The Skin is a Composite of 3 Layers

• This diagram is from the copyright-free collection, The Sourcebook of Medical Illustration, edited by Peter Cull (Park Ridge, NJ: Parthenon, 1989).
• Epidermis: outermost layer, mostly dead keratinized cells (stratified squamous epithelium). No blood vessels, gets nutrition from dermis. Dead cells slough off and are replaced by dividing cells in the stratum basale. Half life of skin cells about 35 days.
• Dermis: contains blood vessels, nerves, sensory receptors for touch, pressure, hot, cold, pain. Also has hair follicles and sweat glands. All this is imbedded in fibrous connective tissue.
• Hypodermis: innermost layer. Has adipose and connective tissue . Connects to underlying muscles.

The Skin is a Selective Barrier to Organisms, Chemicals and Energy

• Microorganisms: the skin is a physical barrier to microbial invasion except where it is damaged by cuts and abrasions. The secretion of an antibacterial enzyme (lysozyme) in sweat and the acidic nature of skin secretions also discourage bacteria.
• Chemicals: oils and other lipids make the skin waterproof and block the flow of both water and water-soluble materials.
• Energy: the black pigment, melanin, absorbs light in the outer layer of skin, protecting the inner layers. Melanin is made in epidermal melanocytes. Skin exposed to light darkens due to increased melanin production. Skin also controls the flow of heat in and out of the body. Heat flow is reduced by layers of adipose tissue and hair (not too important in humans except on the head). Blood flow can be regulated to control heat loss.

Mammals and Birds are Homeotherms

• Mammals and birds maintain a constant body temperature (homeotherms = warm blooded): mammals about 37 deg C (99 deg F); birds about 40 deg C ( 105 deg F).
• Body temperatures of homeotherms are usually above the environmental temperature
• Warm blooded animals have many advantages- faster, more active, etc.
• There is a price to warm bloodedness: higher metabolic rate, more food needed

Body Temperature Results from a Balance Between Production and Loss of Heat

• Heat is constantly produced and lost
• In a balanced state production and loss of heat will be equal and the temperature will be constant

Temperature is Controlled from Sites in the Hypothalamus

• Temperature control requires sensors, a control center, effectors
• Temperature sensor are found throughout the body: skin, body core, brain
  • Two types- respond to hot and cold
• Control center is in the hypothalamus of the brain
  • Hypothalamus acts as a thermostat- has a temperature set point
• Efffectors:
  • Produce more heat (increased metabolic rate, shivering, brown fat metabolism)
  • Change heat loss (blood vessel dilation or constriction, erection of hair, curling up, sweating)

Skin is the Primary Organ for Removal of Metabolic Heat

• About 90% of body heat is lost through the skin
• If body temperature is too high the skin can dilate blood vessels and increase blood flow by 150 times to loose excess heat
  • In cold weather skin will constrict blood vessels and release heat loss
• Heat loss is by radiation, conduction, convection and sweating
• Newton's law of cooling governs heat loss by radiation & conduction
  • Heat loss = (heat conductance)(temperature difference)
  • Temperature difference = (body temp - ambient temp)
• Sweating can be used to lose enormous amounts of heat
  • Sweat glands originate in the dermis- ducts penetrate epidermis, releasing secretion on skin surface
  • Heat of vaporization of water is about 580 Calories/liter
  • If the ambient temperature is higher than the body temperature, sweating is the only way we can lose heat
  • Sweat glands are activated by nerves from the sympathetic nervous system-
• Skin also contains muscles (arrector pili) which erect hair shafts (piloerection), increasing insulation- probably not too important in humans

Failure of Temperature Regulation on Hot Days Can Cause Heat Exhaustion and Heatstroke

• You can lose as much as 1.5 liters of water per hour as sweat
• If this water is not replaced blood pressure will fall and heat regulation will fail and body temperature will rise
• This is heat exhaustion- skin will be wet and cool from sweating- treat by replacing water and salt that has been lost
• Heat stroke is a failure of the sweating mechanism- skin will be dry and hot- very dangerous, treat by rapid cooling

Fever Occurs When the Temperature Set-Point is Raised

• Fevers are caused by in increase in the temperature set point- the thermostat has been set higher
  • Caused by bacterial toxins
  • Fevers are probably beneficial in recovery from infections if they do not get too high
• Fevers can be caused by increased metabolism, reduced heat conduction, or both
• The graph below shows a Madonna computer simulation of temperature regulation
  • At 10 minutes the metabolic rate was increased 10% and the heat conductance was reduced 10%
  • Body temperature rises from 37 deg C (98.6 deg F) to 40.6 deg C (105 deg F) in about 1 hour
  • Bottom line: relatively small changes in metabolic rate and heat conductance can cause significant temperature changes
    

Excessive Heat Loss Can Cause Hypothermia and Frostbite

• Long exposure to cold (especially in water) will cause body temperature to drop
• Below about 33 deg C (92 deg F) body functions start to deteriorate- death due to ventricular fibrillation occurs at about 25 deg C (76 deg F)
• As temperature drops the body will reduce blood flow to hands and feet first to preserve the body core temperature- this can result in frostbite of the extremities

Skin Pigments Regulate UV Light Exposure of the Skin

• Skin pigments, especially melanin, absorb ultraviolet (UV) light and regulate the amount that enters the skin
• UV light:
  • UVA = wavelengths above 320 nm
    • Less dangerous than UVB, but may cause some cancer
  • UVB = 280-320 nm
    • Causes sunburn & cancer
    • UV damages DNA -> mutations -> cancer
    • Required for vitamin D synthesis
  • UVC = wavelengths below 280 nm
    • Doesn't penetrate Earth's atmosphere
    • Absorbed by ozone layer
• Melanin:
  • Produced from the amino acid, tyrosine
  • Made in skin cells called melanocytes
• Protective effects of melanin
  • Reduces skin cancer by absorbing UV
    • In white skin 20-30% of UVB passes through the epidermis
    • In dark black skin only 5% of UVB crosses the epidermis (acts as a sun shield)
  • Protects folic acid from degradation from DNA
    • Folic acid is required for synthesis of N-bases (purines & pyrimidines)
    • Folic acid deficiency during pregnancy can cause neural tube defects (i.e., spina bifida)
• Some UV light is required for synthesis of vitamin D
  • In the skin UV light converts a cholesterol derivative into previtamin D3
  • Previtamin D3 is converted into the active vitamin in the liver and kidney
  • A person with dark skin living at high latitudes, or someone who stays indoors, may become vitamin D deficient due to lack of UV exposure
• Skin melanin may be genetically determined to regulate UV light
• People adapted to the Tropics, where there is a lot of sunlight, tend to have darker skins
• People adapted to the Far North, where there is less light, have lighter skins
• Melanin level seems to be genetically determined
  • Enough to prevent birth defects caused by folic acid deficiency
  • Not so much that vitamin D deficiency occurs

More Information

The theory that skin pigments have evolved to allow vitamin D synthesis while preventing UV degradation of folic acid has been proposed by Nina Jablonski and George Chaplin of the California Academy of Sciences:

• Nina Jablonski and George Chaplin. The evolution of human skin coloration. Journal of Human Evolution 39: 57-106 , 2000

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