Regulation of Body Temperature

Tem­per­a­ture reg­u­la­tion is a great exam­ple of how this home­o­sta­tic reflex works.  If you go to the doc­tor at a hos­pi­tal, the first thing they do is stick a ther­mome­ter in your mouth. They don’t even know why you’re there but they do that because the body temp is just that impor­tant.  Our oral body temp is about 98.6°F.  The core body tem­per­a­ture (rec­tum) is 99.6°F (usu­ally done with babies.)  What about the tem­per­a­ture on the sur­face of our skin?  The air around us is almost always cooler than our bod­ies.  Remem­ber energy pro­duc­tion is con­stantly gen­er­at­ing heat, los­ing about 60% as heat dur­ing oxida­tive phos­pho­ry­la­tion.  The tem­per­a­ture on the sur­face of our skin is usu­ally about 10 degrees lower than our body.  There are ther­mome­ters nowa­days that could check the sur­face of your skin and cor­re­spond that to an oral temp.

Daily (Diur­nal) vari­a­tions in body temperature

Nor­mally your body temp is high­est at the end of your work day, about 7PM.  Your body tem­per­a­ture starts to drop after­wards until you start to sleep and it con­tin­ues to drop until a cou­ple hours before you rise from sleep (about 4am).

Why would the tem­per­a­ture be coolest a cou­ple hours before you awake and not just before it?  Because you have an inter­nal clock in your brain that knows when you’re going to wake up and starts to speed meta­bolic activ­ity up ahead of time.  If you go to sleep con­sis­tently, you will wake up at the same time, usu­ally just before the alarm you have set due to this inter­nal clock.

Since we said your body temp is high­est at the end of your work day… What if you work the grave­yard shift?  Your body tem­per­a­ture is still going to high­est at the end of your work day.  So if you get off work at 2AM, it will be high­est around then.

Monthly vari­a­tions in body temp in women

In women specif­i­cally, there is a monthly vari­a­tion that is affected by the men­strual cycle. Dur­ing the lat­ter two weeks of their men­strual cycle (dur­ing the pos­tovu­la­tory luteal phase), the increase in prog­es­terone causes a 1°F rise in basal body tem­per­a­ture.  The basal body tem­per­a­ture is the low­est body tem­per­a­ture dur­ing sleep.  A woman could take their tem­per­a­ture every morn­ing and the day they wake up and find that their tem­per­a­ture is one degree higher than the day before, then they know they are ovu­lat­ing.  This is cheaper than using an ovu­la­tion test kit.  If a woman gets preg­nant, their body tem­per­a­ture stays ele­vated for the entire 9 months because the prog­es­terone stays at high lev­els to pre­vent men­stru­a­tion from occurring.

Rel­e­vant: The Men­strual Cycle in Detail

Three major sources of heat within the body

1. Cel­lu­lar res­pi­ra­tion is occur­ring in EVERY cell of your body.  When glu­cose releases energy, more than half of the energy (60%!) is given off as heat which is why your body is nearly 100°F in tem­per­a­ture! Just like a light bulb that is fed with elec­tric­ity, most of the energy is given off as heat instead of light.

2. Mus­cu­lar activ­ity gen­er­ates heat for 2 rea­sons. (1) When you start to exer­cise or use your mus­cles, it increases cel­lu­lar res­pi­ra­tion to gen­er­ate more ATP.  (2) It also gen­er­ates heat through some­thing called “fric­tional heat” which is heat due to move­ment.  The same thing hap­pens when you’re dri­ving your car and the tires of your car get hot because they’re touch­ing the asphalt.  This doesn’t gen­er­ate as much heat as cel­lu­lar res­pi­ra­tion when you’re rest­ing, but it’s something.

3. Inges­tion of food. The very process of digest­ing the food that you eat, absorb­ing the nutri­ents, and pro­cess­ing those nutri­ents gen­er­ates heat.  The meta­bolic rate increases 10–20% due to the energy “costs” of these reac­tions.  It’s not only cell res­pi­ra­tion but ALL bio­chem­i­cal reac­tions gen­er­ate heat.   This effect, known as food-induced ther­mo­ge­n­e­sis, is great­est after eat­ing a high pro­tein meal and is less after eat­ing car­bo­hy­drates and lipids.

Exer­cise may also increase the meta­bolic rate up to 15 times the basal rate (pro­fes­sional ath­letes may do up to 20 times).

Four major meth­ods of heat loss from the body

1. Radia­tive heat loss from the skin.  Heat flows from an area that’s warmer to an area that’s colder.  That’s radi­a­tion and it’s just like dif­fu­sion (high pres­sure to lower pres­sure).  Your body tem­per­a­ture is almost always cooler than the air around you.  Only on the rarest of days in Los Ange­les is the air around you warmer.  At 70F (21C), about 60% of heat loss occurs via radi­a­tion.  If the air tem­per­a­ture was actu­ally warmer than you, like say dur­ing a sum­mer day in Las Vegas, then the heat would trans­fer onto you from the air.

