How Does the Human Body Regulate Temperature

7/29/2019 How Does the Human Body Regulate Temperature

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How Does the Human Body Regulate Temperature?The human body normally maintains a set body temperature. How does thishappen? How do we generate additional heat when our body is too cold, andhow do we cool off when we are too hot? Although part of this response isvoluntary, how do we really know that we are cold, and what is our brain

doing involuntarily to keep temperatures constant. At times our temperaturewill move away from the set point, whether through environmental effectssuch as exposure to cold or internal processes including fever and exercise.

How is Heat Distributed Throughout the Body?

Humans are homeotherms, maintaining an average core temperature of37 +/- 0.5 degrees Celsius. Core temperature varies slightly due toenvironmental and metabolic factors. Exercise or fever may raise coretemperature by up to three degrees, while exposure to cold may lower core

temperature by a degree. Beyond these boundaries, the human issusceptible to heat stroke (elevated temperature) or hypothermia, both ofwhich are life threatening conditions.While core temperature is tightly regulated, skin temperature variesgreatly in response to metabolism and the environment. Temperaturereceptors in the skin (cold and hot) detect these changes, initiatingcompensatory mechanisms through the central nervous system.

How Does the Body Produce Heat?

Energy in the form of heat is gained by two methods: production (metabolic)and acquisition (environmental). Heat production and retention are elevatedby a number of factors.

1. Muscular Activity and Shivering the conversion of stored chemicalenergy results in a net increase in heat throughout the skeletal muscle,which spreads through the body.

2. Thyroxin effect on cells thyroid hormones increase the basalmetabolic rate in cells throughout the body, resulting in thermogenesisthrough increased conversion of chemical energy

3. Epinephrine, Norepinephrine and Sympathetic Innervation effect oncells sympathetic effects include the following:

Piloerection contraction of body hair muscles, trapping air next to the skin and reducing heat loss (small effect inhumans)

Increased Metabolic Rate Brown Fat Oxidation In brown fat cells, oxidative

phosphorylation is uncoupled from ATP production,


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resulting in complete conversion of chemical energy toheat energy (little brown fat remaining in adults)

Skin Vasoconstriction decrease in radiation of heat from skin

How Does the Body Remove Heat?

Heat is transferred from the body to the environment through a range ofmechanisms.

Mode of Heat Transfer Percentage Loss (NormalConditions)

Radiation 60%Conduction 15% to air

3% to other objectsEvaporation 22%The body can increase the rate of heat transfer by either increasing itsconduction rate (movement of heat from core to surface) or transfer rate(movement of heat from surface to environment). Conduction rate is greatlyeffected by the dilation and constriction of vessels, while transfer rate is afunction of air temperature, perspiration, clothing, food and body positionamong other factors (learn more about body thermodynamics).When environmental temperature is above that of the core, sweating ratescan be modified to achieve desired cooling. Sweat glands are stimulated bysympathetic cholinergic fibers as well as by epinephrine in the blood. In

times of rapid temperature change, reabsorption of sodium and chloride iscompromised during perspiration. Sweat glands may excrete as high as 60mEq/L of Na and Cl ions (as opposed to 5 mEq/L during times of optimalreabsorption). If the body is exposed to heat for long periods of time,electrolyte levels may fall, leading to systemic dehydration.Given time, thehuman body compensates for this effect through the process ofacclimatization. Through an increase in aldosterone levels, sweat glands arecapable of increasing reabsorption of electrolytes at high temperatures.

How Does the Body Know What to Do?Temperature receptors are located throughout the skin, as well as in the

spinal cord, abdominal viscera, and in and around the great veins. Stimuli tothese receptors triggers heat and cold sensitive neurons in thehypothalamus as well as higher cortical centers in the cerebrum. Thehypothalamic receptors are part of the brains thermostat center, whichregulates responses that promote gain and loss of heat.

Sympathetic Response TRH secretion from the hypothalamus Stimulation of the hypothalamic shivering center.
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Cerebral stimulation elicits a voluntary response.

Curling up to reduce exposed surface area

Change in muscular activity

Food consumption

Change of environment (moving from a cold room to a warm room)

Change of clothing

What Causes a Change in the Set Point?Infection may provide the stimulus required to initiate fever. It is believedthat the presence of bacteria or virus stimulates the release of endogenouspyrogen (a small molecular weight protein) from macrophages. Although noindividual endogenous pyrogen causes all fever, studies have shown a highcorrelation with interleukin-1 beta, interleukin-6, tumor necrosis factor alpha,and interferons beta and gamma, each cytokine independently capable ofcausing the febrile state.

