Life is Hot and Cold (Part II)


Why do we feel flushed and perspire when we’re too hot but not when we’re too cold?

Like everything else in the world, your body is made of matter that must follow the laws of nature.  These laws state that heat is produced by work and that when in contact with each other a warmer object transfers heat to a cooler one.  To prove this to yourself simply put your hand near a working machine and feel the heat it's giving off transfer to your fingers.  Through cellular respiration, your body can only harness one-quarter of the energy released from glucose and the rest is given off as heat.  So, the more work your body does the more heat it produces which increases its core temperature.   And since, in most circumstances, your body is surrounded by cooler air (or water), it’s usually losing heat to its surroundings, which decreases its core temperature.  Your body's core temperature is determined by how much heat it produces from work (metabolism) and how much it loses (or sometimes gains) from its surroundings.

Just as a machine can breakdown if it's too hot or too cold, so too, the proteins that make up the structures and perform the metabolic functions of your body can breakdown or malfunction if its temperature is too high or too low.  Your body has sensors that detect its core temperature and sends this information to the hypothalamus.  The hypothalamus works to keep your core temperature within the normal range of 97oF-99oF (36oC-37oC).

One way it does this is to make you feel too hot or too cold so you can do things to try to correct the situation.  If you're too hot you can get out of the sun, take off your sweater, put on a fan or jump in a cool lake.  And if you’re too cold you can go into the sun, put on a sweater, stand near a roaring fire or jump into a hot tub.  However, just like the thermostat that controls the air conditioning and heating in your home, so too, your body, through the hypothalamus, has ways of automatically trying to prevent its core temperature from going too high or too low.  One way it does this is by sending out nerve messages to promote heat loss if you’re too hot, or limit heat loss, if you’re too cold.  It does this mainly through the skin.

The skin is the outer layer of your body which is in contact with your surroundings.  It’s made up of different types of cells that together serve to protect the body from many aspects of nature, such as friction, chemicals and microbes.  It’s the unique nature of the skin's blood vessels and the presence of millions of sweat glands within it 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’s working.  However this isn’t 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 which allow direct connections between the arterial and venous systems.  These arterio-venous connections allow rapid blood flow through the skin by shunting blood directly from the arteries to the veins, bypassing the capillaries.  Being so close to its surface, the warm blood that travels within the circulation of the skin tends to make the body lose heat.  In general, the more blood flow to the skin surface the more heat loss and the less blood flow to the skin surface the less heat loss.  

When the body’s core temperature changes, the hypothalamus adjusts the messages it sends along specific nerves to the muscles surrounding the blood vessels in the skin.  These messages result in the release of a neurohormone called norepinephrine.  Norepinephrine attaches to specific (adrenergic) receptors on these muscles and tells them to contract.  When your core temperature rises so that you feel too hot, the hypothalamus responds by sending out fewer messages along these specific nerves which causes the release of less norepinephrine.  Less norepinephrine makes the muscles surrounding the blood vessels in your skin relax more.  This causes more blood flow to the skin surface, (which gives you the feeling of being flushed) and more heat loss from your body.  When the body's core temperature drops so that you feel too cold, the hypothalamus responds by sending out more messages along these specific nerves which makes them release more norepinephrine.  More norepinephrine makes the muscles surrounding the blood vessels in your skin contract more.  This causes less blood flow to the skin surface (with no feeling of being flushed) and less heat loss from your body.    

Also, the skin has millions of sweat glands that can release water onto its surface.  Perspiration promotes 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 perspiration through the same specific nerves it uses to control the blood flow in the skin but instead of using norepinephrine as the chemical messenger it uses acetylcholine.  Acetylcholine attaches to specific (cholinergic) receptors on the sweat glands to turn them on.  When your body’s core temperature rises so that you feel too hot, the hypothalamus sends out more messages along these specific nerves making them release more acetylcholine.  More acetylcholine makes the sweat glands secrete more water onto the surface of your skin (making you perspire more) which causes more heat loss by evaporation.  When your body’s core temperature drops so that you feel too cold, the hypothalamus responds by sending out fewer messages along these specific nerves making them release less acetylcholine.  Less acetylcholine makes your sweat glands secrete less water on the surface of your skin (making you barely perspire at all) causing less heat loss by evaporation. 

In summary, your hypothalamus automatically tries to control your core temperature (so you won’t suffer illness or death) by sending nerve messages that tell the blood vessels and sweat glands in your skin to do the right things at the right times.  That’s why you feel flushed and perspire when you’re too hot but not when you’re too cold.

Three Questions for Mr. Darwin

    1. How did my body anticipate its need to place specialized blood vessels and sweat glands within its skin to help it control its core temperature?

    2. How did my body learn how to produce enough norepinephrine and acetylcholine and place enough of their respective specific receptors exactly where they were needed so it could control its core temperature?

    3. How does my hypothalamus know exactly what to automatically do regarding the blood flow and sweat gland function within my skin when my core temperature isn’t just right?

 


Also see Dr. Glicksman's Series on

"Beyond Irreducible Complexity"

"Exercise Your Wonder"


Howard Glicksman M. D. graduated from the University of Toronto in 1978. He practiced primary care medicine for almost 25 yrs in Oakville, Ontario and Spring Hill, Florida. He now practices palliative medicine for a Hospice organization in his community. He has a special interest in how the ethos of our culture has been influenced by modern science’s understanding and promotion of what it means to be a human being.

Comments and questions are welcome.

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