Thermoregulation in Human "Physical Regulation"

The Body's Dynamic Responses: Physical Regulation of Heat

The human body employs a sophisticated array of physical mechanisms to maintain its core temperature, dynamically responding to both internal heat generation and external environmental conditions. These responses can be broadly categorized into those that dissipate excess heat and those that generate or conserve heat.

The Four Main Mechanisms of Heat Transfer

Cooling Down: Dissipating Excess Heat

When the body's internal temperature rises, it activates several physical mechanisms to shed excess heat:

·        Evaporation: This is arguably the most effective cooling mechanism, particularly during physical activity, accounting for approximately 22% of total body heat loss at rest (National Center for Biotechnology Information, n.d.-a). Sweat glands release liquid sweat onto the skin surface, which then cools the body as it evaporates (Houstonmethodist.org, n.d.; Medical News Today, n.d.). This process relies on a principle of physics known as "heat of vaporization," where excess body heat is absorbed and utilized to convert liquid sweat into vapor (Houstonmethodist.org, n.d.). Even without active sweating, a continuous, small amount of water evaporates from the skin and lungs at a rate of 600-700 mL/day, contributing to ongoing heat loss (National Center for Biotechnology Information, n.d.-a).

Vasodilation and Vasoconstriction

·        Vasodilation: The blood vessels located just beneath the skin surface widen, a process called vasodilation (National Blood Service, n.d.; Healthline, 2022). This physiological response increases blood flow to the skin, shunting warm blood away from the body's core (National Blood Service, n.d.; Healthline, 2022). As blood flows closer to the cooler skin surface, heat can more readily dissipate to the environment through radiation and convection (Medical News Today, n.d.; National Center for Biotechnology Information, n.d.-a; Taylor & Francis, n.d.-a).

·        Radiation: This mechanism involves the transfer of heat in the form of infrared rays (JoVE, n.d.; National Center for Biotechnology Information, n.d.-a; Taylor & Francis, n.d.-a). When the body temperature is higher than the surrounding environment, heat is radiated from the body to the cooler surroundings (National Center for Biotechnology Information, n.d.-a). This accounts for approximately 60% of total body heat loss at rest (JoVE, n.d.; National Center for Biotechnology Information, n.d.-a; Taylor & Francis, n.d.-a).

·        Conduction & Convection:

o   Conduction: Heat transfer occurs through direct physical contact between molecules of two materials (JoVE, n.d.; National Center for Biotechnology Information, n.d.-a; Taylor & Francis, n.d.-a). For example, heat is lost when skin comes into direct contact with a cooler object, like sitting on a cold bench (JoVE, n.d.). This mechanism accounts for a relatively small portion of heat loss, about 3% (JoVE, n.d.; National Center for Biotechnology Information, n.d.-a; Taylor & Francis, n.d.-a).

o   Convection: Heat is transferred to the air or water immediately surrounding the skin (JoVE, n.d.; National Center for Biotechnology Information, n.d.-a; Taylor & Francis, n.d.-a). As the air or water warms, it becomes less dense, rises, and moves away from the body, being replaced by cooler fluid, which then absorbs more heat. This continuous circulation helps carry heat away (JoVE, n.d.). Conduction into the air and subsequent convection together account for approximately 15% of total body heat loss (JoVE, n.d.; National Center for Biotechnology Information, n.d.-a).

Warming Up: Generating and Conserving Heat

When the body needs to conserve or generate heat to maintain its core temperature, it activates several mechanisms:

·        Vasoconstriction: The blood vessels under the skin narrow, a process called vasoconstriction (National Blood Service, n.d.; Healthline, 2022). This physiological response decreases blood flow to the skin surface, thereby retaining warm blood closer to the body's core and minimizing heat loss to the cooler external environment (National Blood Service, n.d.; Healthline, 2022; Medical News Today, n.d.). This is a key response activated by the sympathetic nervous system (National Center for Biotechnology Information, n.d.-a).

·        Shivering: This is an involuntary, rapid rhythmic contraction of skeletal muscles (National Center for Biotechnology Information, n.d.-a; Taylor & Francis, n.d.-b; Wikipedia, n.d.). Shivering generates a significant amount of heat as a byproduct of increased muscular metabolic activity (Taylor & Francis, n.d.-b; Wikipedia, n.d.). Maximum shivering can increase basal body heat production by four to five times (Taylor & Francis, n.d.-b). This reflex is typically initiated in response to a drop in skin temperature and intensifies if the core body temperature also begins to fall (Taylor & Francis, n.d.-b).

Shivering or exhibiting piloerection (goosebumps).

 

·        Piloerection (Goosebumps): The erector pili muscles, attached to hair follicles, contract, causing hairs to stand on end and creating "goosebumps" (National Center for Biotechnology Information, n.d.-a). While more effective in animals with dense fur, in humans, this action attempts to trap a layer of insulating air close to the skin, thereby reducing heat loss (Taylor & Francis, n.d.-b).

·        Behavioral Adjustments: Conscious actions play a significant and often immediate role in thermoregulation (Just In Time Medicine, n.d.). These include increasing physical movements, adopting a closed or curled body position to reduce surface area exposure, adding layers of clothing, seeking warmer environments or shelter, and increasing appetite to fuel metabolic heat production (Just In Time Medicine, n.d.; National Center for Biotechnology Information, n.d.-a; Zubair, 2018).

