Mechanisms for sensing internal body temperature and keeping it within the normal range are indispensable for our survival. Thanks to the body’s thermostat, as a house thermostat does, we are able to maintain core temperature with the utmost precision.
The body’s core temperature is maintained within the narrow range of temperatures of 36.1 and 37.8 ºC. To achieve that, information about internal temperature must be sensed and processed so that we can act against heat disturbances and make up for them. Here we will review how and where this information is sensed and processed, and what are the main effectors that allow core temperature to keep stable.
Think of the thermostat that controls the air temperature at home. It has a predetermined temperature that must keep stable. It also has receptors that give the device information about the actual temperature (that should be the predetermined one). When these receptors sense that the house temperature is higher than it should be, it activates the cooling system of the house to compensate for the reduced temperature. On the other side, they sense a lower temperature than the one we assigned, it will activate the heating systems to increase that temperature.
Our body works in a similar way, yet in a more complex manner and generally with greater precision than a home heating and cooling system. Let’s look into that!
Preoptic-anterior hypothalamus: the body’s thermostat
A region of the brain called the preoptic-anterior hypothalamus (POAH) works as the body’s thermostat. It processes information about internal temperature, and in response, the hypothalamus activates mechanisms that regulate the different heating and cooling mechanisms of the body.
Like home thermostats, the hypothalamus has a predetermined temperature that it tries to maintain, which is the normal body temperature. Any deviation from this set point, however small it may be, signals the POAH to readjust the body temperature.
But, how does the POAH receive information about the core temperature?
There are sensory receptors, called thermoreceptors, located throughout the body, but especially in the skin and the central nervous system (the brain and the spinal cord). They monitor both the temperature of the blood and the rate of change as it circulates through these areas with an exceptional sensitivity, and relay this information to the POAH, the body’s thermostat. This allows tiny changes in core temperature to quickly trigger reflexes that help one conserve or eliminate body heat as needed.
When the POAH senses temperatures above or below the normal body temperature, signals are sent through the sympathetic nervous system (SNS) to four sets of thermoregulatory effectors that allow different forms of heat release or conservation.
The 4 main thermoregulatory effectors
Four main effectors are responsible for keeping the internal body temperature within the normal range of temperature, which work through signals from the SNS:
- Skin arterioles – The smooth muscle in the walls of the arterioles that supply blood to the skin are highly innervated by the SNS. When the core temperature increases or decreases, the POAH sends signals via the SNS to the smooth muscle of these arterioles which causes them to dilate or constrict respectively, thus increasing or decreasing skin blood flow. I recommend you check out this article about blood flow redistribution during exercise to learn more about how this happens. Greater skin blood flow allows to dissipate heat to the environment through conduction, convection, and radiation (and indirectly, evaporation as skin temperature goes up).
- Eccrine sweat glands – When the core temperature increases above a certain threshold, the POAH sends signals through the SNS to the eccrine sweat glands to stimulate active secretion of sweat onto the skin surface. As reviewed in a previous article about the 4 mechanisms of heat exchange, the evaporation of the sweat in contact with the skin releases heat from the body to the environment.
- Skeletal muscle – When body temperature decreases, heat needs to be generated in order to reach back a normal body temperature. Thermoreceptors sense the decrease in internal temperature and send signals to the hypothalamus, which activates the brain centers that control muscle tone. These centers stimulate shivering, which is a rapid, involuntary cycle of contraction and relaxation of skeletal muscle. This is ideal for generating heat to either maintain or increase body temperature since no useful work results from the shivering, only heat production.
- Endocrine glands – Several hormones can also alter the metabolic rate of body cells. Increased metabolism results in greater heat production. Then, changes in core temperature also stimulates the secretion of several hormones involved in the control of the metabolic activity of body cells. For example, cooling the body stimulates the release of thyroxine from the thyroid gland, which can elevate the metabolic rate throughout the body by more than 100%.
Our body is amazingly smart. Against any deviation in core temperature from the normal one, it activates different mechanisms of heat conservation, heat production, or heat release that allows for the maintenance of normal body temperature.
To achieve that, thermoreceptors throughout the body sense changes in internal temperature and send the information to what we call the body’s thermostat, made up by the preoptic-anterior hypothalamus (POAH). In response to temperature fluctuations, through the sympathetic nervous system, the POAH calls into action different effectors that allow for balancing heat production with heat loss so that internal temperature can be maintained.
These effectors are the skin arterioles, that control the blood flow reaching the skin; the eccrine sweat glands, that actively secrete sweat to allow heat loss by evaporation; the skeletal muscles, that increase heat production by shivering; and the endocrine glands, that also increase heat production by enhancing the metabolic rate of body cells through the secretion of several hormones.
- Kenny, W. Larry, Jack H. Wilmore, and David L. Costill. 2012. “Exercise in Hot and Cold Environments.” In Physiology of Sport and Exercise, 5th ed., 283–306. Champaign: Human Kinetics Publishers.