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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