Essentials of Human Anatomy & Physiology - Skin and Body Membranes

Body Membranes

• Functions:

  • Cover body surfaces

  • Line body cavities

  • Form protective sheets around organs

• Classified by tissue type

• Two major categories:

  • Epithelial membranes

  • Cutaneous membranes

  • Mucous membranes

  • Serous membranes

  • Connective tissue membranes

  • Synovial membranes

Epithelial Membranes

• Simple organs, also called covering and lining membranes.

• Contain both:

  • Epithelial tissue layer

  • Connective tissue layer

• Cutaneous membrane = skin

  • Dry membrane, outermost protective boundary.

  • Consists of two layers:

  • Epidermis: keratinized stratified squamous epithelium

    • Provides a waterproof barrier

    • Creates our skin tone

  • Dermis: mostly dense (fibrous) connective tissue

    • Supports the epidermis

    • Contains nerve endings, sweat glands and hair follicles

• Mucous membranes (mucosae)

  • Moist membranes, some secrete protective, lubricating mucus.

  • Line all body cavities open to the exterior.

  • Adapted for absorption or secretion.

  • Consists of two layers:

  • Epithelium type depends on site.

  • Loose connective tissue (lamina propria).

    • Location for immune system cells to protect body.

• Serous membranes (serosae)

  • Line compartments in the ventral body cavity closed to the exterior.

  • Occur in pairs, separated by serous fluid, with visceral and parietal layers.

  • Consists of two layers:

  • Simple squamous epithelium

    • Reduces friction between organs and cavities.

  • Areolar connective tissue

    • Provides support and flexibility.

  • Specific serous membranes:

  • Peritoneum: covers organs in the abdominal cavity.

    • Supports and protects abdominal organs

  • Pleurae: surround the lungs.

    • Facilitates breathing by reducing friction.

  • Pericardia: surround the heart.

    • Protects the heart, anchors it and prevents overfilling

Connective Tissue Membranes

• Synovial membranes

  • Loose areolar connective tissue only (no epithelial tissue).

  • Line fibrous capsules surrounding joints, bursae, and tendon sheaths.

  • Secrete a lubricating fluid to cushion organs during muscle activity.

Integumentary System

• Consists of:

  • Skin (cutaneous membrane)

  • Skin appendages

  • Sweat glands

  • Oil glands

  • Hair

  • Nails

• Functions:

  • Insulates and cushions deeper body organs.

  • Protects the entire body from:

  • Mechanical damage (bumps and cuts)

  • Chemical damage (acids and bases)

  • Thermal damage (heat or cold)

  • Ultraviolet (UV) radiation (sunlight)

  • Microbes (bacteria)

  • Water loss

  • Regulates heat loss as controlled by the nervous system.

  • Acts as a mini-excretory system; sweat aids in the loss of:

  • Urea

  • Salts

  • Water

  • Synthesizes vitamin D.

  • Secretions create a protective acid mantle.

• How it protects deeper tissues:

  • Mechanical damage: Physical barrier contains keratin, fat cells, pressure and pain receptors.

  • Chemical damage: Impermeable keratinized cells; pain receptors.

  • Microbe damage: Unbroken surface and "acid mantle"; phagocytes.

• How it aids in body heat loss or retention:

  • Heat loss: Activating sweat glands and allowing blood to flush into skin capillary beds.

  • Heat retention: Not allowing blood to flush into skin capillary beds.

Structure of the Skin

• Two kinds of tissue compose the skin:

  • Epidermis

  • Dermis

• Subcutaneous tissue (hypodermis)

  • Anchors the skin to underlying organs

  • Not technically part of the integumentary system

  • Composed mostly of adipose tissue

  • Serves as a shock absorber and insulates deeper tissues

• Epidermis

  • Outer layer

  • Composed of stratified squamous epithelium

  • Most cells are keratinocytes which produce a fibrous protein called keratin

  • Keratinization makes the epidermis tough

  • Desmosomes connect keratinocytes together

  • Avascular

  • Composed of five layers (strata), from deepest to most superficial:

  • Stratum basale (germinativum)

    • Deepest layer, next to dermis

    • Wavy borderline with the dermis anchors the two together

    • Cells undergoing mitosis

    • Daughter cells are pushed upward

  • Stratum spinosum

    • Cells become increasingly flatter and more keratinized

  • Stratum granulosum

    • Contains granules that help to form keratin

  • Stratum lucidum

    • Formed from dead cells of the deeper strata

    • Occurs only in thick, hairless skin of the palms of hands and soles of feet

    • Provides extra protection in areas subject to wear and tear

  • Stratum corneum

    • Outermost layer

    • Shingle-like dead cells are filled with keratin (protective protein prevents water loss from skin)

