ch 5 5.8 and 5.9

5.8 Functions of Skin

• Skin is first and foremost a barrier• Its main functions include:

– Protection– Body temperature regulation– Cutaneous sensations– Metabolic functions– Blood reservoir– Excretion of wastes

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Protection

• Skin is exposed to microorganisms, abrasions, temperature extremes, and harmful chemicals

• Constitutes three barriers:– Chemical barrier– Physical barrier– Biological barrier


Protection (cont.)

• Chemical barrier– Skin secretes many chemicals, such as:

• Sweat, which contains antimicrobial proteins• Sebum and defensins, which kill bacteria• Cells also secrete antimicrobial defensin

– Acid mantle: low pH of skin retards bacterial multiplication– Melanin provides a chemical barrier against UV radiation damage


Protection (cont.)

• Physical barrier– Flat, dead, keratinized cells of stratum corneum, surrounded by

glycolipids, block most water and water-soluble substances– Some chemicals have limited penetration of skin

• Lipid-soluble substances • Plant oleoresins (e.g., poison ivy)• Organic solvents (acetone, paint thinner)• Salts of heavy metals (lead, mercury)• Some drugs (nitroglycerin)• Drug agents (enhancers that help carry other drugs across skin)


Protection (cont.)

• Biological barriers– Epidermis contains phagocytic cells

• Dendritic cells of epidermis engulf foreign antigens (invaders) and present to white blood cells, activating the immune response

– Dermis contains macrophages• Macrophages also activate immune system by presenting foreign antigens

to white blood cells– DNA can absorb harmful UV radiation, converting it to harmless

heat


Body Temperature Regulation

• Under normal, resting body temperature, sweat glands produce about 500 ml/day of unnoticeable sweat– Called insensible perspiration

• If body temperature rises, dilation of dermal vessels can increase sweat gland activity to produce 12 L (3 gallons) of noticeable sweat– Called sensible perspiration; designed to cool body

• Cold external environment– Dermal blood vessels constrict– Skin temperature drops to slow passive heat loss


Cutaneous Sensations

• Cutaneous sensory receptors are part of the nervous system– Exteroreceptors respond to stimuli outside body, such as

temperature and touch– Free nerve endings sense painful stimuli


Figure 5.1 Skin structure.


Hair shaft

Dermal papillaeEpidermis

Papillarylayer

SubpapillaryplexusSweat pore

Appendages of skin• Eccrine sweat gland• Arrector pili muscle• Sebaceous (oil) gland• Hair follicle• Hair root

Dermis Reticularlayer

Hypodermis(subcutaneoustissue; not partof skin)

Cutaneous plexusNervous structures

• Sensory nerve fiberwith free nerve

endings• Lamellar corpuscle• Hair follicle receptor

(root hair plexus)

Adipose tissue

Metabolic Functions

• Skin can synthesize vitamin D needed for calcium absorption in intestine

• Chemicals from keratinocytes can disarm some carcinogens• Keratinocytes can activate some hormones

– Example: convert cortisone into hydrocortisone• Skin makes collagenase, which aids in natural turnover of

collagen to prevent wrinkles


Blood Reservoir

• Skin can hold up to 5% of the body’s total blood volume• Skin vessels can be constricted to shunt blood to other organs,

such as an exercising muscle


Excretion

• Skin can secrete limited amounts of nitrogenous wastes, such as ammonia, urea, and uric acid

• Sweating can cause salt and water loss


5.9 Skin Cancer and Burns

• Skin can develop over 1000 different conditions and ailments• Many internal diseases reveal themselves on skin• Most common disorders are infections• Less common, but more damaging, are:

– Skin cancer– Burns


Skin Cancer

• Most skin tumors are benign (not cancerous) and do not spread (metastasize)

• Risk factors– Overexposure to UV radiation– Frequent irritation of skin

• Some skin lotions contain enzymes that can repair damaged DNA

• Three major types of skin cancer– Basal cell carcinoma– Squamous cell carcinoma– Melanoma


Skin Cancer (cont.)