The body's thermoregulatory strategy involves a sophisticated interplay between involuntary physiological responses (such as sweating, vasoconstriction, and shivering) controlled by the autonomic nervous system (Medical News Today, n.d.; Oxford Research Encyclopedias, n.d.; The Royal Society, n.d.) and voluntary behavioral responses (like dressing warmer or seeking shade) (Just In Time Medicine, n.d.; National Center for Biotechnology Information, n.d.-a). This dynamic relationship reveals a hierarchical and complementary nature. Behavioral adaptations often serve as the first line of defense against thermal stress, providing rapid, conscious adjustments to the environment (Just In Time Medicine, n.d.). These conscious choices can significantly reduce the physiological load on the body's involuntary systems. For instance, simply putting on a jacket often suffices to maintain warmth, preventing the need for the more metabolically demanding process of shivering. If behavioral responses are insufficient or unavailable, the more energy-intensive physiological mechanisms are then activated. This highlights the profound adaptive capacity of humans, extending beyond pure biological reflexes to include sophisticated cognitive and cultural strategies, demonstrating an efficient, multi-layered thermoregulatory system (Castellani & Young, 2016; Just In Time Medicine, n.d.).

Various Behavioural Adjustments for Thermoregulation

Table: Physical Mechanisms of Heat Exchange

Mechanism	Type	Description	Approx. % of Heat Loss (at rest)
Radiation	Heat Loss / Gain	Transfer of heat via infrared waves between objects of different temperatures. (JoVE, n.d.; National Center for Biotechnology Information, n.d.-a)	~60% (Loss) (JoVE, n.d.; National Center for Biotechnology Information, n.d.-a)
Conduction	Heat Loss / Gain	Transfer of heat through direct contact between two materials. (JoVE, n.d.; National Center for Biotechnology Information, n.d.-a)	~3% (Loss) (JoVE, n.d.; National Center for Biotechnology Information, n.d.-a)
Convection	Heat Loss / Gain	Transfer of heat to air or fluid currents moving away from the body surface. (JoVE, n.d.; National Center for Biotechnology Information, n.d.-a)	~15% (Loss, with conduction) (National Center for Biotechnology Information, n.d.-a)
Evaporation	Heat Loss	Cooling through the vaporization of sweat from the skin surface. (Houstonmethodist.org, n.d.; National Center for Biotechnology Information, n.d.-a; Sustainability Workshop, n.d.)	~22% (Loss) (National Center for Biotechnology Information, n.d.-a)
Vasodilation	Heat Loss	Widening of blood vessels near the skin to increase blood flow and heat dissipation. (Healthline, 2022; National Blood Service, n.d.)	N/A
Vasoconstriction	Heat Gain / Conservation	Narrowing of blood vessels near the skin to decrease blood flow and retain heat in the core. (Healthline, 2022; National Blood Service, n.d.)	N/A
Shivering	Heat Gain	Involuntary, rapid contractions of skeletal muscles to generate heat. (National Center for Biotechnology Information, n.d.-a; Taylor & Francis, n.d.-b; Wikipedia, n.d.)	N/A
Piloerection	Heat Gain / Conservation	Contraction of tiny muscles causing hairs to stand, attempting to trap an insulating layer of air. (National Center for Biotechnology Information, n.d.-a; Taylor & Francis, n.d.-b)	N/A
Behavioral Adjustments	Heat Loss / Gain	Conscious actions like changing clothing, seeking shade/shelter, or altering body posture. (Just In Time Medicine, n.d.; National Center for Biotechnology Information, n.d.-a)	N/A

References

American Society of Heating, Refrigerating and Air-Conditioning Engineers. (2010). ANSI/ASHRAE Standard 55-2010: Thermal Environmental Conditions for Human Occupancy.

Britannica.com. (n.d.). Neural thermoreceptive pathways. Retrieved from https://www.britannica.com/science/thermoreception/Neural-thermoreceptive-pathways

Castellani, J. W., & Young, A. J. (2016). Human physiological adaptations to the cold. In L. E. Armstrong & J. R. L. D. C. Castellani (Eds.), Environmental Physiology (pp. 165-188). Springer.

Centers for Disease Control and Prevention. (n.d.). Acclimatization | Heat. Retrieved from https://www.cdc.gov/niosh/heat-stress/recommendations/acclimatization.html

Cleveland Clinic. (n.d.). Heat-Related Illness (Hyperthermia). Retrieved from https://my.clevelandclinic.org/health/diseases/22111-hyperthermia

Healthline. (2022, October 18). Thermoregulation. Retrieved from https://www.healthline.com/health/thermoregulation

Hopkins Medicine. (n.d.). Fever. Retrieved from https://www.hopkinsmedicine.org/health/conditions-and-diseases/fever

Houstonmethodist.org. (n.d.). How sweat works why we sweat when we are hot as well as when we are not. Retrieved from https://www.houstonmethodist.org/blog/articles/2020/aug/how-sweat-works-why-we-sweat-when-we-are-hot-as-well-as-when-we-are-not/