2. Cuta­neous vasodi­la­tion. All blood ves­sels in the body are capa­ble of dilat­ing or restricting.

Our body can affect or con­trol how much blood flows through our cuta­neous ves­sels (the ves­sels close to our skin).  If the ves­sel is dilated, that’s vasodi­la­tion.  If it con­stricts, that’s vaso­con­stric­tion.  When we’re hot, we could increase the rate at which we lose heat to the air by dilat­ing the ves­sels, which means more hot blood flows through our skin and our skin will look flushed.  If you immerse your hands in hot water, the ves­sels dilate to get rid of the heat.  Con­versely, if we con­strict the blood ves­sels, and reduce the amount of blood that’s flow­ing through our skin, that will reduce the amount of heat loss.  If you put your hands in ice cold water, your hands will look white because of the vasoconstriction.

3.  Evap­o­ra­tion of sweat from the skin.  In order for sweat to work, the heat from your body has to cause the sweat from your body to evap­o­rate.  Let me give you an anal­ogy, let’s say you put a pot of water on the stove and you turn on the flame.  The heat from the flame is trans­ferred to the water.  As the water gains heat, the water mol­e­cules start to vibrate and move faster and faster and as they gain this kinetic energy, the water under­goes a phase change, going from liq­uid to a vapor/gas state.  It takes energy to make some­thing go from solid to liq­uid to gas.  The water has gained enough energy for it to evap­o­rate.  When we sweat, we release water onto our skin, the heat of our body trans­fers to our sweat and causes the sweat to undergo a phase change from liq­uid to vapor and we get rid of the sweat and heat at the same time.

One fac­tor slows this effec­tive­ness of this:  Humid­ity.  The more humid the air, the slower the rate at which sweat evap­o­rates.  When you have 100% humid­ity, instead of water evap­o­rat­ing into the air, the water will con­dense and fall as rain.  This is why most peo­ple can han­dle dry desert heat bet­ter than humid heat.  Any­where on the east-coast or the south in the sum­mer feels like a steam room.  The sweat doesn’t evap­o­rate and every­thing just rolls down and your clothes stick to you.  Sweat­ing only cools you down if the heat of your body causes that water to evap­o­rate.  If the air tem­per­a­ture is hot­ter than you AND it’s really humid, then it feels like you’re going to die (haha).

4.  Breath­ing.  Go ahead and exhale on your hand.  Feel how warm and humid it is?  Every time we exhale, we lose water and heat from our body.  About 20% of our body heat is lost due to exhal­ing.  The lungs are the coolest parts of our body.  Ani­mals that can­not sweat, such as dogs, change their breath­ing pat­tern to get rid of heat and pant.  If you have expe­ri­ence with a horse you’ll know that horses sweat and don’t pant.  Since we aren’t aware of this water loss through the skin and mucous mem­branes of the mouth and res­pi­ra­tory tract, it is called insen­si­ble water loss.

The Ther­moreg­u­la­tory Reflex Center

The temp con­trol cen­ter is found in the hypo­thal­a­mus of our brain.  When­ever the body temp becomes lower or higher than the set point level, the TRC acti­vates effec­tors to com­pen­sate and return the body tem­per­a­ture back to the set point level.  The job of this TRC cen­ter is to reg­u­late the body of the tem­per­a­ture at a set point of 98.6 degrees F. In order to do this it needs to get this infor­ma­tion from ther­more­cep­tors (through an affer­ent path­way) in the rest of our body.

All the con­trol cen­ter does is it COMPARES the actual body tem­per­a­ture with the desired body tem­per­a­ture.  If they match, the con­trol cen­ter doesn’t have to do any­thing.  If they don’t match, then its job is to get the temp back to the set point.  The TRC will send out­put sig­nals across a motor path­way (an effer­ent path­way) to effec­tors such as cuta­neous blood ves­sels, sweat glands, skele­tal mus­cles (shiv­er­ing) and glands (that secrete hor­mones that speed up cel­lu­lar respiration).

Remem­ber the mnemonic: A comes before E in the alpha­bet, so sig­nals are sent through an Affer­ent path­way (input) to the con­trol cen­ter and then the out­put goes through an Effer­ent path­way to the effec­tors (mus­cles and glands).