Endogenous pyrogen travels through the blood, arriving at the hypothalamusto cause an increase in the set-point, possibly through a subsequent increasein prostaglandin levels. The brain appears to recognize the presence ofendogenous pyrogens at certain sites known as circumventricular organsthat lack a blood-brain barrier.How does the body protect itself from high fever? It remains unclear as tothe mechanism of regulating a new set-point. Arginine vasopressin andalpha-melanocyte-stimulating hormone are two peptides that have beenshown to reduce fever, but there are many candidates for the process ofcontrolling fever levels, either by mediating effects of endogenous pyrogensor acting independently to reduce the set-point.

What Are the Effects of Fever? Is it Beneficial?Fever is an energetically costly process, especially in the compromised stateof infection, and some studies have shown that the administration ofantipyretic (fever-reducing) drugs appear to have little effect on the courseof certain diseases.However, the fever state is shown to have a number of effects on host-defense.

Enhanced neutrophil migration Increased production of antibacterial substances by neutrophils (eg,

superoxide anion) Increased production of interferon Increased antiviral and antitumor activity of interferon Increased T-cell proliferation Decreased growth of microorganisms in iron-poor environment


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How Does Aspirin Lower Fever?Hippocrates wrote of a bitter powder extracted from willow bark that easedaches and pains and reduce fevers as long ago as the fifth century B.C.Eventually, it was discovered that the active ingredient in willow bark issalycin, which is converted by the body into salycilic acid.

Aspirin works by inhibiting the effects of pyrogens on the hypothalamus.Aspirin is acetylsalycilic acid, used because it is believed to be less harmfulto the stomach.

Acetylsalycilic AcidReferences:Guyton, A.C., M.D and J.E. Hall M.D., Textbook of Medical Physiology(9thEdition). W.B. Saunders Company, 1996.Kapit, Macey and Meisami. The Physiology Coloring Book. Pp. 133-134. 1987.Harper Collins Publishing.Klugger, Mathew J. Fever Revisited. Pediatrics 1992; 90: 846-850.Saper, Clifford B.; Breder, Christopher D. The Neurologic Basis Of Fever. N

Engl J Med 1994; 330: 1880-1886.


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KEY CONCEPTS

1. Energy balance occurs when the energy going into a system equals thatcoming out. If energy in (dietary intake) exceeds energy out (exercise,

metabolism, processing of food, etc), weight gain occurs. The internationalunit of heat is the calorie, which is defined as the amount of heat needed toraise 1 gram of water 1o C. Usually energy units are expressed in Kilocalories(=1,000 calories); a Kilocalorie is also called a Calorie (Note the capital C todistinguish it from calorie).

2. Food intake is regulated by the "satiety" center in the hypothalamus andinvolves both short term (hunger bouts between meals) and long term(overall weight regulation) control. Short term responses include hormonalrelease (CCK) from the small intestine and distention of the stomach toinhibit feeding following a meal, as well as Neuropeptide Y (NPY) release in

the hypothalamus between meals to stimulate feeding. Long term regulationof weight involves Leptin, a hormone secreted by white fat cells, whichinhibits feeding. Leptin acts by inhibiting a stimulatory brain neuropeptide,Melanocortin. Orexin A and B are other brain neuropeptides that stimulatefeeding.

3. Metabolism is the sum of all of the cellular reactions in the body. At thelevel of the organism, metabolism is usually measured as the rate of oxygenconsumption. A standardized estimate of metabolism is Basal Metabolic Rate(BMR), which is the minimal rate of energy expenditure in an awakeindividual. To obtain an accurate BMR, energy requiring activities are

minimized (e.g., absence of physical activity, sweating, shivering, foodprocessing, etc.).

4. Core body temperature is tightly regulated at about 99oF even under coldand hot environmental conditions. The preoptic/anterior hypothalamus isresponsible for regulating body temperature. This brain area contains athermostat which acts like a set point to control various effector systems.Constant body temperature is achieved by balancing heat gain with heat losseffector systems. When body temperature is slightly elevated relative to theset point, heat loss mechanisms are stimulated (sweating, shallow and morerapid breathing, increased blood flow to the skin). When body temperature

decreases relative to the set point, heat production or heat conservationmechanisms are activated (shivering, nonshivering thermogenesis-NST,peripheral cooling of the limbs, and modification of behavior, such as puttingon a sweater). The hypothalamic set point, however, can change (e.g.,during a fever) due to infective agents, such as bacterial pyrogens.

