Heat Transfer — Skin

Skin

• Largest organ in body ~15% body weight
• Resting blood flow ~500ml/min in 70kg male
  • Can increase 30-fold or decrease 10-fold
• Largest barrier between human body and external environment
• Composed of epidermis, dermis and subcutaneous tissue

Role of Skin

• Homeostasis
  • Relatively impermeable
  • Prevents fluid loss
    • Only ~300ml/day lost through skin
• Immunological
  • Physical barrier
  • Low pH and commensal organisms
  • Abundant Antigen Presenting Cells (APC) such as dendritic cells
• Sensory function
  • Embedded with nerve endings
    • Sensory signals to CNS and spinal reflex arcs
      • Touch
      • Pain
      • Temperature
• Thermoregulation (SEE BELOW FOR DETAILS)
  • Sensory
    • C fibres – heat
    • A δ fibres – cold
  • Effector
    • Heat loss
      • Occurs via
        • Radiation
        • Conduction
        • Convection
        • Evaporation
      • Requires transfer of core body heat to skin
        • Regulated by AV shunts (glomus body)
    • Heat gain
      • Opening of AV shunts prevents loss of core body heat to skin
• Synthesis of cholecalciferol
• Protection against UV

Role of Skin in Thermoregulation

Sensors:

• Thermoreceptors, on free endings of sensory nerves
  • Aδ fibres (myelinated, cold, sense below 38°c)
  • C fibres (unmyelinated, warm, sense above 30°c)
• Anterior hypothalamus is sensitive to changes in blood temperature

Controller:

• Hypothalamus
  • Maintains temperature around a set temperature
  • Influenced by many factors including circadian rhythms, thyroid function and ambient temperature

Effector:

• Heat exchange is due to convection, conduction, radiation and evaporation
• Depends on the direction and size of the gradient between skin and the environment

Skin blood flow:

• Heat loss through radiation and convection
• Increased blood flow to skin increases convection and conduction (blood is warm, if temperature gradient present will lose heat to air at increased rate when more flow)
• Assuming skin temperature is below ambient temperature

• When too cold: Sympathetic outflow from hypothalamus
  • Noradrenaline acts on α adrenoreceptors (abundant in skins)
    • Constriction of cutaneous precapillary sphincters
      • Shunting of blood through arteriovenous shunts, bypassing skin
• When too warm: Decreased sympathetic outflow
  • Cutaneous vasodilation

Sweating:

• Heat lost through evaporation and convection
• Energy of molecules in a liquid is unevenly distributed
  • High energy molecules will enter gas phase, be liberated from the liquid
  • Lowering the average temperature of the liquid
• Sweat glands in dermis
  • Secretion of precursor fluid from the gland
  • Ducts ascend through the dermis to exit from the dermis
• Pre-ganglionic sympathetic (cholinergic) innervation
• When too cold, Decreased stimulation
  • Fluid moves slowly through duct
  • Fluid mostly absorbed, minimal sweat
• When too hot, increased sympathetic stimulation
  • Fluid moves quickly through duct
  • Fluid not resorbed, lots of sweat

Galwin 2016