Inci­den­tally the anal­ogy we’re going to draw from this is the ther­mo­stat in your home.  If you have your ther­mo­stat set to 70, the actual tem­per­a­ture of your home will look like this above.  The heater doesn’t turn on until it reaches a lit­tle bit below that.  When it does turn on, it doesn’t turn off the moment it hits that set tem­per­a­ture.  It will con­tinue to blow heat a lit­tle above it.  So the aver­age tem­per­a­ture of your home is oscu­lat­ing at a “steady state.”  This is show your body tem­per­a­ture works as well.

What are the home­o­sta­tic reflexes that are acti­vated when­ever the body tem­per­a­ture is LOWER than the set point?

a) Per­son feels cold.  When you’re colder than the set point, then you feel cold.  Why do we say this?  Well, you’re not a dog. We have behav­ioral responses.  Maybe you will put a sweater on or make your­self a hot drink or have some hot soup.  It’s not like you’re going to wait until you shiver.  Or you might turn up the heat in your home.  If that’s not suf­fi­cient or you don’t have a coat to put on, then the next thing that hap­pens is…

b) Cuta­neous vaso­con­stric­tion.  When your blood ves­sels con­strict, that reduces blood flow to the skin, min­i­miz­ing heat loss. Inci­den­tally you will tend to look pale because there’s less warm red blood flow­ing through your skin.  If that’s not sufficient…

c) Shiv­er­ing.  Your skele­tal mus­cles will be acti­vated to con­strict and that’s known as shivering.

All the above three things are acti­vated by your ner­vous sys­tem.  These are all caused by nerve impulses and hap­pen very quickly.

One quick point before we con­tinue. The next time you’re shiv­er­ing, you could stop shiv­er­ing any­time you want.  All you need to do is start mov­ing.  Start run­ning in place, use your mus­cles, do jump­ing jacks.  The only rea­son you’re shiv­er­ing is because you’re an idiot telling your­self “I’m so cold, look at me, I’m so cold I’m shiv­er­ing.”  You could either move your mus­cles vol­un­tar­ily or invol­un­tar­ily.  Luck­ily, your brain has built-in over­rides to do it for you if you’re an idiot.

d) Secre­tion of thy­roxin and adren­a­lin.   These involve the use of hor­mones.  The ner­vous sys­tem works very quickly, it’s like send­ing e-mails elec­tron­i­cally.  The endocrine sys­tem is like send­ing snail mail.  With hor­mones, you have to man­u­fac­ture, secrete, and have them cir­cu­late in the blood stream to then finally cause changes in the body.  This may take hours in some regards.  If you’re cold for an extended num­ber of days, your ther­mo­stat in your brain will secrete thy­roxin (from thy­roid gland) and adren­a­lin (from adrenal gland) and both have very dif­fer­ent pur­poses but they both increase the rate of cel­lu­lar res­pi­ra­tion to break food down at a faster rate, in part to pro­duce heat.  (We already learned that 60% of the cell res­pi­ra­tion energy is given off as heat.)  This is known as acclima­ti­za­tion.

In the win­ter time, as we are con­tin­u­ously exposed to cooler weather, and if you live in VERY cold places like Anchor­age, Alaska, or Min­neapo­lis, Min­nesota, this is called “Meat and Pota­toes Coun­try.“  Peo­ple eat heavy foods to keep up with the increase in metab­o­lism.  When the tem­per­a­tures start to warm up, our meta­bolic rate slows down, we gen­er­ate less heat.  In the warm, sum­mer months, peo­ple don’t feel like eat­ing heav­ier foods and will choose the yogurt and sal­ads because it gives off less heat.  So that’s known as acclimatizing.

What are the home­o­sta­tic reflexes that are acti­vated when­ever the body tem­per­a­ture is HIGHER than the set point?

a) Per­son feels hot.  Maybe you’ll take a jacket off, have a cold drink, and stand in front of a fridge. If that’s not enough…

b Cuta­neous vasodi­la­tion.  The blood ves­sels will dilate to give off heat and the skin will look flushed.  If that’s not enough…

c) Sweat­ing.  We will start to sweat to get rid of heat.

d) Acclima­ti­za­tion.  Again, in terms of longer term acclima­ti­za­tion, our thy­roid and adren­als will secrete less thy­roxin and adren­a­lin to lower the meta­bolic rate.

Next we take a patho­phys­i­o­log­i­cal appli­ca­tion of this and talk about Inflam­ma­tion and Fever!

Basic Phys­i­ol­ogy Primers

• The Con­cept of a Ref­er­ence Person
• Review of Cell Mem­brane Structure
• The Pro­teins Embed­ded in Cell Membranes
• Fluid Com­part­ments within the Human Body
• Move­ment of sub­stances across cell membranes
• The Prin­ci­ple of Homeostasis
• Reg­u­la­tion of Body Temperature
• Inflam­ma­tion and the Patho­phys­i­ol­ogy of Fever
• The Men­strual Cycle