Keeping Cool, Warming Up: Appreciating the Body's Temperature Control System
Howard Glicksman
Editor's note: Physicians have a special place among the thinkers who have elaborated the argument for intelligent design. Perhaps that's because, more than evolutionary biologists, they are familiar with the challenges of maintaining a functioning complex system, the human body. With that in mind, Evolution News is delighted to offer this series, "The Designed Body." For the complete series, see here. Dr. Glicksman practices palliative medicine for a hospice organization.
As I showed in the last two articles in this series, heat is the transfer of energy from one object to another, whereas temperature is a measure of an object's internal energy or its degree of random molecular motion. The body must control the temperature of its internal organs (core temperature) because the molecules that make up the cells and perform the functions of life work best within a certain temperature range: 97o-99oF (36o-37oC).
When it comes to heat and temperature, the laws of nature make two demands on the body. First, they demand that heat be released when energy is used to do work. At complete rest, the body releases a minimum amount of heat from its basal metabolic rate (BMR), which is mostly under control of thyroid hormone. However, since the body must remain fairly active to survive, it releases even more heat due to this work. And second, the laws of nature demand that a warmer object transfer heat energy to a cooler one when they come in contact with each other. Sit in cold water or a hot sauna and your body will either lose heat to, or gain heat from, its surroundings.
In summary, the body's core temperature is determined by how much heat it produces through metabolism, whether it is at complete rest or not, and how much heat it loses to, or gains from, its environment. My last article looked at how the body, through thyroid hormone regulation, takes control of the BMR to help keep the core temperature within the normal range. However, there are other very important mechanisms the body uses to take control to keep the real numbers of core temperature where they need to be to survive within the laws of nature.
Any activity causes the body to use more energy and release more heat, above and beyond the level of the BMR. And since the body is always in contact with its surroundings (usually air, but sometimes water), it is always losing or gaining heat from its environment. Since these changes can take place rapidly, the body must have the ability to react quickly enough to correct the situation and keep its core temperature under control. In other words, besides using thyroid hormone to control the BMR, moment-to-moment thermoregulation must take into account, not only the heat released by the body's activity, but also the heat lost to, or gained from, its surroundings. This requires all three control components.
The first thing you need to take control is a sensor to detect what needs to be controlled. The body has two different sets of temperature sensors which are called thermoreceptors. There are peripheral thermoreceptors in the skin that detect either hot or cold. Their main function is to warn the body when it is being exposed to very high or very low temperatures which may result in tissue damage (thermal burn or frostbite). In addition, the body has central thermoreceptors, which detect the core temperature, and are located within the chest, the abdomen, and the hypothalamus.
The second thing you need to take control is an integrator that can take the data it receives from the sensors, compare it with a standard, decide what must be done, and then send out orders. The hypothalamus is the integrator for core temperature control. Currently, we don't fully understand how it knows what the proper core temperature should be for survival. It is thought that the hypothalamus acts like a thermostat and keeps the body's core temperature around a set-point, which for most healthy people is 97o-99oF (36o-37oC).
If the core temperature rises above the set-point, the hypothalamus sends out messages to limit heat production and promote heat loss. If the core temperature drops below the set-point, the hypothalamus sends out messages to promote heat production and limit heat loss. Besides sending messages to make you aware of being too hot or too cold, the hypothalamus also uses neurohormones in the sympathetic nervous system to keep moment-to-moment control of your core temperature.
The third thing you need to control something is an effector that can do something about the situation. When it comes to thermoregulation, the effectors the body uses can either be voluntaryor involuntary.
When the hypothalamus makes you conscious of a significant rise or fall in the core temperature, making you feel too hot or too cold, you can voluntarily do something to try to correct the situation. If you are too hot you can reduce the amount of heat your body produces by stopping your present activity and coming to a complete rest. You can remove some of your clothing to allow the heat to leave your body easier. You can get out of direct sunlight to prevent its heat from warming you too much or turn on a fan or pour cold water on yourself to help your body lose more heat. In contrast, if you are too cold, you can increase the amount of heat your body produces by increasing your activity level, like rubbing your hands together, stamping your feet, or moving around more. You can put on heavier clothing to prevent your body from losing too much heat. You can go out into the sunshine or stand near something hot, like a fire or wood stove or jump into a hot tub so you can receive more heat.
Besides doing things that promote heat loss and limit its production when we feel too hot or promote heat production and limit heat loss when we feel too cold, our body has several involuntary (automatic) mechanisms in place to achieve this as well.
When, despite all efforts, the body is still too cold, the hypothalamus can activate two other effectors to promote heat production. One of them is to make the muscles shiver. This shaking activity does not move the bones to perform work, but instead produces more heat for the body. The other effector for increased heat production causes the release of certain hormones to increase the body's metabolic rate and release more heat from cellular respiration and fat.
