The Integumentary System: Structure, Function, and Thermoregulation

The Integumentary System

Overview

The integumentary system is the body's largest organ system, primarily composed of the skin and its associated structures such as hair, nails, and glands. It serves as the first line of defense against environmental hazards and plays a crucial role in maintaining homeostasis.

• Main components: Skin, hair, nails, sweat glands, sebaceous (oil) glands

• Primary functions: Protection, sensation, thermoregulation, metabolic functions, excretion

Gidden's Concept 10: Thermoregulation

Definition and Importance

Thermoregulation is the process of maintaining core body temperature at a near constant value, which is essential for optimal body function. The hypothalamus acts as the body's thermostat, setting and regulating the temperature for proper physiological activity.

• Key mechanisms: Skin, glands, and blood vessels in the skin are central to temperature regulation.

• Compensatory responses: Vasodilation, vasoconstriction, sweating, and shivering.

• Clinical relevance: Disorders of thermoregulation can lead to conditions such as hyperthermia or hypothermia.

Additional info: The hypothalamus integrates signals from thermoreceptors and initiates responses to maintain temperature homeostasis.

Levels of Organization in the Integumentary System

Hierarchical Structure

The integumentary system is organized from the cellular level up to the organ system level, reflecting increasing complexity and specialization.

• Cells: Epithelial cells, fibroblasts, melanocytes, keratinocytes

• Tissues: Epidermis (epithelial tissue), dermis (connective tissue)

• Organs: Skin

• Organ System: Integumentary system (skin, hair, nails, glands)

Structure of the Skin (Integument)

Major Layers

The skin consists of two main regions: the epidermis and the dermis. Beneath these lies the hypodermis (subcutaneous layer), which is not technically part of the skin but shares some functions.

• Epidermis: Superficial layer, composed of keratinized stratified squamous epithelium

• Dermis: Underlies the epidermis, mostly fibrous connective tissue

• Hypodermis: Subcutaneous layer, primarily adipose tissue; absorbs shock, insulates, and anchors skin to underlying structures (mainly muscles)

Epidermis: Layers and Cell Types

Layers of the Epidermis

The epidermis is made up of four or five distinct layers, depending on the location (thick or thin skin):

• Stratum corneum: Outermost layer; dead, keratinized cells

• Stratum lucidum: Present only in thick skin (palms, soles); clear, dead cells

• Stratum granulosum: Cells begin keratinization; nuclei and organelles disintegrate

• Stratum spinosum: Several layers of keratinocytes held together by desmosomes; contains dendritic cells

• Stratum basale: Deepest layer; single row of stem cells, actively mitotic; contains melanocytes

Mnemonic: Come Let's Get Sun Burnt (Corneum, Lucidum, Granulosum, Spinosum, Basale)

Principal Cell Types in the Epidermis

• Keratinocytes: Most numerous; produce keratin for protection and waterproofing

• Melanocytes: Produce melanin pigment, which protects against UV radiation

• Dendritic (Langerhans) cells: Macrophages; key activators of the immune system

• Tactile (Merkel) cells: Sensory touch receptors; involved in light touch and texture discrimination

Dermis

Structure and Function

The dermis is a strong, flexible connective tissue layer beneath the epidermis. It contains fibroblasts, macrophages, mast cells, and white blood cells, as well as blood vessels, lymphatic vessels, hair follicles, and glands.

• Papillary layer: Areolar connective tissue with collagen and elastic fibers; contains dermal papillae (fingerprints)

• Reticular layer: Dense irregular connective tissue; provides strength, resiliency, and elasticity

Skin Color

Contributing Pigments

Skin color is determined by three main pigments:

• Melanin: Only pigment produced in the skin; protects against UV damage

• Carotene: Obtained from diet (e.g., carrots); precursor to vitamin A

• Hemoglobin: Oxygen-carrying pigment in red blood cells; contributes to pinkish hue

Clinical note: Local accumulations of melanin result in freckles and pigmented moles. Increased exposure to sunlight stimulates melanin production.

Appendages of the Skin

Hair

Hair consists of dead, keratinized cells and serves various protective and sensory functions.

• Types: Lanugo (fetal hair), vellus (fine body hair), terminal (coarse hair of scalp, eyebrows, and after puberty, axillary and pubic regions)

• Functions: Protection from trauma, heat loss, sunlight; sensory detection

• Pigments: Melanins (yellow, rust, brown, black); trichosiderin, pheomelanin, eumelanin; gray/white hair results from decreased melanin and increased air bubbles in shaft

Nails

Nails are scale-like modifications of the epidermis, providing protection for the distal, dorsal surfaces of fingers and toes.

• Composition: Hard keratin

• Nail matrix: Responsible for nail growth

Sweat Glands (Sudoriferous Glands)

Sweat glands are distributed over most skin surfaces and play a key role in thermoregulation.

• Eccrine sweat glands: Most abundant; found on palms, soles, forehead; secrete sweat (99% water, salts, vitamin C, antibodies, dermcidin, metabolic wastes); function in thermoregulation

• Apocrine sweat glands: Larger; confined to axillary and anogenital areas; ducts empty into hair follicles; begin functioning at puberty; may act as scent glands

• Modified apocrine glands: Ceruminous glands (ear wax), mammary glands (milk)

Sebaceous (Oil) Glands

These glands secrete sebum, an oily substance that lubricates and protects the skin and hair.

• Distribution: Widely distributed except in thick skin (palms, soles)

• Function: Softens hair and skin, bactericidal properties

• Regulation: Stimulated by hormones, especially androgens; become active at puberty

Functions of the Integumentary System

Protection

• Chemical barriers: Skin secretions (low pH, sebum, defensins), melanin (UV protection)

• Physical barriers: Stratum corneum blocks most water and water-soluble substances

• Biological barriers: Dendritic cells, macrophages

Body Temperature Regulation

• Insensible perspiration: ~500 ml/day under normal conditions

• Sensible perspiration: Increased sweat gland activity during heat; dermal vessel dilation

• Cold response: Dermal blood vessels constrict; skin temperature drops to slow heat loss

Other Functions

• Cutaneous sensation: Sensory receptors detect temperature, touch, pain

• Metabolic functions: Synthesis of vitamin D, activation of hormones, detoxification

• Blood reservoir: Up to 5% of body's blood volume in skin

• Excretion: Nitrogenous wastes and salt in sweat

Developmental Aspects and Aging

Changes Over the Lifespan

• Childhood to adulthood: Skin thickens, accumulates more subcutaneous fat; increased gland activity

• After age 30: Effects of environmental assaults accumulate; scaling and dermatitis more common

• Aging skin: Epidermal replacement slows, skin becomes thin, dry, and itchy; decreased elasticity and sebaceous gland activity; increased risk of cancer due to fewer melanocytes and dendritic cells; hair thinning and alopecia (baldness)

Prevention: UV protection, good nutrition, hydration, hygiene

Summary Table: Layers of the Skin

Layer	Location	Main Features
Stratum corneum	Outermost	Dead, keratinized cells; protection
Stratum lucidum	Only in thick skin	Clear, dead cells; extra protection
Stratum granulosum	Middle	Keratinization begins; water barrier
Stratum spinosum	Above basale	Desmosomes; dendritic cells; flexibility
Stratum basale	Deepest	Stem cells; melanocytes; mitosis

Key Equations

• Heat loss by evaporation (sweating):

$Q = mL$

Where $Q$ is the heat lost, $m$ is the mass of water evaporated, and $L$ is the latent heat of vaporization.