Integumentary System: Skin, Hair, and Nails

• The integumentary system consists of the skin, hair, nails, and associated glands
• It is the body's largest organ system by surface area and weight
• The skin has three major layers: the epidermis, dermis, and hypodermis
• The epidermis is the outermost layer, composed of keratinized stratified squamous epithelium
• It is avascular, meaning it lacks blood vessels
• The epidermis has four or five layers, depending on the region of the body: stratum basale, stratum spinosum, stratum granulosum, stratum lucidum (only in thick skin), and stratum corneum
• Keratinocytes, melanocytes, Langerhans cells, and Merkel cells are the four main cell types in the epidermis
• Keratinocytes produce keratin, a fibrous protein that provides protection
• Melanocytes produce melanin, a pigment that protects against UV radiation
• Langerhans cells are immune cells that protect against pathogens
• Merkel cells are touch receptors

Dermis

• The dermis is the middle layer of the skin, composed of dense irregular connective tissue
• It is vascularized and contains sensory receptors, hair follicles, and glands
• The dermis has two layers: the papillary layer and the reticular layer
• The papillary layer is the superficial layer, containing dermal papillae that project into the epidermis
• The reticular layer is the deeper layer, containing collagen and elastic fibers that provide strength and elasticity

Hypodermis

• The hypodermis, also known as the subcutaneous layer, is the deepest layer of the skin
• It is composed of areolar and adipose tissue
• The hypodermis provides insulation, cushioning, and energy storage
• It connects the skin to underlying tissues and organs

Functions of the Integumentary System

• Protection: The skin acts as a barrier against physical, chemical, and biological damage
• Thermoregulation: The skin helps regulate body temperature through sweating and blood flow
• Sensation: The skin contains sensory receptors that detect touch, pressure, pain, and temperature
• Vitamin D synthesis: The skin synthesizes vitamin D when exposed to sunlight
• Excretion: The skin excretes small amounts of waste products through sweat
• Absorption: The skin can absorb certain substances, such as medications

Skin Color

• Melanin, carotene, and hemoglobin are the three pigments that contribute to skin color
• Melanin is produced by melanocytes and ranges in color from yellow to brown to black
• Carotene is a yellow-orange pigment found in some vegetables and fruits
• Hemoglobin is the red pigment in red blood cells

Hair

• Hair is a filamentous structure made of keratinized cells
• It protects the scalp from UV radiation and provides insulation
• Hair follicles are located in the dermis
• Hair has three parts: the hair shaft, hair root, and hair bulb

Nails

• Nails are hard, keratinized plates that protect the fingers and toes
• They consist of the nail plate, nail bed, and nail matrix
• The nail matrix is responsible for nail growth

Glands

• Sebaceous glands secrete sebum, an oily substance that lubricates the skin and hair
• Sweat glands secrete sweat, which helps regulate body temperature
• There are two types of sweat glands: eccrine and apocrine
• Eccrine sweat glands are found all over the body and secrete watery sweat
• Apocrine sweat glands are found in the armpits and groin and secrete thicker sweat that contains proteins and fats

Aging and the Integumentary System

• With age, the skin becomes thinner, drier, and less elastic
• Collagen and elastin fibers in the dermis decrease
• Melanocyte activity decreases, leading to paler skin
• Hair follicles produce thinner, less pigmented hair
• The number of sweat glands decreases, making it harder to regulate body temperature

Skin Disorders

• Acne: A common skin condition caused by clogged hair follicles
• Eczema: A chronic inflammatory skin condition
• Psoriasis: A chronic autoimmune skin condition that causes scaly, red patches
• Skin cancer: The most common type of cancer, often caused by UV radiation

Thermoregulation

• The skin plays a crucial role in thermoregulation, maintaining a stable internal body temperature
• When body temperature rises, blood vessels in the skin dilate, increasing blood flow to the surface
• This allows heat to dissipate into the environment
• Sweat glands also secrete sweat, which evaporates and cools the skin
• When body temperature falls, blood vessels in the skin constrict, reducing blood flow to the surface
• This conserves heat
• Shivering, a form of muscle contraction, generates heat

Wound Healing

• Wound healing is the process of repairing damaged skin
• It involves several stages: inflammation, proliferation, and remodeling
• Inflammation is the initial response, characterized by redness, swelling, and pain
• Proliferation involves the growth of new tissue
• Remodeling involves the reorganization of collagen fibers
• Factors that affect wound healing include age, nutrition, and infection

Sensory Receptors

• The skin contains a variety of sensory receptors that detect different stimuli
• Meissner's corpuscles are sensitive to light touch
• Pacinian corpuscles are sensitive to deep pressure and vibration
• Merkel cells are associated with fine touch and pressure
• Free nerve endings detect pain and temperature

Vitamin D Synthesis

• The skin synthesizes vitamin D3 (cholecalciferol) when exposed to ultraviolet (UV) radiation from sunlight
• This process begins when UV radiation converts 7-dehydrocholesterol in the skin into vitamin D3
• Vitamin D3 then undergoes further processing in the liver and kidneys to become the active form of vitamin D, calcitriol
• Vitamin D is essential for calcium absorption and bone health
• It also plays a role in immune function and other physiological processes

Excretion and Absorption

• The skin excretes small amounts of waste products through sweat glands
• Water, salts, urea, and ammonia are among the substances excreted
• The skin can also absorb certain substances, such as medications applied topically
• Absorption depends on factors such as the size and lipid solubility of the substance
• Some medications are designed to be absorbed through the skin for localized or systemic effects

Clinical Significance

• The integumentary system provides valuable diagnostic information
• Changes in skin color, such as jaundice or cyanosis, can indicate underlying medical conditions
• Skin lesions, rashes, and other abnormalities can be signs of infection, allergy, or disease
• Biopsies of skin tissue can be used to diagnose skin cancer and other disorders

Homeostasis

• The integumentary system plays a vital role in maintaining homeostasis, the body's stable internal environment
• By regulating body temperature, protecting against pathogens, and synthesizing vitamin D, the skin helps to ensure the proper functioning of other organ systems
• The skin also communicates with other systems through sensory receptors and the release of hormones and other signaling molecules.