Hsqeconsultancy.co.uk. (n.d.). The six basic factors. Retrieved from https://hsqeconsultancy.co.uk/the-six-basic-factors/

Iowa State University. (n.d.). Body Temperature Homeostasis: Cold Pressor Test. In CURE Human Physiology. Retrieved from https://iastate.pressbooks.pub/curehumanphysiology/chapter/body-temperature-homeostasis/

JoVE. (n.d.). Mechanisms of Heat Transfer | Anatomy and Physiology. Retrieved from https://www.jove.com/science-education/v/16236/mechanisms-of-heat-transfer

Just In Time Medicine. (n.d.). Overview of Thermoregulation. Retrieved from https://www.justintimemedicine.com/curriculum/6935

Khan Academy. (n.d.). An introduction to cellular respiration (article). Retrieved from https://www.khanacademy.org/science/hs-bio/x230b3ff252126bb6:energy-and-matter-in-biological-systems/x230b3ff252126bb6:cellular-respiration/a/cellular-respiration-overview

Kenhub. (n.d.). Thermoreceptors. Retrieved from https://www.kenhub.com/en/library/physiology/thermoreceptors

Labster. (n.d.). Metabolic Heat Production. Retrieved from https://theory.labster.com/metabolic_heat_production/

Lumen Learning. (n.d.). Energy and Heat Balance. Retrieved from https://courses.lumenlearning.com/suny-ap2/chapter/energy-and-heat-balance/

Mayo Clinic. (n.d.). Hypothermia symptoms causes effects. Retrieved from https://www.mayoclinic.org/diseases-conditions/hypothermia/symptoms-causes/syc-20352682

Medical News Today. (n.d.). Thermoregulation. Retrieved from https://www.medicalnewstoday.com/articles/thermoregulation

Medlineplus.gov. (n.d.). Body temperature norms. Retrieved from https://medlineplus.gov/ency/article/001982.htm

Middel, A. (n.d.). What is Thermal Comfort & What Influences It?. Sparks.learning.asu.edu. Retrieved from https://sparks.learning.asu.edu/videos/thermal-comfort

National Blood Service. (n.d.). Functions of blood: regulation. NHS Blood Donation. Retrieved from https://www.blood.co.uk/news-and-campaigns/the-donor/latest-stories/functions-of-blood-regulation/

National Center for Biotechnology Information. (n.d.-a). Physiology, Temperature Regulation - StatPearls - NCBI Bookshelf. Retrieved from(https://www.ncbi.nlm.nih.gov/books/NBK507838/)

National Center for Biotechnology Information. (n.d.-b). Hypothermia. In StatPearls. Retrieved from(https://www.ncbi.nlm.nih.gov/books/NBK545239/)

National Center for Biotechnology Information. (n.d.-c). Nonshivering thermogenesis. Retrieved from https://pubmed.ncbi.nlm.nih.gov/6722594/

National Center for Biotechnology Information. (n.d.-d). Mitochondrial ROS support non-shivering thermogenesis. Retrieved from https://pmc.ncbi.nlm.nih.gov/articles/PMC6599457/

National Center for Biotechnology Information. (n.d.-e). Brown adipose tissue (BAT) is a thermogenic organ contributing to non-shivering thermogenesis. Retrieved from https://pmc.ncbi.nlm.nih.gov/articles/PMC10164504/

Oxford Research Encyclopedias. (n.d.). Autonomic Thermoregulation. Retrieved from https://oxfordre.com/neuroscience/abstract/10.1093/acrefore/9780190264086.001.0001/acrefore-9780190264086-e-15

PubMed. (n.d.). Nonshivering thermogenesis. Retrieved from https://pubmed.ncbi.nlm.nih.gov/6722594/

ResearchGate. (n.d.). Autonomic Nervous System Central Thermoregulatory Control. Retrieved from(https://www.researchgate.net/publication/285946287_Autonomic_Nervous_System_Central_Thermoregulatory_Control)

Romanovsky, A. A. (2011). Neural control of shivering pathway. Journal of Physiology, 589(Pt 16), 3927–3930. https://pmc.ncbi.nlm.nih.gov/articles/PMC3167123/

SA Health. (n.d.). Heat-related illness signs symptoms and treatment. Retrieved from https://www.sahealth.sa.gov.au/wps/wcm/connect/public+content/sa+health+internet/healthy+living/protecting+your+health/environmental+health/healthy+in+the+heat/heat-related+illness+signs+symptoms+and+treatment

SimScale. (n.d.). What is ASHRAE 55 thermal comfort?. Retrieved from https://www.simscale.com/blog/what-is-ashrae-55-thermal-comfort/

Sustainability Workshop. (n.d.). Human Thermal Comfort. Retrieved from https://sustainabilityworkshop.venturewell.org/node/811.html

Taylor & Francis. (n.d.-a). Metabolic heat. Retrieved from https://taylorandfrancis.com/knowledge/Medicine_and_healthcare/Endocrinology/Metabolic_heat/

Taylor & Francis. (n.d.-b). Shivering. Retrieved from(https://taylorandfrancis.com/knowledge/Engineering_and_technology/Biomedical_engineering/Shivering/)

