physio_thermoregulation

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Thermoregulation

Overview

First we will discuss variations of normal temperature

We will talk about the balance of heat among human

beings, the physiological factors that can affect the

production of heat as well as the physical factors that

affect the exchange of heat between the body and the

environment.

Also, we will discuss the integration of the various

responses of the human body in so far as the handling

of heat as well as cold is concern.

And of course, we will talk about fever and other

disorders of the regulation of temperature.

Lecture Proper

We all know that physical and chemical reactions aredependent on temperature. With this, physiological

reactions are actually sensitive to changes. And of

course, we want all these reactions to be relatively high

in terms of rate. And we want these reactions to be

optimal. So that means we really need to regulate

temperature and if we do not there are certain

consequences to it. If it is very cold, even the heart can

be affected and the heart can undergo fibrillation. If it is

very hot and we cannot control this, we can develop

brain lesions. Of course we all know that the nerves areaffected by heat and denaturation of proteins can occur

in extreme settings. So the normal range of body

temperature is 36-38°C (37°C or 98.6°F).

Temperature Physiological response

27-29°C Cardiac fibrillation

30-34°C Impairment of

temperature regulation

34-36°C Mild hypothermia

36-38° Normal38-40°C Fever or exercise

40-44°C Heat stroke, brain lesion

Normal Variations of Human Body Temperature

The body tries to maintain the normal temperature but

nevertheless there are still variations in the normal

body temperature. When we talk about temperatures,

there are certain temperatures that we can actually

obtain. There is what is known as the core body

temperature (temperature within the body especially

your internal organs such as the liver, heart) and skin

temperature.

It is really the core temperature that is as much as

possible maintained at a constant level where as the

skin temperature can be varied depending upon the

temperature of the environment.

Core body temperature: homeothermic body

temperature

Core Temperatures:

a. Core temperature differences : 0.2 C -1.2 C; brain exhibits radial temp.gradient of > 1 C from center to cortex.

b. Rectal temperature - highesttemperature

c. sublingual (0ral) temperature - mostpreferred clinically, 0.2 C - 0.5 C lowerthan rectal; affected by inspired airand by food and drink

d. esophageal temperature - (abovecardia) - used in sports medicine, withthermosensors; shorter delay than

rectal to measure body-temp.

e. axillary temperature - can representcore temperature but a delay of 30minutes for equilibration to measureaccurately; readings affected byvasoconstriction especially when feveris building up, in a low ambienttemperature

Skin temperature: poikilothermic shell

Skin Temperaturea. Skin temperature- represents shell

temperatureb. mean skin temperature - from

measurements of forehead, chest,abdomen, upper arm, forearm, back ofhand, thigh, calf, and dorsum of foot;

SUBJECT: PHYSIOLOGY

TOPIC: THERMOREGULATION

LECTURER: DR. MENDOZA

DATE: MARCH 2011



in calculating mean, mean iscalculated in accordance with body-surface area they represent. Themean skin temperature measured inthis way for a nude person in acomfortable ambient temperature is 33- 34 C.

But there are variations even in the core body

temperature. And there are several factors that can

affect this. One is time of day. Second is among females,

it is where they are in terms of their menstrual cycle

that can affect the temperature. Another is physical

activity. Fourth is the age of the subject because usually

infants as well as the elderly have very poor control of

temperature. For instance, one way in which we can

dissipate heat is by sweating. However, infants cannot

sweat. So obviously infants are like other organisms

whose temperature are actually dependent upon the

external temperature. Then among the elderly, there

temperature sensors are already deranged and at the

same time, their metabolic rate is very low therefore

their ability to produce heat is not that good anymore.

Secondly, in terms of heat production, one of the ways

in which heat can be produced within the body is by

muscular contraction. We know that the muscle mass of

the elderly is significantly reduced. So age can actually

affect the core temperature.

Factors that affect the core body temperature:

Time of day

Menstrual cycle

Physical activity

Age of the subject

Degree of alertness

Periodic Fluctuations of Core Temperature

Diurnal Rhythm

The core temperature can fluctuated depending on

the time of day. Normally, the lowest temperature is

at 3:00-6:00 am and the highest temperature is at

3:00-6:00 pm.