However, the main effector for thermoregulation is the skin. The skin is the outer layer of the body, which is in direct contact with its surroundings. It is made up of many different types of cells that together serve to protect the body from many aspects of nature, like friction, chemicals, and microbes. It is the unique nature of the skin's circulation and the presence of millions of sweat glands that provide it with the equipment to help the body control its core temperature.
The blood flow within a given tissue or organ is usually related to its metabolic needs -- in other words, how hard it is working. However, this is not the case for the skin. In fact, the amount of blood flow in the skin is usually much more than its metabolic needs demand. The skin, particularly in the hands, feet, ears, nose, and lips, has blood vessels that allow direct connections between the arterial and venous systems. These arterio-venous connections facilitate rapid blood flow by shunting blood directly from the arteries to the veins while bypassing the capillaries. Being so close to the surface of the body, the warm blood that travels in the circulation of the skin has a tendency to cause the body to lose heat by radiation and conduction aided by convection. In general, the more blood flow to the skin surface, the more heat loss from the body, and the less blood flow to the skin surface the less heat loss.
When the body's core temperature changes, the hypothalamus adjusts the amount of messages it sends along the sympathetic nerves to the muscles surrounding the blood vessels in the skin. These nerve impulses result in the release of a neurohormone called norepinephrine. Norepinephrine attaches to specific receptors on these muscles and tells them to contract. When the body's core temperature drops so that you feel cold, the hypothalamus responds by sending out more messages along these sympathetic nerves, which makes them release more norepinephrine. More norepinephrine makes the blood vessels in the skin contract more. This results in less blood flow to the skin surface and less heat loss from the body.
When the body's core temperature rises so that you feel hot, the hypothalamus responds by sending out fewer messages along these sympathetic nerves, which causes the release of less norepinephrine. Less norepinephrine makes the blood vessels in the skin relax more. This results in more blood flow to the skin surface and more heat loss from the body by radiation and conduction aided by convection.
In addition, the skin has millions of sweat glands that can release perspiration onto its surface. This promotes further heat loss by evaporation as the water on the skin picks up heat from the body and is turned into water vapor. The hypothalamus triggers sweating through the sympathetic nerves, but instead of using norepinephrine as the chemical messenger, it uses a neurohormone called acetylcholine.
Acetylcholine attaches to specific receptors on the sweat glands to turn them on. When the body's core temperature rises and you feel hot, the hypothalamus sends out more messages along the sympathetic nerves that supply the sweat glands, making them release more acetylcholine. More acetylcholine makes the sweat glands secrete more perspiration. This results in more heat loss from the body by evaporation. When the body's core temperature drops so that you feel cold, the hypothalamus responds by sending out fewer messages along the sympathetic nerves that supply the sweat glands, making them release less acetylcholine. This makes the sweat glands secrete less perspiration, resulting in less heat loss from the body.
In summary, the body's control of its core temperature involves not only thyroid function and the BMR, but also the sympathetic nervous system. By necessity, life is a dynamic process in which the body must stay active and internally produce heat while at the same time externally losing or gaining it from its surroundings. When the hypothalamus receives data from the central thermoreceptors, compares it to the set-point, and determines that the body is too hot or too cold, it tells the conscious mind to do something to try to correct the situation. In addition, it sends out messages along sympathetic nerves to the blood vessels and the sweat glands in the skin which either promotes or limits heat loss. The result is that the body is usually able to control its core temperature and thereby survive within the laws of nature.
It would seem that the system the body uses for thermoregulation knows what it's doing. However, to do its job properly the hypothalamus needs (1) central thermoreceptors throughout the body to detect its core temperature, (2) the ability to adjust the messages it sends out along the sympathetic nerves based on the set-point using (3) norepinephrine for the blood vessels in the skin and (4) acetylcholine for the sweat glands which, to have an effect, need (5) norepinephrine and (6) acetylcholine receptors, respectively. If any one of these six parts were to be missing, or not working properly, the whole system would fail and the body would not be able to control its core temperature. Biochemist Michael Behe has described such a system, where the absence of any one part renders it useless, as irreducibly complex -- a hallmark of intelligent design. The system our body uses to control its core temperature demonstrates irreducible complexity.
However, this is not enough to explain how human life can survive within the laws of nature. Real numbers have real consequences and, if the core temperature rises too high or drops too low, it causes severe impairment of enzyme activity and with it, the metabolism. When it comes to thermoregulation, the body would seem to have natural survival capacity because the hypothalamus seems to inherently know, not only what the thyroid hormone level should be, but alsowhat the set-point should be.
Next time, in the context of thyroid function, we'll look at what happens when the numbers don't add up as they should.