The Royal Society. (n.d.). Sympathetic regulation during thermal stress in human aging. Retrieved from https://pmc.ncbi.nlm.nih.gov/articles/PMC4846507/

UF Health. (n.d.). Sweating. Retrieved from https://ufhealth.org/conditions-and-treatments/sweating

Wikipedia. (n.d.). Shivering. Retrieved from(https://en.wikipedia.org/wiki/Shivering)

World Health Organization. (n.d.). How is body temperature controlled?. Retrieved from(https://www.ncbi.nlm.nih.gov/books/NBK279457/)

Zhu, X., & Li, F. (2023). Tonic inhibitory influence of neurons in the vLPO on skeletal muscle shivering. International Journal of Advanced Research in Biological Sciences, 10(11), 1-10. https://doaj.org/article/eefd79c4839043a9851d9b74294cb3d3

Zubair, M. (2018). The physiology role played by the hypothalamus during the thermoregulation in exercise. International Journal of Advanced Research in Biological Sciences, 5(11), 161-165. https://ijarbs.com/pdfcopy/nov2018/ijarbs14.pdf

Zubair, M. (2023). Autonomic control of sweating. PMC, 9884722. https://pmc.ncbi.nlm.nih.gov/articles/PMC9884722/

Zubair, M. (n.d.). Thermoregulation. EBSCO. Retrieved from https://www.ebsco.com/research-starters/zoology/thermoregulation

Thermoregulation in Human " Chemical Regulation"

The Inner Furnace: Chemical Regulation of Body Temperature

Beyond the physical exchange of heat with the environment, the human body possesses sophisticated chemical mechanisms to regulate its internal temperature, primarily through metabolic processes and hormonal signaling.

Metabolic Heat Production: The Byproduct of Life

Cellular Respiration and Heat as a Byproduct of ATP Production.

The human body continuously generates heat as an unavoidable byproduct of its metabolic processes, primarily cellular respiration (Khan Academy, n.d.; National Center for Biotechnology Information, n.d.-a; Taylor & Francis, n.d.-a). This fundamental biological process involves the breakdown of carbohydrates, proteins, and fats (metabolites) to produce adenosine triphosphate (ATP), the body's universal energy currency (Khan Academy, n.d.; National Center for Biotechnology Information, n.d.-a). A significant portion of the chemical energy released during these intricate reactions—approximately 60%—is dissipated as heat, which is essential for maintaining core body temperature (National Center for Biotechnology Information, n.d.-a; Taylor & Francis, n.d.-a). The rate of metabolic heat production is directly influenced by activity level; increased physical work or "work rate" leads to a higher metabolic rate and thus greater heat production, necessitating enhanced heat loss mechanisms to prevent overheating (Hsqeconsultancy.co.uk, n.d.; Taylor & Francis, n.d.-a).

Non-Shivering Thermogenesis (NST): The Role of Brown Adipose Tissue (BAT)

Beyond the heat generated by shivering, the body can produce heat through non-shivering thermogenesis (NST), a metabolic process primarily involving brown adipose tissue (BAT) (Medical News Today, n.d.; National Center for Biotechnology Information, n.d.-a; National Center for Biotechnology Information, n.d.-c; National Center for Biotechnology Information, n.d.-e; PubMed, n.d.; Wikipedia, n.d.). BAT is distinctive due to its rich vascular supply, multiple small lipid droplets, and a high concentration of mitochondria containing uncoupling protein-1 (UCP1) (National Center for Biotechnology Information, n.d.-e; PubMed, n.d.). Unlike typical cellular respiration where energy is captured as ATP, UCP1 in BAT mitochondria uncouples the proton gradient from ATP synthesis, allowing energy to be dissipated directly as heat (National Center for Biotechnology Information, n.d.-d; National Center for Biotechnology Information, n.d.-e).

Brown Adipose Tissue (BAT) and White Adipose Tissue (WAT) cells

While more prominent in human neonates and infants (who cannot shiver effectively) (National Center for Biotechnology Information, n.d.-a; National Center for Biotechnology Information, n.d.-e; Wikipedia, n.d.), active BAT depots have been rediscovered in adult humans (National Center for Biotechnology Information, n.d.-e). BAT is primarily activated by cold stress via the sympathetic nervous system, specifically through norepinephrine (National Center for Biotechnology Information, n.d.-c; National Center for Biotechnology Information, n.d.-e; PubMed, n.d.). Emerging evidence also suggests BAT contributes to "diet-induced thermogenesis" following meal ingestion, acting as an energy sink that helps maintain energy balance (National Center for Biotechnology Information, n.d.-e; PubMed, n.d.). The superior energy dissipation capacity of BAT and its potential to increase the overall energy-expending capacity in humans suggest it could significantly improve current whole-body weight management strategies (National Center for Biotechnology Information, n.d.-e; PubMed, n.d.). This observation extends BAT's relevance beyond its role solely in thermoregulation to its broader implications for systemic metabolic health (National Center for Biotechnology Information, n.d.-e). The mention of its vital roles in glucose and lipid homeostasis and its designation as an important therapeutic target for treating metabolic disorders related to morbidities such as obesity and type 2 diabetes (National Center for Biotechnology Information, n.d.-e) reveals a significant, emerging area of research. This connection implies that a biological mechanism primarily evolved for cold defense also holds promise for addressing prevalent modern health challenges, suggesting that modulating BAT activity (e.g., through cold exposure or pharmacological agents) could be a future strategy for weight management and improving glucose metabolism, showcasing the interconnectedness of physiological systems.

