Body Temperature Regulation

• Body temperature is regulated differently in different organisms, some maintain constant temperatures, others are dependent on ambient temperature.

Body Temperature Types

• Homoiothermic organisms maintain a constant body temperature.
• Poikilotherm organisms' body temperature depends on the ambient temperature.

Heat Production

• Heat is produced via metabolism, physical activity, and active transport.

Heat Loss

• Heat dissipates through radiation, conduction, convection, and evaporation.

Core vs. Shell Temperature

• Core temperature, found in the head and inner abdominal/thoracic organs, remains almost constant.
• Shell temperature, in the skin and superficial tissues, changes with the surrounding temperature.
• Core volume increases as ambient temperature rises.

Measurement of Core Temperature

• Axillary (under the arm) temperature averages 36.5°C.
• Oral temperature averages 37°C.
• Rectal temperature averages 37.5°C.
• Vaginal temperature averages 37.5°C.
• Contactless (forehead) temperature averages 36.5°C.

Fluctuations in Body Temperature

• Circadian rhythms cause endogenous temperature fluctuations, lowest in the morning and highest in the late afternoon, adapting over 1-2 weeks with a range of 0.5-0.7°C.
• Infradian rhythms, approximately 28 days, are influenced by menstruation, where temperature is 0.5°C higher during the second half of the menstrual cycle due to progesterone.

Circadian Rhythm Components

• Suprachiasmatic nucleus (hypothalamus)
• Epiphysis produces melatonin which affects serotonin levels
• GCS
• Lateral geniculate body
• Retina

Mechanisms of Heat Loss/Gain

• Conduction is the transfer of thermal energy during collisions between adjacent molecules via direct contact.
• Convection involves conductive heat loss/gain and the movement of air/water, where water is a better conductor.
• Radiation involves the emission of heat through electromagnetic waves, which depends on temperature gradient.
• Evaporation is the cooling of the body surface as water evaporates from the skin and mucosa.

Heat Loss at Room Temperature

• Conduction and convection account for 18% of heat loss.
• Radiation accounts for 60% of heat loss at room temperature (20-22°C).
• Evaporation accounts for 22% of heat loss at room temperature (20-22°C).

Evaporation Types

• Perspiratio insensibilis is the insensible water loss through skin diffusion and respiratory tract evaporation, totaling about 600 ml/day.
• Perspiration (sweating) is the only mechanism for heat loss when ambient temperature is higher than body temperature.
• Sweat gland activity is controlled by sympathetic cholinergic fibers (M-receptors).
• Sweat composition is altered in ducts through Na+ and Cl- reabsorption via Aldosterone.
• Maximum sweat production is about 700 ml/hour.
• Evaporation of 1 liter of sweat results in a heat loss of 2400 kJ.
• Sweat glands are innervated by sympathetic postganglionic fibers releasing norepinephrine epinephrine and acetylcholine.

Heat Transport

• Body fat acts as an insulator against conduction.
• Heat is convected by blood, with skin blood flow accounting for up to 30% of cardiac output.

Thermoregulatory Center

• The thermoregulatory center is located in the hypothalamus, which maintains a "set point" of 37.0°C.
• The ventral hypothalamus integrates information from peripheral and central thermoreceptors.
• Peripheral thermoreceptors are cold receptors (most numerous) and heat receptors in the skin.
• Central thermoreceptors are heat (most numerous) and cold receptors found in deep body structures, including the preoptic part of the hypothalamus, abdominal organs and spinal cord.
• The posterior hypothalamus activates temperature-regulating processes for decreasing temperature when the body is too hot or increasing temperature when it is too cold.

Temperature decreasing mechanisms stimulated by heat

• Skin vasodilation inhibits sympathetic vasoconstriction (30% of cardiac output), thus ↑ heat transfer to the skin is increased 8 times
• Evaporation activates sympathetic cholinergic fibers and the secretion of bradykinin by sweat glands.
• Decreased food appetite
• Apathy and inactivity
• Decreased muscle tone

Temperature Increasing Mechanisms Stimulated by Cold

• Skin vasoconstriction is stimulated by sympathetic vasoconstrictor neurons.
• Behavior adaptations includes huddling and clothing, which decreases surface area.
• Piloerection (horripilation) contracts arrector pili muscles, raising hairs to create an insulating layer, though this is less effective in humans.

Mechanisms to Increase Heat Production

• Increased muscle tone generates heat
• Shivering thermogenesis involves rhythmic muscle contractions and relaxations without external work which are influenced by the hypothalamus.
• Nonshivering thermogenesis activates the adrenal medulla to release (nor)epinephrine, uncoupling oxidative phosphorylation in brown fat.
• Behavioral adjustments such as increasing motor activity or moving to warmer surroundings.

Central Nervous System Control

• CNS control involves the autonomic nervous system (sympathetic part) and somatomotor nerves, for thermoregulation.
• ANS stimulates skin vessels and sweat glands.
• Somatomotor Nerves control muscoloskeletal behavior.

Thermoregulation During Physical Activity

• Increased heat production occurs doing physical activity
• Increased heat loss is facilitated via evaporation.
• Core temperature correspondingly increases.
• Simultaneously, skin temperature decreases due to sweating.

Temperature Regulation in Newborns

• Newborns have 4-5x higher relative heat production versus adults.
• They have high heat loss due to thin shells with little subcutaneous adipose tissue
• Newborns have a surface-to-volume ratio 3x higher than adults.
• Their neutral temperature range is 32-34°C
• Brown adipose tissue is used for heat generation.

Adaptations to Heat

• Sweating begins sooner
• Increase in sweat volume
• Decrease in Na+ and Cl- concentration.

Adaptations to Cold

• Probably stimulation of thyroid gland secretion which will increase metabolic rate
• Ability to Increase the insulating ability of a given amount of skin fat
• Behavior is the most important

Hyperthermia

• Heat cramps can lead to heat syncope, then heat exhaustion, heat injury, and ultimately heat stroke.
• It is Characterized by breakdown in heat-regulating systems which increase body temperature
• Can cause Collapse, delirium, and unconsciousness
• Damage to the liver, kidney, and impaired cerebral circulation can occur
• Risk populations include children, old persons, those with obesity or hyperthyroidism, and those undergoing intense physical activity.

Fever

• Infection occurs
• Pyrogens are released
• Macrophages respond
• Interleukin-1 and 6 are released (endogenous pyrogens)
• This affects the ventral hypothalamus
• Prostaglandins are synthesised which increase the temperature set point
• Which leads to increased heat production and decreased heat loss to try and reach the new set point, leading to a fever

Hypothermia

• Decreased ventilation,BPM, BP, and metabolism, which can lead to death
• 34°C - Disruption of hypothalamic thermostatic function
• 29°C - It becomes impossible to return to normal temperature without external intervention
• Risk populations include cardiac failure, anorexia, alcohol use, and hypothyroidism
• Artificial hypothermia (strong sedative) may be induced during heart surgery to depress metabolism and reduce oxygen needs.