AssessmentsBody temperature regulation (thermoregulation)
Body temperature regulation (thermoregulation)
USMLE® Step 1 style questions USMLE
USMLE® Step 2 style questions USMLE
Which of the following reasons explains why infants are more likely to experience hypothermia than adults while undergoing a surgical procedure under general anesthesia?
For humans, the normal body temperature ranges between 36.1°C, or 97 °F, and 37°C, or 98.6°F.
When body temperature increases above 38.5° C, or 101.3°F, that’s called hyperthermia.
The opposite condition, when body temperature decreases below 35 °C, or 95 °F, is known as hypothermia.
Thermoregulation is needed in response to internal and external temperature variations.
Internal temperature variations are sensed by specialized nerve cells, called thermoreceptors, located in the anterior hypothalamus.
Now, the hypothalamus works as a thermostat.
The front part or the anterior hypothalamus responds to increased environmental temperatures and it also controls the core temperature of the body.
The back part or the posterior hypothalamus, on the other hand, responds to decreased environmental temperatures.
Changes in the external temperature are sensed by the skin thermoreceptors, which are specialized nerve cells located in the skin.
For example, during winter, when the environmental temperature is less than the body temperature, the skin receptors sense these variations and send the information to the anterior hypothalamus which will then inform the posterior hypothalamus that the body has to generate heat.
Now, besides the behavioral habits, such as putting more clothes on or drinking hot tea, there are several other physiologic mechanisms through which heat production is increased.
First, thyroid hormone action is stimulated.
Thyroid hormones are thyroxine or T4 and triiodothyronine or T3, which is the active form.
One of their roles is to increase heat production and they do that by stimulating conversion of T4 to T3.
ATP is basically the energy currency in the cell and the more we have, the more of it can be used to generate heat.
Because these hormones are thermogenic hormones, any excess or deficit of these hormones will disturb the thermoregulation.
For example, with hypothyroidism, when there are low levels of thyroid hormones, there is an extreme sensitivity to cold.
With hyperthyroidism, on the other hand, there are high levels of thyroid hormones, and this leads to heat intolerance.
Catecholamines bind to β receptors in brown fat, which is also referred to as “good fat” because brown adipose cells can burn calories and generate heat - instead of energy, like the other kind of fat would.
The activation of the sympathetic nervous system also stimulates the α1 receptors in vascular smooth muscle of skin blood vessels, causing vasoconstriction, or narrowing of the arterioles.
Finally, the posterior hypothalamus signals the skeletal muscles, causing rhythmic contractions known as shivering.
When we shiver and these muscles contract, ATP breaks into adenosine diphosphate or ADP, which means that one of 3 phosphoryl groups from ATP gets popped off and turned into a phosphate molecule.
Now, normally, these phosphoryl groups are bonded with the adenosine and these bonds have a lot of energy, meaning that the electrons in this bond are in a high energy state.
Well, when a bond is broken by a chemical reaction, such as hydrolysis, the electrons go into a lower energy state.
As they do that, they release that high energy they previously had and we get what is called an exothermic reaction.
Exo means exit and thermo refers to heat or energy.
- "Hypothalamic regulation of sleep and circadian rhythms" Nature (2005)
- "Warm fibers innervating palmar and digital skin of the monkey: responses to thermal stimuli" Journal of Neurophysiology (1979)
- "Enzyme-Catalyzed Phosphoryl Transfer Reactions" Annual Review of Biochemistry (1980)
- "Opening and closing the metabolite gate" Proceedings of the National Academy of Sciences (2008)
- "Textbook of Anatomy" Wife Goes On (2011)
- "Williams Textbook of Endocrinology" Saunders (2010)
- "Dorland's Illustrated Medical Dictionary" Elsevier Health Sciences (2011)
- "Thyroidal and peripheral production of 3,5,3’-triiodothyronine in humans by multicompartmental analysis." Am J Physio (1990)