Role of the Skin in Thermoregulation

fever

During this episode, the body's temperature control system or Tset is increased caused by PGE2 or other pyrogens that have an upward resetting of anterior hypothalamic "thermostat". The consequence is that the anterior hypothalamus now responds to normal core temperature as if it were cold, causing sweating to stop, vasoconstriction of skin, and may initiate shivering

non-apical skin (non-glabrous)

This hairy skin is located pretty much everywhere on the body. These areas are innervated by sympathetic vasoconstrictors and vasodilator nerves. These areas have few, if any AVAs. Instead, sympathetic vasoconstrictor nerves release NE which interact with alpha-1 and alpha-2 adrenergic receptors on cutaneous arterioles and the few AVAs. Here, blood flow is primarily nutritive in function instead of for thermoregulation

conduction

This is heat transfer by direct physical contact between two objects

radiation

This is heat transfer via electromagnetic infrared radiation

convection

This is heat transferred by flowing (moving) fluid

passive vasodilation

This is the main cause of vasodilation in apical skin This means that there is a relief of sympathetic tone to the AVAs

active vasodliation

This is the main cause of vasodilation in non-apical skin increased Tcore directly leads to vasodilation and subsequent heat loss in non-apical skin. Interestingly, sweating-related vasodilation accounts for over 75% of the total reflex increase in blood flow in non-apical skin

evaporation

This is the process of losing heat through the conversion of water to gas. Sweating provides a mechanism for this heat loss When air temperature equals or exceeds skin temperature, no body heat can be lost by conduction or convection. Radiation is generally not effective either. This is the only mode for heat loss under these conditions

apical skin (glabrous)

This non-hairy skin is located on the palms, soles and lips. These areas are innervated only by sympathetic vasoconstrictor nerves. These areas also have numerous arteriovenous anastomoses (AVA) which are richly innervated by these sympathetic vasoconstrictor nerves. Therefore, in these areas, opening or closing of these AVAs can cause substantial changes in skin blood flow Here, blood flow provides a major function in thermoregulation

hypothalamus

This part of the CNS has temperature-sensitive neurons that monitor core body temperature while skin temperature is detected by temperature sensing somatic sensory neuron endings within the skin This integrates all these sensory inputs and orchestrates responses including reflex changes in sympathetic activity to skin blood vessels, sweat glands and arrector pili, reflex changes in somatic nerve activity to skeletal muscles initiating or curtailing shivering, and initiating behavioral changes to get away from heat or towards it

anterior hypothalamus

This part of the hypothalamus acts as a thermostat, in that it has temperature-sensing hypothalamic neurons that detect current deep brain (ie. core) temperature (Tcore) and compares Tcore with the "set-point" temperature (Tset). If Tcore and Tset are not matched, this part of the hypothalamus sends an "error signal" to the posterior hypothalamus

posterior hypothalamus

This part of the hypothalamus receives an "error signal" from the anterior hypothalamus, signaling that there is a discrepancy between Tcore and Tset. This part of the hypothalamus also receives info about skin temperature (Tskin) from sensory nerve endings in the skin, so therefore it takes Tskin into account when responding to errors in Tcore. This part of the hypothalamus then orchestrates coordinated responses to the temperature errors through behavioral changes and physiological (reflex) responses

vasodilator system

This system in human skin is not tonically active in normothermia and is only activated during increases in internal temperature, such as those that occur during exercise or environmental heat exposure The activity of this system is mediated by co-transmission from sympathetic cholinergic (Ach) nerves, although the co-transmitter hasn't been identified. The mechanism also includes a moderate role for nitric oxide (NO).

vasoconstrictor system

This system in human skin is tonically active in theromoneutral environments. Therefore, subtle changes in activity in this system during most daily activities are responsible for maintenance of normal body temperature with slight changes in activity or ambient temperature This is possible because small changes in skin blood flow can cause relatively large changes in heat dissipation

clear cells

The coiled ducts of eccrine sweat glands ascend towards epidermal surface. These cells then secrete isotonic fluid into the lumen, containing mainly Na and Cl, but also some ammonia, urea and uric acid Most of the Na and Cl- is reabsorbed by the duct cells and as a consequence, the fluid/sweat reaching the skin surface is a hypotonic solution of NaCl

arterio-venous anastomosies (AVAs)

These are shunts between arteries and venous plexuses that are composed almost exclusively of smooth muscle. They are numerous in the palms, soles, and skin of the ears, nose and lips (ie. Apical skin) Blood flow through these is non-nutrient because there are few exchange vessels. These vessels have low level of basal tone and are innervated exclusively by sympathetic fibers, to which they are very responsive

eccrine sweat glands

These sweat glands are located in higher density on the soles of the feet, the forehead, the palms and the cheeks. The major neurotransmitter for sympathetic signaling of these glands are acetylcholine (Ach) These sweat glands secrete an odorless, clear fluid that serves to aid in the regulation of body temperature by allowing heat loss through evaporation. This secretion occurs directly onto epidermal surface Therefore, these glands have a primary function in thermoregulation

apocrine sweat glands

These sweat glands are located in limited areas such as the axilla and urogenital sinus. The major neurotransmitters for sympathetic signaling of these glands are catecholamines These glands produce a thick, odorless fluid that undergoes bacterial decomposition, leading to substances with strong odors. These glands open into the shaft of hair follicles in these regions

Heat stress

This can be particularly threatening for an elderly person with pre-existing heart disease because venous blood that normally returns to the core is transferred to the skin in this state since the skin is trying to dissipate heat. This works against the maintenance of central venous pressure, cardiac filling pressure (preload), stroke volume, cardiac output, and arterial blood pressure Thus, this state can threaten cardiac output and arterial blood pressure, particularly in a compromised patient

increased Tcore

This change in core body temperature causes a relief of tonic sympathetic outflow to the apical skin by acting on AVAs through relieving NE signaling, causing passive vasodilation (ie. taking away vasoconstriction) and subsequent heat loss

decreased Tcore

This change in core body temperature causes increased sympathetic outflow to apical skin through NE signaling to AVAs, causing active vasoconstriction and subsequent heat conservation