Thermoregulation

Radiation

direct transfer of heat (as electromagnetic waves) through the air. All objects emit heat, but lose or absorb (gain) it based on how cool/warm their environment is.

Conduction

loss/gain of heat due to transfer of energy from collisions of adjacent molecules making direct contact with each other. 

Convection

loss/gain of heat due to the movement of water or air.

Evaporation

loss of heat due to evaporating perspiration (sweat) being replace by cool air.

Internal “core”

consists primarily of organs inside the head and trunk

Outer “shell”

consists of skin and subcutaneous fat

In cold conditions

the shell insulates the core from the environment 🡪 preserves body heat 

in warm conditions

the shell dissipates excess body heat

the body will Priortize

keeping the internal organs in homeostasis 

process of adjusting body temperature in response to environmental temperature is controlled by

the brain (specifically the hypothalamus) and is an example of homeostasis   

Countercurrent exchange

Mechanism of minimizing heat loss through heat recycling within the body (also prioritizes the core)

• Warm blood passes down arteries and transfers heat to colder blood returning from the veins

• Allows extremities to receive oxygen and nutrients with minimal heat loss

• Built-in; not controlled by the brain

body temperature also follows a

• circadian rhythm and homeostasis 

• These are separate regulatory processes, but they do interact.

• They are both controlled by the hypothalamus but involve different hypothalamic sub-regions and different circuits

Thermoneutral zone

range of environmental temperatures in which basal rate of heat production is in equilibrium with the rate of heat loss. Between 26 °C (78.8 °F)  and 37 °C (98.6 °F) for humans

BMR: basal metabolic rate

amount of energy expended daily at rest

Humans (and other mammals) have both

behavioral and physiological thermoregulatory responses. They are endotherms.

responses are motivated by the perception of thermal discomfort

• when the climate is perceived to push us away from the our thermoneutral zone

• These perceptions are detected peripherally (skin thermoreceptors) & centrally (core thermoreceptors)

Peripheral thermal sensations arise by

• comparing the temp of objects and ambient air molecules to the temp of skin (~32°C / 89.6 °F)

• Humans are sensitive to sudden changes in skin temp outside the range of 31- 36°C (87.8 – 96.8 °F)

Humans tolerate a narrow range of core temperatures because of

Speed of chemical reactions within the body

The hypothalamus: the ‘body’s thermostat’

• Distinct subsets of hypothalamic neurons:

  • Have intrinsic thermosensitivity

  • Receive inputs from neurons in the periphery (e.g., dorsal root ganglia) and elsewhere in the core (including blood supply):

    • Esophagus

    • Stomach

    • large intra-abdominal veins

    • Head

Transient receptor potential (TRP) channels 

• Have 6 transmembrane domains

• Form tetramers and an ion-conducting pore

• Nine thermosensitive TRP channels have been identified, each with a different temp sensitivity

Some TRP channels are also activated by chemical ligands, which facilitated their discovery

 A=ankyrin; M=melastatin; V=vanilloid

Sweating

• Engaged when body temperature is too high to be cooled by maximal vasodilation alone

• Sweat is composed of blood plasma (minus proteins): Essentially an aqueous solution of electrolytes (mostly NaCl)

• Sweating appears to be controlled by neurons in the anterior hypothalamus

• Cools skin through evaporative heat loss

Non-shivering thermogenesis

• Occurs in brown adipose tissue (BAT), between the shoulder blades

• Regulated by sympathetic nervous system mainly via norepinephrine

• Mostly important in human infants (can’t shiver), but exists in adults to a lesser degree

• Controlled by areas in the posterior hypothalamus Due to prolongedcold exposure

In normal respiration, the energy in glucose drives

• Formation of a proton gradient in the mitochondria

• Production of ATP via the enzyme ATP synthase

In non-shivering themogenesis, the H+ gradient is uncoupled to ATP production 

• Uncoupling protein 1 (a.k.a. thermogenin) enables movement of protons across mitochondrial membrane without ATP production

  • Fat is burned without producing ATP, and heat is released instead

Extreme cold response: shivering

• Temp threshold ~1°C < vasoconstriction; considered a “last-resort” mechanism to generate heat

• Antagonistic muscle pairs are rhythmically activated

• Burns energy without productive work 🡪 heat

• Trunk and neck muscles preferentially recruited (located in core)

• Controlled by “shivering center” located in the posterior hypothalamus (sends projections to spinal motor neurons)

Cold core temps

• feeling cold, mild to moderate shivering 

• (hypothermia) intense shivering, numbness 

• extreme shivering, loss of movement, confusion

• loss of shivering, slow heart beat, shallow breath

• (medical emergency) delirium, sleepiness 🡪 coma

• comatose, little breathing, slow heart rate 

• death due to respiratory arrest

Hot core temps

• (hyperthermia if not caused by fever) sweating, discomfort

• extreme sweating, breathlessness, fast heart rate, convulsions in infants 

• medical emergency) fainting, vomiting, dizziness, delirium

• brain damage, shock, convulsions, respiratory collapse, likely death

• certain death