Hormonal Orchestration: Thyroid Hormones and Catecholamines

The endocrine system, through the release of specific hormones, plays a crucial role in chemical thermoregulation:

·        Thyroid Hormones: The hypothalamus influences the thyroid gland to release thyroid hormones (T3 and T4) (Medical News Today, n.d.; National Center for Biotechnology Information, n.d.-a). These hormones significantly increase the body's basal metabolic rate (BMR), leading to enhanced energy utilization and subsequent heat production across body cells (Medical News Today, n.d.; National Center for Biotechnology Information, n.d.-a; Taylor & Francis, n.d.-a). Conversely, an underactive thyroid (hypothyroidism) can lead to a lower body temperature, while an overactive thyroid (hyperthyroidism) can cause a higher body temperature due to increased metabolism (Medical News Today, n.d.).

Endocrine System's Role in Thermoregulation

·        Catecholamines: The adrenal glands, stimulated by the sympathetic nervous system, release catecholamines such as epinephrine (adrenaline) and norepinephrine (noradrenaline) (Medical News Today, n.d.; National Center for Biotechnology Information, n.d.-a). These hormones increase the metabolic rate and directly contribute to heat production (Medical News Today, n.d.; National Center for Biotechnology Information, n.d.-a). Norepinephrine is particularly vital in stimulating brown adipose tissue (BAT) activity for non-shivering thermogenesis (National Center for Biotechnology Information, n.d.-c; National Center for Biotechnology Information, n.d.-e; PubMed, n.d.).

Fever: A Controlled Rise in Temperature

Fever is a distinct and regulated thermoregulatory response, characterized by an elevation in core body temperature above the body's normal set point (National Center for Biotechnology Information, n.d.-a). Unlike hyperthermia, which is an uncontrolled and dangerous overheating, fever is a controlled process orchestrated by the hypothalamus (Healthline, 2022; Hopkins Medicine, n.d.; National Center for Biotechnology Information, n.d.-a). It occurs when substances called pyrogens (originating from infections, inflammation, or other immune processes) act on the hypothalamus (National Center for Biotechnology Information, n.d.-a). These pyrogens trigger the release of prostaglandins, specifically prostaglandin E2 (PGE2), in the hypothalamus (Just In Time Medicine, n.d.; National Center for Biotechnology Information, n.d.-a). PGE2 then "resets" the hypothalamic temperature set point to a higher level, prompting the body to generate and conserve heat until this new, elevated baseline is reached (Just In Time Medicine, n.d.; National Center for Biotechnology Information, n.d.-a).

The Process of Fever, showing Pyrogens interacting with the Hypothalamus

This explains the common experience of feeling cold and shivering during the onset of a fever, even as body temperature is actively rising, as the body perceives itself to be below the new, higher set point (Taylor & Francis, n.d.-b; Wikipedia, n.d.). This active, controlled elevation of temperature, mediated by immunological mediators (National Center for Biotechnology Information, n.d.-a) and prostaglandins to fight off germs (Hopkins Medicine, n.d.), suggests an adaptive, beneficial role. Higher temperatures are known to inhibit the growth of many pathogens and enhance the activity of various immune cells. This reframes fever from a purely negative symptom of illness to a sophisticated, chemically mediated defense mechanism, highlighting the body's intricate strategies for survival against infection and differentiating fever from uncontrolled, dangerous hyperthermia.

References

American Society of Heating, Refrigerating and Air-Conditioning Engineers. (2010). ANSI/ASHRAE Standard 55-2010: Thermal Environmental Conditions for Human Occupancy.

Britannica.com. (n.d.). Neural thermoreceptive pathways. Retrieved from https://www.britannica.com/science/thermoreception/Neural-thermoreceptive-pathways

Castellani, J. W., & Young, A. J. (2016). Human physiological adaptations to the cold. In L. E. Armstrong & J. R. L. D. C. Castellani (Eds.), Environmental Physiology (pp. 165-188). Springer.

Centers for Disease Control and Prevention. (n.d.). Acclimatization | Heat. Retrieved from https://www.cdc.gov/niosh/heat-stress/recommendations/acclimatization.html

Cleveland Clinic. (n.d.). Heat-Related Illness (Hyperthermia). Retrieved from https://my.clevelandclinic.org/health/diseases/22111-hyperthermia

Healthline. (2022, October 18). Thermoregulation. Retrieved from https://www.healthline.com/health/thermoregulation

Hopkins Medicine. (n.d.). Fever. Retrieved from https://www.hopkinsmedicine.org/health/conditions-and-diseases/fever

Houstonmethodist.org. (n.d.). How sweat works why we sweat when we are hot as well as when we are not. Retrieved from https://www.houstonmethodist.org/blog/articles/2020/aug/how-sweat-works-why-we-sweat-when-we-are-hot-as-well-as-when-we-are-not/

Hsqeconsultancy.co.uk. (n.d.). The six basic factors. Retrieved from https://hsqeconsultancy.co.uk/the-six-basic-factors/

