Chapter 6: The Integumentary System

Overview

The integumentary system is composed of the skin and its associated structures, including hair, nails, and glands. It serves as the body's primary barrier against the external environment and plays a crucial role in protection, sensation, thermoregulation, and homeostasis.

Key Terms and Definitions

• Keratin: A tough, fibrous protein found in the epidermis, hair, and nails, providing structural strength and waterproofing.

• Melanin: A pigment produced by melanocytes that gives skin, hair, and eyes their color and protects against UV radiation.

• Carotene: A yellow-orange pigment stored in the skin, contributing to skin color and serving as a precursor to vitamin A.

• Papilla: A small, nipple-like projection; in hair, the dermal papilla supplies nutrients to the growing hair.

• Collagen: A structural protein abundant in the dermis, providing strength and elasticity to the skin.

• Flexure lines: Natural skin folds found at joints, where the dermis is tightly attached to underlying structures.

Structure of the Skin

Layers of the Skin

The skin consists of two main layers, each with distinct structures and functions:

• Epidermis: The outermost layer, composed primarily of epithelial cells. It provides a waterproof barrier and creates our skin tone.

• Dermis: The deeper layer, made up of connective tissue, containing blood vessels, nerves, hair follicles, and glands.

• Subcutaneous tissue (Hypodermis): Located beneath the dermis, this layer consists mainly of adipose tissue and provides insulation and cushioning. Additional info: The hypodermis is not technically part of the skin but is closely associated with its functions.

Tissue Types in the Skin

• Epidermis: Composed of keratinized stratified squamous epithelium.

• Dermis: Contains dense irregular connective tissue (mainly collagen and elastin fibers), as well as blood vessels, nerves, and various cell types.

• Hypodermis: Primarily adipose tissue and some areolar connective tissue.

Layers of the Epidermis

Structure, Location, and Function

The epidermis is organized into several layers (from deepest to most superficial):

• Stratum basale (germinativum): Deepest layer; contains stem cells that divide to produce new keratinocytes and melanocytes.

• Stratum spinosum: Provides strength and flexibility; cells begin to accumulate keratin.

• Stratum granulosum: Cells undergo apoptosis; keratinization begins.

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

• Stratum corneum: Outermost layer; consists of dead, flattened keratinocytes that form a protective barrier.

Cell Types in the Epidermis:

• Keratinocytes: Produce keratin; most abundant cell type.

• Melanocytes: Produce melanin pigment.

• Langerhans cells: Immune cells involved in defense.

• Merkel cells: Sensory receptors for touch.

Layers and Functions of the Dermis

• Papillary layer: Thin, superficial layer; contains dermal papillae that increase surface area for exchange of nutrients and waste with the epidermis.

• Reticular layer: Thicker, deeper layer; composed of dense irregular connective tissue, providing strength and elasticity.

Cell Types in the Dermis: Fibroblasts (produce collagen and elastin), macrophages, mast cells.

Skin Color

Factors Contributing to Skin Color

• Melanin: Amount and type determine skin color; increased production leads to darker skin.

• Carotene: Accumulates in the stratum corneum and hypodermis, imparting a yellow-orange hue.

• Hemoglobin: Oxygenated blood in dermal capillaries gives a pinkish tint to fair skin.

• Additional info: Other factors include genetic variation, exposure to sunlight, and certain medical conditions.

Hair Structure and Function

Parts of the Hair and Hair Follicle

• Shaft: The visible part of the hair above the skin surface.

• Root: The portion of hair below the skin surface.

• Bulb: The enlarged base of the hair follicle; contains the hair matrix and papilla.

• Matrix: Region of actively dividing cells responsible for hair growth.

• Sheath: Surrounds the hair root and follicle, providing protection and support.

Medulla, Cortex, Shaft/Root: The medulla is the innermost layer, the cortex provides strength and color, and the shaft/root refers to the location of the hair relative to the skin surface.

Arrector Pili Muscle

• Function: Smooth muscle attached to hair follicles; contraction causes hair to stand up (goosebumps) in response to cold or emotional stimuli.

• Functional Relationship: Helps trap air for insulation and can signal emotional states.

Nails

Structure of Nails

• Nail plate: Visible, hard part of the nail composed of keratin.

• Nail bed: Skin beneath the nail plate.

• Matrix: Area of nail growth located under the cuticle.

• Lunula: Crescent-shaped whitish area at the base of the nail.

Glands of the Skin

Sudoriferous (Sweat) Glands

• Eccrine glands: Widely distributed; secrete watery sweat for thermoregulation.

• Apocrine glands: Located in axillary and genital areas; secrete thicker, milky sweat that can produce odor when broken down by bacteria.

Sebaceous (Oil) Glands

• Location: Associated with hair follicles; found throughout the skin except palms and soles.

• Secretion: Produce sebum, an oily substance that lubricates and waterproofs skin and hair.

Comparison of Gland Types

Functions of the Skin and Homeostasis

Major Functions

• Protection: Acts as a barrier against pathogens, chemicals, and physical injury.

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

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

• Excretion: Removes waste products via sweat.

• Vitamin D synthesis: Initiates production of vitamin D when exposed to sunlight.

• Homeostasis: Maintains internal balance by regulating water loss, temperature, and protection from environmental hazards.

Homeostatic Mechanisms

• Sweating: Evaporation of sweat cools the body.

• Vasodilation/Vasoconstriction: Blood vessels in the dermis widen or narrow to release or conserve heat.

Example Equation:

Heat loss by evaporation:

Where is the heat lost, is the mass of water evaporated, and is the latent heat of vaporization.

Additional info: The skin also plays a role in immune defense and sensory perception, contributing to overall health and well-being.