and Physiology Unit 5 – Integumentary System

Overview and Structure

• Integumentary system consists of skin, hair, nails, and associated glands
• Skin is the largest organ in the body covering an area of 1.5-2.0 square meters in adults
• Composed of two main layers: epidermis (outer layer) and dermis (inner layer)
• Epidermis is avascular (lacks blood vessels) and relies on diffusion from the dermis for nutrients
• Dermis is highly vascularized (contains blood vessels) and provides support, nutrients, and sensation to the skin
• Subcutaneous layer (hypodermis) lies beneath the dermis and consists mainly of adipose tissue (fat) for insulation and energy storage
• Skin thickness varies depending on location (eyelids: 0.5mm, palms and soles: 4mm)

Layers of the Skin

• Epidermis is the outermost layer of the skin composed of keratinized stratified squamous epithelium
  • Consists of five sublayers (strata): basale, spinosum, granulosum, lucidum (only in thick skin), and corneum
  • Stratum basale contains stem cells that continuously divide to replenish cells lost from the surface
• Dermis is the inner layer of the skin composed of connective tissue (collagen and elastic fibers)
  • Divided into two regions: papillary dermis (upper) and reticular dermis (lower)
  • Papillary dermis contains dermal papillae that increase surface area for nutrient exchange and anchoring of the epidermis
  • Reticular dermis is thicker and provides strength and elasticity to the skin
• Dermo-epidermal junction (DEJ) is a basement membrane that separates the epidermis from the dermis
  • Consists of hemidesmosomes and anchoring fibrils that prevent the layers from separating (shearing)

Functions of the Integumentary System

• Protection against external factors (UV radiation, pathogens, chemicals, mechanical damage)
  • Keratin in the epidermis provides a waterproof barrier and mechanical protection
  • Melanin pigment in the epidermis absorbs UV radiation and protects against DNA damage
  • Langerhans cells in the epidermis are part of the immune system and help defend against pathogens
• Temperature regulation through sweat production, vasodilation, and vasoconstriction
• Sensation via various receptors (touch, pressure, temperature, pain) in the dermis
• Excretion of water, salts, and organic compounds through sweat glands
• Synthesis of vitamin D3 in the epidermis upon exposure to UV radiation
• Aesthetics and communication through changes in color (blushing, pallor) and texture (goosebumps)

Accessory Structures

• Hair is a keratinized filament that grows from hair follicles in the dermis
  • Consists of a shaft (visible part) and a root (within the follicle)
  • Hair growth occurs in cycles (anagen, catagen, telogen) regulated by hormones and genetics
• Nails are keratinized plates that cover the dorsal surface of the distal phalanges of fingers and toes
  • Consist of a nail plate, nail bed, nail matrix (site of nail growth), and cuticle (protective seal)
• Sweat glands are coiled tubular glands that produce sweat for thermoregulation and excretion
  • Two types: eccrine (most numerous, open directly onto skin surface) and apocrine (limited distribution, open into hair follicles)
• Sebaceous glands are holocrine glands that secrete sebum (oily substance) into hair follicles to lubricate and waterproof the skin and hair
  • Sebum also has antimicrobial properties and maintains an acidic pH on the skin surface

Homeostasis and Thermoregulation

• Skin plays a crucial role in maintaining homeostasis, particularly in regulating body temperature
• Thermoregulation involves a balance between heat production and heat loss
  • Heat is produced by metabolism and muscle activity
  • Heat is lost through radiation, conduction, convection, and evaporation
• In hot conditions, the skin helps cool the body through vasodilation (increased blood flow to the skin) and sweating (evaporation of sweat)
• In cold conditions, the skin helps conserve heat through vasoconstriction (decreased blood flow to the skin) and piloerection (raised hair follicles, "goosebumps")
• Skin also helps maintain fluid and electrolyte balance through the controlled release of sweat

Wound Healing and Repair

• Wound healing is a complex process involving multiple overlapping stages: hemostasis, inflammation, proliferation, and remodeling
  • Hemostasis: clotting cascade activated to stop bleeding and form a provisional matrix for cell migration
  • Inflammation: neutrophils and macrophages clean the wound and release growth factors
  • Proliferation: fibroblasts produce collagen and other extracellular matrix components, epithelial cells migrate to cover the wound (re-epithelialization)
  • Remodeling: collagen fibers reorganize, scar tissue forms, tensile strength of the skin increases
• Factors affecting wound healing include age, nutrition, oxygenation, infection, and underlying medical conditions (diabetes, immunodeficiency)
• Chronic wounds (pressure ulcers, diabetic foot ulcers) may fail to progress through the normal stages of healing due to repeated trauma, ischemia, or infection

Common Disorders and Diseases

• Acne: chronic inflammatory condition of the pilosebaceous unit (hair follicle and associated sebaceous gland) characterized by comedones (blackheads and whiteheads), papules, pustules, and nodules
• Atopic dermatitis (eczema): chronic inflammatory skin condition characterized by pruritic (itchy), erythematous (red), and scaly patches
• Psoriasis: autoimmune disorder characterized by the rapid proliferation of keratinocytes, resulting in thick, scaly plaques
• Skin cancer: malignant neoplasms arising from the epidermis (melanoma, basal cell carcinoma, squamous cell carcinoma) or dermis (dermatofibrosarcoma protuberans)
  • Risk factors include UV exposure, fair skin, family history, and immunosuppression
• Infections: bacterial (cellulitis, impetigo), viral (herpes simplex, varicella-zoster), fungal (tinea, candidiasis), and parasitic (scabies, lice) infections can affect the skin

Clinical Applications and Relevance

• Skin assessment is an essential component of the physical examination and can provide valuable diagnostic clues for systemic diseases
  • Changes in color (jaundice, cyanosis), texture (xerosis, lichenification), and integrity (ulcers, fissures) may indicate underlying pathology
• Dermatologic procedures include biopsy (punch, shave, excisional), cryotherapy, electrosurgery, and laser therapy for the diagnosis and treatment of skin lesions
• Topical medications (creams, ointments, gels) are commonly used to deliver drugs directly to the skin for local effects (corticosteroids, antibiotics, retinoids)
• Cosmetic procedures (chemical peels, microdermabrasion, botulinum toxin injections, dermal fillers) are used to improve the appearance of the skin and treat signs of aging
• Skin substitutes (autografts, allografts, xenografts, synthetic materials) are used to promote wound healing and provide coverage for large wounds or burns
• Sunscreen and protective clothing are important for preventing skin damage and reducing the risk of skin cancer