Iowa State University. (n.d.). Body Temperature Homeostasis: Cold Pressor Test. In CURE Human Physiology. Retrieved from https://iastate.pressbooks.pub/curehumanphysiology/chapter/body-temperature-homeostasis/

JoVE. (n.d.). Mechanisms of Heat Transfer | Anatomy and Physiology. Retrieved from https://www.jove.com/science-education/v/16236/mechanisms-of-heat-transfer

Just In Time Medicine. (n.d.). Overview of Thermoregulation. Retrieved from https://www.justintimemedicine.com/curriculum/6935

Khan Academy. (n.d.). An introduction to cellular respiration (article). Retrieved from https://www.khanacademy.org/science/hs-bio/x230b3ff252126bb6:energy-and-matter-in-biological-systems/x230b3ff252126bb6:cellular-respiration/a/cellular-respiration-overview

Kenhub. (n.d.). Thermoreceptors. Retrieved from https://www.kenhub.com/en/library/physiology/thermoreceptors

Labster. (n.d.). Metabolic Heat Production. Retrieved from https://theory.labster.com/metabolic_heat_production/

Lumen Learning. (n.d.). Energy and Heat Balance. Retrieved from https://courses.lumenlearning.com/suny-ap2/chapter/energy-and-heat-balance/

Mayo Clinic. (n.d.). Hypothermia symptoms causes effects. Retrieved from https://www.mayoclinic.org/diseases-conditions/hypothermia/symptoms-causes/syc-20352682

Medical News Today. (n.d.). Thermoregulation. Retrieved from https://www.medicalnewstoday.com/articles/thermoregulation

Medlineplus.gov. (n.d.). Body temperature norms. Retrieved from https://medlineplus.gov/ency/article/001982.htm

Middel, A. (n.d.). What is Thermal Comfort & What Influences It?. Sparks.learning.asu.edu. Retrieved from https://sparks.learning.asu.edu/videos/thermal-comfort

National Blood Service. (n.d.). Functions of blood: regulation. NHS Blood Donation. Retrieved from https://www.blood.co.uk/news-and-campaigns/the-donor/latest-stories/functions-of-blood-regulation/

National Center for Biotechnology Information. (n.d.-a). Physiology, Temperature Regulation - StatPearls - NCBI Bookshelf. Retrieved from(https://www.ncbi.nlm.nih.gov/books/NBK507838/)

National Center for Biotechnology Information. (n.d.-b). Hypothermia. In StatPearls. Retrieved from(https://www.ncbi.nlm.nih.gov/books/NBK545239/)

National Center for Biotechnology Information. (n.d.-c). Nonshivering thermogenesis. Retrieved from https://pubmed.ncbi.nlm.nih.gov/6722594/

National Center for Biotechnology Information. (n.d.-d). Mitochondrial ROS support non-shivering thermogenesis. Retrieved from https://pmc.ncbi.nlm.nih.gov/articles/PMC6599457/

National Center for Biotechnology Information. (n.d.-e). Brown adipose tissue (BAT) is a thermogenic organ contributing to non-shivering thermogenesis. Retrieved from https://pmc.ncbi.nlm.nih.gov/articles/PMC10164504/

Oxford Research Encyclopedias. (n.d.). Autonomic Thermoregulation. Retrieved from https://oxfordre.com/neuroscience/abstract/10.1093/acrefore/9780190264086.001.0001/acrefore-9780190264086-e-15

PubMed. (n.d.). Nonshivering thermogenesis. Retrieved from https://pubmed.ncbi.nlm.nih.gov/6722594/

ResearchGate. (n.d.). Autonomic Nervous System Central Thermoregulatory Control. Retrieved from(https://www.researchgate.net/publication/285946287_Autonomic_Nervous_System_Central_Thermoregulatory_Control)

Romanovsky, A. A. (2011). Neural control of shivering pathway. Journal of Physiology, 589(Pt 16), 3927–3930. https://pmc.ncbi.nlm.nih.gov/articles/PMC3167123/

SA Health. (n.d.). Heat-related illness signs symptoms and treatment. Retrieved from https://www.sahealth.sa.gov.au/wps/wcm/connect/public+content/sa+health+internet/healthy+living/protecting+your+health/environmental+health/healthy+in+the+heat/heat-related+illness+signs+symptoms+and+treatment

SimScale. (n.d.). What is ASHRAE 55 thermal comfort?. Retrieved from https://www.simscale.com/blog/what-is-ashrae-55-thermal-comfort/

Sustainability Workshop. (n.d.). Human Thermal Comfort. Retrieved from https://sustainabilityworkshop.venturewell.org/node/811.html

Taylor & Francis. (n.d.-a). Metabolic heat. Retrieved from https://taylorandfrancis.com/knowledge/Medicine_and_healthcare/Endocrinology/Metabolic_heat/

Taylor & Francis. (n.d.-b). Shivering. Retrieved from(https://taylorandfrancis.com/knowledge/Engineering_and_technology/Biomedical_engineering/Shivering/)

The Royal Society. (n.d.). Sympathetic regulation during thermal stress in human aging. Retrieved from https://pmc.ncbi.nlm.nih.gov/articles/PMC4846507/