So, this is known as the diurnal rhythm as far as the

core body temperature is concerned. It is not really

related to the night-day cycle but in the 24-hour

cycle and some believe that this is synchronized with

the earth’s rotation along its axis and this occurs in the

absence of signals such as light stimulus. It is dependent

upon the sleep-wake pattern. So if you are moving to

another time zone, we neec 1-2 weeks in order for the

temperature rhythm to adjust to the normal living

patterns set by the new time zone.

Degree of alertness

When you are alert, your temperature is relatively

higher.

Menstrual cycle

Then among females, the rhythm method is actually aform of contraceptives or birth control. What is done

here is that the female actually measures her body

temperature. When ovulation is about to happen, body

temperature actually increases. When this happen is it

safe or not? NOT. Traditionally, it has been thought that

the reason of this increase in temperature is the surge

in progesterone. (new researche: undetermined cause)

Exercise

With exercise, we produce more heat. As a result, you

get to see an increase in core body temperature as

reflected by the temperature taken from the rectum.

As you can see, in work, there is an increase in the core

body temperature but there is a drop in the skin

temperature. This is because in exercise, there is an

increase in sweating. Sweat evaporates which displace

the heat. This is why the skin temperature is muchlower.

By the way there are several ways in which we can

measure the body temperature. You can use oral

thermometer. Nowadays, there is what we call the

automatic thermometer that uses the heat generated

by ear. This is correlated with the oral temperature. We



can also take the core body temperature through the

rectum. Among the two, the more reliable but not

practical is the rectal temperature because it is not

affected by the environment or the air.

Why does the body maintain its core temperature

constant at 37° 0.6°C?

This is because a lot of chemical and physical factors are

depedent upon the body temperature. Also,physiological functions are sensitive to temparature

fluctuations. And we want as much as possible for these

reactions to be optimal.

The rate of enzymatic reaction increases with an

increase in body temperature. primarily because of this

increase in energy of substrates otherwise known as the

Brownian motion. But beyond a certain temperature,

you can actually damage, especially proteins

(denaturation). So as much as possible we want all

reactions to operate at an optimal range.

How is it that the body maintains a constant

temperature?

There are several factors that come into play to regulate

the core body temperature.

Physical factors and physiological factors

For us to have a constant temperature there must be

heat balance

Heat loss = heat gain

Heat productionPHYSIOLOGIOCAL

Heat lossPHYSICAL

Metabolism

Muscle contraction

Heat transfer at the skin’s

surface

1) Basal metabolic rate of all cells (biological

and chemical processes of the cells utilized

energy and the by product of this is heat)

2) Increased metabolic rate due to muscle

activity (voluntary as well as involuntary). When

muscles are at work (contraction), the consume

much energy (ATP) which produces heat.

Exercise can also increase metabolic rate 100x

to 1000x compared to the basal metabolic rate.

3) Sympathetic Nervous Excitation which leads

to release of (i) Norephinephrine andEpinephrine (produced by adrenal medulla; can

actually induce glycogenolysis which in turn

produces heat) and (ii) increased Metabolism of

brown adipose tissue (brown fatin neck,

scapular, nape area)

Brown fat metabolism induce heat production

through the stimulation of the sympathetic

nervous system which induces the release of

free fatty acids which undergoes β-oxidation.

In infants, heat production rate can increase by

100% with chemical thermogenesis while in

adults the increase is only 10-15%.

**infant cannot shiver so they generate heat

through brown fat metabolism.

4) hermogenic effects of hormones : (i)

glucagon (induces glycogenolysis); basal and

long-term effects of (ii) thyroxine (affect

metabolism by increasing transcription,translationprotein synthesis enzymatic

activity fuel utilization heat production); (iii)

progesterone (menstrual cycle); (iv) growth

hormone; (v) catecholamines.

A.5) Temperature-dependent increase in cell

metabolism; Brownian motion and temperature

effects on enzyme kinetics.

As you increase the temperature in tissues,

the metabolic rate of tissue itself increases. Thisproduces more heat. An increase in temperature

in tissues of about 10°C will double the

metabolic rate of that tissue.

6) the metabolism of food that we take in

produces heat. This is known as the specific

dynamic action of food intake. It is the obligatory

energy expenditure that occurs in assimilation

into the body.

