Skin System

Functions in homeostasis and osmoregulation

• Heat homeostasis:
  • Too cold: hair stands up (goose bumps), vasoconstriction decreases blood supply at skin (less heat loss).
  • Too hot: sweat (evaporative cooling), vasodilation increases blood supply at skin (more heat loss).
• Water homeostasis: Insulates body against water loss.
• Osmoregulation: sweat excretes salts and nitrogenous wastes (urea, uric acid, ammonia)
• Some other functions of the skin:
  • protect against UV radiation by making melanin (absorbs UV)
  • make vitamin D upon exposure to sunlight.
  • Act as blood reservoir. Vasoconstriction in skin shunts blood to other organs.
  • Sense touch, pressure, pain, heat, cold.
  • Protection.

Functions in thermoregulation

• hair, erectile musculature
  • hairs help insulate the body by trapping air in them.
  • Normally hair lies at an angle to the skin, with erectile muscle attaching to it.
  • When it's cold, erectile muscles contract, and the hair stands up. This erect position helps hair to trap more air, providing better insulation.
• fat layer for insulation: fat in hypodermis act as insulation.
• sweat glands, location in dermis: produce sweat, cools the body by evaporative cooling.
• vasoconstriction and vasodilation in surface capillaries
  • When it's cold: vasoconstriction of arterioles reduce blood supply to skin capillaries. Leads to less heat loss at skin surface.
  • When it's hot: vasodilation of arterioles increase blood supply to the skin capillaries. Leads to more heat loss at skin surface.

Physical protection

• nails, calluses, hair
  • nail = hard keratin = tougher than the soft keratin on skin.
  • calluses = extra thick layer of dead keratin-packed cells on the surface of skin.
  • hair = hard keratin.
• protection against abrasion, disease organisms
  • Keratin protect skin against abrasion.
  • The tight seal made from keratin-packed cells and glycolipids form a barrier against pathogens.
• Chemical protection: Sweat is acidic, contains antibodies, and antimicrobial agents. Sebum (skin oil) kills bacteria.
• Natural flora: good bacteria on the surface of skin don't cause harm to you, and they fight off bad bacteria that can harm you.

Structure

• layer differentiation, cell types, tissue types (epithelial, connective)
  • Epidermis = stratified squamous epithelial tissue = protection
    • Keratinocytes = cells that produce keratin = dominates the epidermis.
    • Keratinocytes start off like normal cells at the bottom of the epidermis, but gets flatter as you go up, and becomes dead, keratin plates at the surface of the skin.
    • Melanocytes = cells that make melanin, the skin pigment.
    • Dendritic cells (Langerhans cells) = phagocytes that eat pathogen and present foreign antigens to activate immune response.
  • Dermis = connective tissue = blood, nerve supply
    • Fibroblasts = make fiber and ground substance (glue) for the extracellular matrix that makes up connective tissue.
    • Hair follicles, sweat glands, and oil (sebum) glands.
    • Blood vessels and nerves.
  • Hypodermis = adipose tissue = absorbs shock and provides insulation.
  • Review tissue types here
• relative impermeability to water: due to layer of dead, keratin-packed cells sealed with glycolipids.
  • Keratin is water insoluble, and layers of dead, keratin-packed cells reside on the skin surface.
  • Glycolipids seal the space between the dead keratin-packed cells.
  • Sebum (skin oil) contribute some. But oil glands are not present everywhere (absent in palms and soles).