UF Health. (n.d.). Sweating. Retrieved from https://ufhealth.org/conditions-and-treatments/sweating

Wikipedia. (n.d.). Shivering. Retrieved from(https://en.wikipedia.org/wiki/Shivering)

World Health Organization. (n.d.). How is body temperature controlled?. Retrieved from(https://www.ncbi.nlm.nih.gov/books/NBK279457/)

Zhu, X., & Li, F. (2023). Tonic inhibitory influence of neurons in the vLPO on skeletal muscle shivering. International Journal of Advanced Research in Biological Sciences, 10(11), 1-10. https://doaj.org/article/eefd79c4839043a9851d9b74294cb3d3

Zubair, M. (2018). The physiology role played by the hypothalamus during the thermoregulation in exercise. International Journal of Advanced Research in Biological Sciences, 5(11), 161-165. https://ijarbs.com/pdfcopy/nov2018/ijarbs14.pdf

Zubair, M. (2023). Autonomic control of sweating. PMC, 9884722. https://pmc.ncbi.nlm.nih.gov/articles/PMC9884722/

Zubair, M. (n.d.). Thermoregulation. EBSCO. Retrieved from https://www.ebsco.com/research-starters/zoology/thermoregulation

Thermoregulation in Human "Neural Regulation"

 

The Brain's Command Center: Neural Regulation

The central nervous system, particularly the hypothalamus, serves as the ultimate command center for thermoregulation, orchestrating the body's responses to maintain a stable internal temperature.

The Hypothalamus: The Body's Master Thermostat

Human Brain Highlighting the Hypothalamus

The hypothalamus, a small but profoundly vital region located deep within the brain, functions as the body's primary thermoregulatory center, effectively acting as its "thermostat" (Healthline, 2022; Just In Time Medicine, n.d.; Kenhub, n.d.; Medical News Today, n.d.; National Center for Biotechnology Information, n.d.-a; World Health Organization, n.d.; Zubair, 2018). More specifically, the preoptic area of the hypothalamus is responsible for setting the body's temperature "set point" and integrating a multitude of signals to maintain precise temperature homeostasis (Just In Time Medicine, n.d.; National Center for Biotechnology Information, n.d.-a). This central control unit contains specialized neurons that continuously monitor and respond to changes in core body temperature (Kenhub, n.d.; Zubair, 2018). Different regions within the hypothalamus are specialized for distinct thermoregulatory responses: the posterior hypothalamus primarily controls heat-generating and heat-conserving reactions (such as shivering and vasoconstriction) in response to cold stimuli, while the anterior hypothalamus governs heat-dissipating responses (like sweating and vasodilation) when the body needs to cool down (Just In Time Medicine, n.d.; Zubair, 2018).

Thermoreceptors: Sensing Internal and External Temperatures

Temperature information from both the internal and external environments is continuously gathered by specialized sensory nerve endings known as thermoreceptors, which are distributed throughout the body (Kenhub, n.d.; National Center for Biotechnology Information, n.d.-a; Zubair, 2018).

·        Peripheral Thermoreceptors: These receptors are primarily located in the skin and mucous membranes, where they sense surface temperatures (Kenhub, n.d.; National Center for Biotechnology Information, n.d.-a). They are crucial for providing rapid feedback about environmental temperature changes, allowing for quick behavioral and physiological adjustments (ResearchGate, n.d.).

·        Central Thermoreceptors: Found in deeper structures such as the viscera, spinal cord, and within the hypothalamus itself, these receptors continuously monitor the body's core temperature (Kenhub, n.d.; National Center for Biotechnology Information, n.d.-a).

These thermoreceptors transduce temperature changes into neural action potentials, which are then transmitted to the central nervous system, predominantly to the hypothalamic thermoregulatory center (Kenhub, n.d.; National Center for Biotechnology Information, n.d.-a).

The Neural Pathways: Afferent Sensing, Central Integration, Efferent Responses

Thermoregulation operates through a sophisticated neural feedback loop, a complex mechanism involving three main components (Just In Time Medicine, n.d.; National Center for Biotechnology Information, n.d.-a; Zubair, 2018):

·        Afferent Sensing (Input): Peripheral and central thermoreceptors detect temperature deviations and transmit this sensory information to the hypothalamus (Kenhub, n.d.; Just In Time Medicine, n.d.; National Center for Biotechnology Information, n.d.-a; Zubair, 2018). These signals travel via specific neural pathways, such as the spinothalamic tract, which relays temperature information to higher brain centers like the brainstem, thalamus, and somatosensory cortex for conscious perception of temperature (Britannica.com, n.d.; Kenhub, n.d.; Just In Time Medicine, n.d.). Crucially, these afferent signals also feed directly into the hypothalamus for the initiation of involuntary, autonomic control responses (Just In Time Medicine, n.d.).

The Simplified Neural Pathway of Thermoregulation

·        Central Integration: The hypothalamus, particularly its preoptic area, serves as the primary site for central integration (Just In Time Medicine, n.d.; Kenhub, n.d.; National Center for Biotechnology Information, n.d.-a). Here, all incoming thermal information (from both peripheral and core receptors) is processed, compared against the body's set point, and integrated with other physiological data (Just In Time Medicine, n.d.; Zubair, 2018). This comprehensive integration allows the hypothalamus to compute the precise and appropriate thermoregulatory response needed (Just In Time Medicine, n.d.; Zubair, 2018).