    • Protects from abrasion and penetration

  • Melanin

  • Pigment produced by melanocytes in the stratum basale

  • Color is yellow to brown to black

    • Protects skin from UV damage

  • Epidermal dendritic cells

  • Alert and activate immune cells to a threat (bacterial or viral invasion)

  • Merkel cells

  • Associated with sensory nerve endings

  • Serve as touch receptors called Merkel discs

• Dermis

  • Connective tissue

  • Underlies the epidermis

  • Two regions

  • Papillary

    • Upper dermal region

    • Contain projections called dermal papillae

    • Areolar connective tissue

    • Indent the epidermis above

    • Many projections contain capillary loops, and others house pain and touch receptors

    • Capillary loops provide nutrients to the epidermis

    • On palm and sole surfaces, papillae increase friction and gripping ability

    • Fingerprints are identifying films of sweat

  • Reticular

    • Deepest skin layer

    • Dense irregular connective tissue

    • Blood vessels

    • Sweat and oil glands

    • Deep pressure receptors (lamellar corpuscles)

    • Enable sensations of deep touch and vibrations

  • Other dermal features

  • Cutaneous sensory receptors

  • Phagocytes

  • Collagen and elastic fibers

  • Blood vessels

  • Nerve supply

Skin Color

• Three pigments contribute to skin color

  • Melanin

  • Yellow, reddish brown, or black pigments

  • Produced in response to sunlight exposure

  • Carotene

  • Orange-yellow pigment (also found in some vegetables)

  • Accumulates in the stratum corneum and adipose tissue

  • Hemoglobin

  • Red coloring from blood cells in dermal capillaries

  • Oxygen content determines the extent of red coloring

• Abnormal skin colors

  • Redness (erythema): embarrassment, inflammation, hypertension, fever, or allergy

  • May indicate various underlying conditions

  • Pallor (blanching): emotional stress (such as fear), anemia, low blood pressure, impaired blood flow to an area

  • Can be a sign of shock or anemia

  • Jaundice (yellow cast): indicates a liver disorder

  • Bilirubin build-up in the blood

  • Bruises (black and blue marks): hematomas

  • Blood has escaped from circulation and clotted in the tissue spaces

Appendages of the Skin

• Cutaneous glands are all exocrine glands

  • Sebaceous glands

  • Sweat glands

  • Hair and hair follicles

  • Nails

• Sebaceous (oil) glands

  • Located all over the skin except for palms and soles

  • Produce sebum (oil)

  • Makes skin soft and moist

  • Prevents hair from becoming brittle

  • Kills bacteria

  • Most have ducts that empty into hair follicles; others open directly onto skin surface

  • Glands are activated at puberty with increased androgens

  • Contribute to acne

• Sweat (sudoriferous) glands

  • Produce sweat

  • Widely distributed in skin

  • Two types

  • Eccrine glands

    • More numerous, located all over the body

    • Open via duct to sweat pores on the skin’s surface

    • Produce acidic sweat (water, salts, vitamin C, traces of metabolic waste)

    • Function in body temperature regulation

  • Apocrine glands

    • Ducts empty into hair follicles in the armpit and genitals

    • Begin to function at puberty

    • Release sweat that also contains fatty acids and proteins (milky or yellowish color)

    • Play a minimal role in body temperature regulation

    • Cause body odor

• Hair

  • Located body-wide except for palms, soles, nipples, lips

  • Produced by hair follicle

  • Root is enclosed in the follicle

  • Shaft projects from the surface of the scalp or skin

  • Consists of hard keratinized epithelial cells

  • Melanocytes provide pigment for hair color

  • Different concentrations produce different hair colors

  • Hair grows in the matrix of the hair bulb in stratum basale

• Hair anatomy

  • Central medulla

  • Core layer, might not be present in thinner hairs

  • Cortex surrounds medulla

  • Bulkiest layer containing melanin

  • Cuticle on outside of cortex (most heavily keratinized region)

  • Protects the cortex from damage

  • Melanin provides color

• Associated hair structures

  • Hair follicle

  • Composed of an inner epithelial root sheath and an outer fibrous sheath

  • Dermal region provides a blood supply to the hair bulb (deepest part of the follicle)

  • Arrector pili muscle

  • Connects to the hair follicle to pull hairs upright when we are cold or frightened.

  • Produces "goosebumps"

• Nails

  • Heavily keratinized, scalelike modifications of the epidermis