LECTURE OBJECTIVES


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1. What is energy balance and how is it important?2. Establish how set point is important to temperature regulation3. Describe how the body actively maintains a constant body temperaturedespite changes in ambient temperature

LECTURE OUTLINE

I. ENERGY BALANCE

A. Energy Balance is when energy in = energy out1. Most of our daily energy budget goes to maintaining

Tb2. Calorie, a measure of heat quantity

B. Regulation of food intake is a complex of interactions

1. Neuropeptides are important in long term regulationof body weighta. NPY and the Orexins stimulate feedingb. Melanocortin inhibits feeding

2. Leptin, a hormone, regulates these neuropeptidesa. Parabiosis experiment (Ob/Ob mice)

3. Summary of short & long term regulation of bodyweight

C. Metabolic rate

1. Oxygen consumption as an measure of metabolism2. Stadardizing metabolic measurements3. Basal metabolic rate (BMR)

a. Conditions that have to be meet for anaccurate BMR (inactive, thermoneutral, fasted, etc.)

b. Scaling4. Ambient temperature has a major effect on metabolism

D. Daily energy balance

II. TEMPERATURE REGULATION--GENERAL CONCEPTS

A. Heat exchange and the individual1. Heat flux goes down a temperature gradient2. Modes of heat transfer (Radiation, Conduction,

Convection,and Evaporation)
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B. Body temperature is regulated at 99oF

C. Energy expenditure changes with environmentaltemperature

1. Zone of thermoneutrality2. Sweating in the hot and shivering in the cold use

energy

III. THERMOREGULATORY CENTER AND SET POINT

A. Thermostat is in PO/AH region of hypothalamus

B. Set Point Theory

1. ,,,,,,,,,,,HC1[D0============= from set point2. Shifts in the set pointa. Fever as a change in set pointb. Causes of fever--roles of pyrogens and

Prostaglandins

IV. PHYSIOLOGICAL MECHANISMS -- THE SPECIFICS

A. Heat gain and heat loss determine body temperature1. Only core temperature is regulated at 99oF

B. Specific Physiological Mechanisms1. Vasoconstriction and vasodilation of vessels2. Shivering3. Nonshivering thermogenesis (NST)

a. Role ofbrown fatb. Molecular mechanism for heat production--

Thermogenin4. Sweating

a. Heat acclimation and Aldosterone
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Internal Regulation: Temperature Regulation & Thirst

How do human beings adapt to the differences in the environmentalextremes represented by the images above (desert) and below (Le Moyne)?

For about five minutes in small groups, the class should list as manyadaptations as you can think of for living in each of these environmental

settings.

I. Temperature Regulation

A. Basal Metabolism = energy used each day to maintain constantbody temperature while at rest

1700 calories (kcal) of 2600 calories of average total daily energyexpenditure goes to basal metabolism.
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B. Homeostasis

= biological processes whichkeep temperature & otherbody variables within a

certain range

For example, temperature,weight, hydration (fluids; H2O)content of blood (acidity, oxygen,fat, glucose)

Homeostasis is maintained byboth (1) internal biologicalprocesses and (2) externalbehaviors.

Set Point: homeostasis reliesupon set points for each bodyvariable, that is, a specificnarrow range of acceptablevalues which the body mustmaintain. For example, thehuman body temperaturegenerally stay between roughly36.5 and 37.5 degrees Celsius(97.7 and 99.5 degrees

Fahrenheit)

Negative Feedback: Theprocesses the body uses tomaintain set points relies uponreducing the discrepancybetween the current value ofthe variable and the set pointvalue. This approach is callednegative feedback. (See graphic

and example on right).

C. Controlling Body Temperature


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Poikilothermic Animals: their body temperature is the same as theenvironment's

Reptiles, amphibians (turtles, etc.), and most marine animals like fish

They do not have internal mechanisms (generally) to affect their bodytemperature, e.g., they lack the ability to shudder or sweat

They move within the environment (use behavioral means) to keeptheir body temperature in proper range.

Some poikilothermic animals such as frogs, fish, and insects usemolecules of antifreeze (glycerol and others) to survive intemperatures at 40 degrees F. Others effectively dehydrate theirtissues before actually freezing and minimize tissue damage due to icecrystals.

Homeothermic Animals: Maintain an almost constant bodytemperature despite variations in the environment's by use of

physical mechanisms

Birds, mammals, humans

Homeothermic animals employ energy requiring fuel (food) in orderto maintain their body temperature, a major component of basalmetabolism. In humans the constant body temperature hovers near37.0 C (98.6 F)

Cattle = 38.0 C(100.4 F)

Dog = 39.0 C (102.2 F) Goat = 39.0 C(102.2 F)


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Horse = 38.3 C(100.9 F)

Mongolian Gerbil = 37.5 C(99.5 F)

Mouse = 36.9 C(98.4 F)

The Advantages of Constant High Body Temperature (ca. 37 Celsius)

1. Mechanisms for cooling are less efficient than for heating. It is importantfor body to be warmer than air.

2. High body heat keeps animal ready for rapid movement.

Note: Above 40 C (104 F) some proteins become unstable. Hence,body heat needs to be lower than 40 C.