·        Efferent Responses (Output): Based on the integrated thermal information, the hypothalamus sends out command signals to various effector tissues and systems throughout the body to either dissipate (lose) or generate (produce) heat (Just In Time Medicine, n.d.; National Center for Biotechnology Information, n.d.-a; Zubair, 2018). These efferent signals travel via distinct neural pathways, often involving relay stations in caudal parts of the hypothalamus and brainstem, ultimately reaching target organs like sweat glands, blood vessels, and skeletal muscles (Britannica.com, n.d.; Just In Time Medicine, n.d.; Zubair, 2018).

The Autonomic Nervous System: Sympathetic Control of Heat Production and Loss

The autonomic nervous system (ANS), particularly its sympathetic branch, plays a critical and direct role in executing involuntary thermoregulatory responses (Medical News Today, n.d.; Oxford Research Encyclopedias, n.d.; ResearchGate, n.d.; The Royal Society, n.d.).

·        Response to Cold: When the body is exposed to cold temperatures, the hypothalamus stimulates efferent nerves within the sympathetic nervous system (ResearchGate, n.d.). This activation leads to several heat-conserving and heat-generating responses:

o   Vasoconstriction: Intense constriction of skin arterioles and veins occurs, significantly reducing blood flow to the skin surface and thereby minimizing heat loss to the environment (Medical News Today, n.d.; National Blood Service, n.d.; National Center for Biotechnology Information, n.d.-a; ResearchGate, n.d.).

o   Increased Metabolic Rate: The adrenal glands release catecholamines (epinephrine and norepinephrine), which increase the overall metabolic rate and consequently boost heat production (Medical News Today, n.d.; National Center for Biotechnology Information, n.d.-a). Norepinephrine is also crucial for stimulating brown adipose tissue (BAT) activity for non-shivering thermogenesis (National Center for Biotechnology Information, n.d.-c; National Center for Biotechnology Information, n.d.-e; PubMed, n.d.).

o   Shivering: The primary motor center in the posterior hypothalamus is activated, driving skeletal muscle contraction, which generates heat through involuntary shivering (National Center for Biotechnology Information, n.d.-a; Taylor & Francis, n.d.-b; Wikipedia, n.d.). The neural pathway for shivering involves descending excitatory signals from the preoptic area, relayed through hypothalamic and medullary sites (such as the rostral medullary raphe region, rRPa), ultimately activating somatomotor neurons that control skeletal muscles (Romanovsky, 2011; Zhu & Li, 2023).

·        Response to Heat: In warm conditions, the sympathetic nervous system either inhibits specific responses or activates others to promote heat dissipation:

o   Vasodilation: Sympathetic activity to blood vessels in the skin is inhibited, causing them to widen (vasodilation). This shunts warm blood to the skin surface, increasing heat loss via radiation and convection (Medical News Today, n.d.; National Center for Biotechnology Information, n.d.-a).

o   Sweating: Sympathetic cholinergic fibers innervating eccrine sweat glands are activated, leading to increased sweat production for evaporative cooling (National Center for Biotechnology Information, n.d.-a; UF Health, n.d.; Zubair, 2023). The hypothalamus directly controls this sweat secretion via efferent sympathetic sudomotor pathways (Zubair, 2023).

o   Decreased Metabolic Rate: The release of catecholamines from the adrenal glands and thyroid hormones from the hypothalamus is reduced, leading to a decreased metabolic rate and less heat production (Medical News Today, n.d.; National Center for Biotechnology Information, n.d.-a).

While the sympathetic nervous system's role in "fight-or-flight" responses and cold thermoregulation is extensively detailed, the parasympathetic nervous system also contributes to autonomic thermoregulation, though its specific mechanisms in heat dissipation are less elaborated in the available information (Oxford Research Encyclopedias, n.d.; ResearchGate, n.d.).

The consistent emphasis on the hypothalamus's role in integrating information from both peripheral and central thermoreceptors (Kenhub, n.d.; Just In Time Medicine, n.d.; National Center for Biotechnology Information, n.d.-a) highlights that this is not a passive reception of data but an active process of synthesis. The resulting efferent responses (vasoconstriction, shivering, sweating, etc.) are not isolated reflexes but rather highly coordinated outputs orchestrated by this central processing unit (Just In Time Medicine, n.d.; Zubair, 2018). The mention of "parallel but distinct effector-specific core efferent pathways" (Just In Time Medicine, n.d.; Romanovsky, 2011) further supports this highly organized, integrated control mechanism. This implies a sophisticated neural network within the hypothalamus that can fine-tune multiple bodily functions simultaneously to achieve the singular goal of maintaining the set point. This remarkable sophistication of the brain's thermoregulatory system, capable of precisely coordinating diverse physiological functions in concert to respond to thermal challenges, rather than simply triggering isolated reflexes, explains why damage to the hypothalamus or its connections (e.g., from spinal cord injury about the level of T6 or traumatic brain injury) can severely impair the body's ability to thermoregulate, leading to life-threatening conditions (National Center for Biotechnology Information, n.d.-a).

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