• Basal cell carcinoma – Least malignant and most common– Stratum basale cells proliferate and slowly invade dermis and

hypodermis– Cured by surgical excision in 99% of cases


Skin Cancer (cont.)

• Squamous cell carcinoma– Second most common type; can metastasize– Involves keratinocytes of stratum spinosum– Usually is a scaly reddened papule on scalp, ears, lower lip, or

hands– Good prognosis if treated by radiation therapy or removed

surgically


Skin Cancer (cont.)

• Melanoma– Cancer of melanocytes; is most dangerous type because it is highly

metastatic and resistant to chemotherapy– Treated by wide surgical excision accompanied by immunotherapy– Key to survival is early detection: ABCD rule

• A: asymmetry; the two sides of the pigmented area do not match • B: border irregularity; exhibits indentations• C: color; contains several colors (black, brown, tan, sometimes red or blue)• D: diameter; larger than 6 mm (size of pencil eraser)


Burns

• Tissue damage caused by heat, electricity, radiation, or certain chemicals– Damage caused by denaturation of proteins, which destroys cells

• Immediate threat is dehydration and electrolyte imbalance– Leads to renal shutdown and circulatory shock

• To evaluate burns, the Rule of Nines is used– Body is broken into 11 sections, with each section representing 9% of

body surface (except genitals, which account for 1%)– Used to estimate volume of fluid loss


Figure 5.11 Estimating the extent and severity of burns using the rule of nines.


Totals41⁄2%

Anterior and posteriorhead and neck, 9%

Anterior and posteriorupper limbs, 18%

41⁄2%41⁄2% Anteriortrunk, 18%

Anterior and posteriortrunk, 36%

9% 9% Perineum, 1%

Anterior and posteriorlower limbs, 36%

100%

Burns (cont.)

• Burns can be classified by severity– First-degree

• Epidermal damage only– Localized redness, edema (swelling), and pain

– Second-degree• Epidermal and upper dermal damage

– Blisters appear– First- and second-degree burns are referred to as partial-thickness burns

because only the epidermis and upper dermis are involved


Burns (cont.)

• Burns can be classified by severity (cont.)– Third-degree

• Entire thickness of skin involved (referred to as full-thickness burns)• Skin color turns gray-white, cherry red, or blackened• No edema is seen and area is not painful because nerve endings are

destroyed• Skin grafting usually necessary


Figure 5.12 Partial-thickness and full-thickness burns.


1st-degreeburn 3rd-degree

burn

2nd-degreeburn

Skin bearing partial-thicknessburn (1st- and 2nd-degree burns)

Skin bearing full-thicknessburn (3rd-degree burn)

Burns (cont.)

• Burns are considered critical if:– >25% of body has second-degree burns– >10% of body has third-degree burns– Face, hands, or feet bear third-degree burns

• Treatment includes:– Debridement (removal) of burned skin– Antibiotics– Temporary covering– Skin grafts


Developmental Aspects of the Integumentary System

• Fetal: by end of 4th month, skin of fetus is developed– Lanugo coat: delicate hairs in 5th and 6th month– Vernix caseosa: sebaceous gland secretion that protects skin of

fetus while in watery amniotic fluid



• Infancy to adulthood: skin thickens and accumulates more subcutaneous fat; sweat and sebaceous gland activity increases, leading to acne– Optimal appearance during 20s and 30s– After age 30, effects of cumulative environmental assaults start to

show– Scaling and dermatitis become more common



• Aging skin– Epidermal replacement slows; skin becomes thin, dry, and itchy

(decreased sebaceous gland activity)– Subcutaneous fat and elasticity decrease, leading to cold

intolerance and wrinkles– Increased risk of cancer due to decreased numbers of melanocytes

and dendritic cells – Hair thinning

• Ways to delay aging:– UV protection, good nutrition, lots of fluids, good hygiene