Cells for reproduction (esp. sperm in males) must be kept cooler than

rest of body.

Mechanisms for raising heat include going to a warmer place, putting onmore clothing, shivering, decreasing blood flow to skin, becoming moreactive, increasing metabolic rate, huddling with others, etc.

Mechanisms for lowering heat include finding a cooler place, becoming lessactive, sweating or panting, taking off clothing or shedding coat of fur, anddiverting blood to the skin.

Physiological Mechanisms of Temperature Regulation


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The major brain mechanisms for heat regulation are found in two nuclei ofthe hypothalamus: (2) preoptic area(POA) and (2) anteriorhypothalamus (AH)= POA/AH (treated as one area)

POA/AH monitors its own temperature + receives input from receptors

in skin & spinal cord. It then sends signals to the body to engage inactivities like shivering, raising body metabolism, etc. Mechanisms for heat control -- shivering, sweating, etc., -- gradually

develop in maturing mammal and are not present at birth. Forexample, room temperature of 68-73 F is comfortable for people, buttoo cold for newborn rat.

Fever & Infection

Infection > Leukocytes -> interleuken-1 >

Prostaglandins >POA/AH > Bodytemperature goes up

Moderate temperature risemay help to kiss orweaken some bacteria.

Fevers of 105-109 F canbe fatal

II. Thirst

Water = 70% of the body of a mammal. The chemical processes of the bodyrequire that there be different concentrations of fluid. Thus, water isnecessary for all bodily functions.

Mechanisms of Water Regulation

When body needs water, posterior pituitarygland releases vasopressin (= antidiruetichormone [ADH]) which causes the kidneys toreabsorb water & increases concentration ofurine.


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Most humans drink more water than we need and excrete the excess.

It is possible to drink so much water to cause hyponatremia (lack ofsodium) and, possibly, death.

Police investigating death of fraternitypledge

By GREG WELTER - Staff Writer

"Thursday, February 03, 2005 - A ChicoState University student died Wednesdaymorning following what police have termed"fraternity pledge activities." Matthew WilliamCarrington, 21, formerly of Concord, waspledging Chi Tau fraternity and collapsed in the

basement of the house at 440 W. Fourth St.sometime before 5 a.m.

Authorities are looking into reports that he andother pledges may have been forced to drinklarge quantities of water and exercise heavily,inducing a sometimes deadly condition calledhyponatremia."

--Chico (CA) Enterprise-Record

Two types of thirst: Osmotic (due to eating salty foods) and hypovolemic(due to loss of fluids)

Osmotic Thirst

The set point for solutes(molecules in fluid) in bodyis constant at ca. 0.1 M

Osmotic Pressure =

water tends to flowacross a semipermeablemembrane from area oflow soluteconcentration to highsolute concentration.(The membrane is"semipermeable" because
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the water can pass through but the solute can't.)

Cell membranes are semipermeable

When a person eats very salty food, the result is a higher

concentration of sodium ion solutes in the extracellular fluid. Osmoticpressure draws water out of cells. Certain neurons detect the loweringof their water content and this triggers osmotic thirst. We tend todrink more water and kidneys excrete more salt.

Where are the osmotic pressure detectors?o OVLT (organum vasculosum laminae terminalis) -- a set of

neurons in the third ventricle.o Stomach receptors detect high levels of sodium

Detectors affect (1) the supraoptic nucleus & paraventricular

nucleus of the hypothalamus which, in turn, signal the posteriorpituitary to control release of vasopressin, and (2) the lateralpreoptic area of the hypothalamus which controls drinking.

Hypovolemic Thirst

= thirst based on low blood volume (therefore, low bloodpressure) so that nutrients & water cannot get to body's cells.Need to replenish water and lost solutes (e.g., salt).

Sodium craving => Aldosterone released => kidneys, salivary

glands, sweat glands preserve sodium & excrete less concentratedurine.

Mechanisms in hypovolemic thirst

o Baroreceptors: receptors attached to large veins detecepressure of blood returning to the heart

o Kidneys: release the hormone renin when blood volume is low=> splits a blood protein, angiotensinogen to form angiotensinI => converted to the hormone angiotensin II => constrictsblood vessels

o Subfonical organ (SFO) = neurons near the third ventricle ofthe brain signal the preoptic area when they are stimulated byangiotensin.


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o Synergistic effect: if baroreceptors signal very low bloodpressure, much less angiotensin is required to stimulate thirst.

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How Does the Human Body Regulate Temperature