Skin Color

• Melanin– Basic determinant of skin color is quantity, type, and

distribution of melanin– Types of melanin

• Eumelanin—group of dark brown (almost black) melanins• Pheomelanin—group of reddish and orange melanins

Skin Color

• Melanin (cont.)– Melanin formed from tyrosine by melanocytes

• Regulated by the enzyme tyrosinase

• Albinism—congenital absence of tyrosinase

– Other factors• Genetics (4-6 pairs of genes control the amount of melanin

produced)

• Sunlight

• Adrenocorticotropic hormone (ACTH)

• Melanocyte-stimulating hormone (MSH)

• Increasing age…decreasing tyrosinase activity

• Other pigments

Skin Color

• Other pigments– Beta carotene (group of yellowish pigments from food)

can also contribute to skin color– Hemoglobin—color changes also occur as a result of changes in

blood flow • Redder skin color when blood flow to skin increases• Cyanosis—bluish color caused by darkening of hemoglobin

when it loses oxygen and gains carbon dioxide • Bruising can cause a rainbow of different colors to appear in

the skin– Other pigments—from cosmetics, tattoos, and bile pigments in

jaundice

Functions of the Skin (Table 6-1)

• Protection– Physical barrier to microorganisms– Barrier to chemical hazards– Reduces potential for mechanical trauma– Prevents dehydration– Protects (via melanin) excess UV exposure

Functions of the Skin

• Protection (cont.)– Surface film

• Emulsified protective barrier • Formed by mixing of residue and secretions of

sweat and sebaceous glands with sloughed epithelial cells from skin surface

• Shedding of epithelial elements is called desquamation

Functions of the Skin

– Surface film (cont.)• Functions

– Antibacterial, antifungal activity– Lubrication– Hydration of skin surface– Buffer of caustic irritants– Blockade of toxic agents

Functions of the Skin

• Surface film (cont.)– Chemical composition

• From epithelial elements– Amino acids

– Sterols

– Complex phospholipids

• From sebum– Fatty acids

– Triglycerides

– Waxes

• From sweat– Water

– Ammonia

– Urea

– Lactic acid

– Uric acid

Functions of the Skin

• Sensation– Skin acts as a sophisticated sense organ– Somatic sensory receptors detect stimuli that permit

us to detect pressure, touch, temperature, pain, and other general sensations

– When activated, the receptors make it possible for the body to respond to changes occurring in both the external and internal environments.

Functions of the Skin

• Movement Without Injury– Skin is supple and elastic

• Permitting change in body contours without injury

– It grows as we grow– Exhibits stretch and recoil characteristics

Functions of the Skin

• Excretion– Regulation of volume and chemical content of sweat

influences the body’s total fluid volume and the amounts of certain waste products

– Water– Waste Products

• Urea• Ammonia• Uric acid

– Overall, the skin plays a minor role in excretion of body wastes

Functions of the Skin

• Vitamin D Production (Endocrine Function)– Skin is exposed to UV light– 7-dehydrocholesterol is converted to cholecalciferol

• Vitamin D precursor

– Blood transports precursor to liver and kidneys– Vitamin D is produced– Process and end result fulfill the necessary steps

required for vitamin D to be classified as a hormone

Functions of the Skin

• Immunity– Phagocytic cells destroy bacteria– Langerhans cells trigger helpful immune reaction

working with “helper T cells”

Functions of the Skin

• Homeostasis of body temperatures– Body temperature changes very little in the course of a day

• Set point is ~37° C

– Healthy survival depends on biochemical reactions taking place at certain rates…requires normal enzyme functioning

– To maintain homeostasis of body temperature, heat production must equal heat loss

– Heat production• By metabolism of foods in skeletal muscles and liver• Chief determinant of heat production is the amount of muscular

work being performed

Functions of the Skin

– Heat production• By metabolism of foods in skeletal muscles and liver• Chief determinant of heat production is the amount of

muscular work being performed• During exercise and shivering, metabolism and heat

production increase greatly• During sleep, very little muscular work results in decreasing

metabolism and heat production

Functions of the Skin

– Heat loss• Approximately 80% of heat loss occurs through the skin• Remaining 20% occurs through the mucosa of the

respiratory, digestive, and urinary tracts • Vasoconstriction

– Dermal vessels constrict– Warm blood circulates deeper in the body

• Vasodilation– Dermal vessels widen– Warm blood is moved from deeper tissues to the skin– Heat can then be lost to external environment

» Evaportation» Radiation» Conduction» Convection

Functions of the Skin

– Heat loss• Evaporation

– To evaporate any fluid, heat energy must be expended– This method of heat loss is especially important at high environmental

temperatures, when it is the only method by which heat can be lost from the skin

• Radiation– Transfer of heat from one object to another without actual contact– Important method of heat loss in cool environmental temperatures

• Conduction– Transfer of heat to any substance actually in contact with the body– Accounts for relatively small amounts of heat loss

• Convection– Transfer of heat away from a surface by movement of air– Usually accounts for a small amount of heat loss

Functions of the Skin

– Homeostatic regulation of heat loss (Figure 6-7)• Heat loss by the skin is controlled by a negative feedback

loop• Receptors in the hypothalamus monitor the body’s internal

temperature• If the body temperature is increased, the hypothalamus

sends a nervous signal to the sweat glands and blood vessels of the skin

• The hypothalamus continues to act until the body’s temperature returns to normal