For instance, when protein is ingested, we

increase the metabolic rate by 30 kcal,

carbohydrates by 6 kcal, and fats by 4 kcal. So

oxidation of food itself will bring about an increase in

heat production.

Physiologic Factors Affecting Heat Production



Physical Factors Affecting Heat Exchange withthe Environment

When heat produced is already in excess you need to

have a mechanism by which you are able to lose the

heat. These are the factors in which the body can

dissipate heat towards the environment:

1. conduction to objects (3% of heat loss) –

whenever you are in direct contact with an

object which has a lower temperature than

your skin, then there is actually heat transfer

(conduction).

If the temperature of the object is higher than

the skin temperature then the opposite can

occur.

2. air currents - heat dissipation by convection

3. evaporation (accounts for 22% of heat loss)

4. radiation (60% emmission of infrared)

Every object emmits infrared radiation. So if the

environment is lower than the human

temperature then the body is able to radiate

this infrared electromagnetic waves.5. conduction to air or fluid (15%)

if air or water surrounding the body has lower

temperature then the body, then you lose heat

by direct conduction.

As far as air is concern, when you lose heat, the

air around you increases in temperature. When

there is equal temperature between the body

and the environment, then there is no more

heat exchange.

With air, when it is heated up, air rises so it isreplaced by cooler air. With this, heat exchange

can continue.

Another way to displace heat in the air is

through air currents. So when there is wind the

that displaces the air surrounding your body

which is a little bit higher because of the heat

exchange that occurs. This is the reason why

when you are in a cold temperature and there is

wind you feel colder still. That is because the air

that surrounds you that is heated up by your

body is replaced by new air.

Effects of clothing on External Heat Flow Conduction and convection can be affected by

insulation (clothes that you wear).

***the percentages presented above occurs in a person

without clothing (KINKY!!!)***

the human body has its own insulation in the form of fat

content. That is the reason why females are more

resistant to changes in temperature (females have a

higher fat content than males).

Thermal resistance or insulation - Trapped airspaces in the weaved material decreasesconduction, evaporation, and convective heattransfer.

Half the heat from skin to clothing is radiated. Thisis further radiated from clothing to air.

Specialized clothing can prevent radiative heatloss,such as insulating the inner lining with gold.

Body temperature during exercise

During exercise, the body produce more heat. As a

result you need more heat dissipation. Evaporation

accounts for heat dissipation during exercise. However,

evaporation is not dependent on temperature gradient.

It depends upon the water content of air. If humidity is

very high, then significant less evaporation will occur

because water content of air is high already. If humidity

is very low then evaporation can occur efficiently.

Formula: Htot = Hc + Hr + He + Hs

Legend:

Htot = rate of metabolic heat production

Hc = rate of conductive and convective heat exchange*

Hr = rate of radiative heat exchange*

He = rate of evaporative heat exchange*

Hs = rate of storage of heat in the body*



Therefore heat disspation is much greater. (During

exercise, do not wipe of the sweat, let it evaporate for

this will cause a greater amount of heat exchange).

Countercurrent principle and thermal conductance

Going to the extremeties you have your warm arterial

blood.

Going away from the extremeties you have your cold

arterial blood.

So there is actually an exchange because usually, your

arteries and veins run parallel with each other so there

can be a counter exchange (heat transfer).

Arterial blood is warm because it came you’re your core

organs especially your the heart.

Body size

Increased metabolic rate per unit mass therefore

produces more heat

Among infants the surface area: volume ratio is greateras compared to adults. Therefore there is a tendency

for infants to lose more heat as compared to adults.

Unfortunately, infants do not have a lot of the

mechanisms that are available for adults to produce

heat. That is the reason why when you look into the

nursery, infants are placed under a lamp to keep them

heated up.

This relationship is called the Law of metabolic

reduction.

Age matters when it comes to controlling the core body

temperature. infants have a hard time controlling therecore body temperature because they do not have

mechanisms of heat production same with the adults.

Their only consolation is that they do have a significant

amount of brown fat so they can induce naturing heat

generation. Infants have higher metabolic rate.

Whereas in elderly, they have a lower metabolic rate

compared to normal adult.

The greater metabolic rate among infants is really to

match the greater loss of heat as a result of the freater

surface area:volume body ratio.

In terms of the integration of all of these, in matching

heat production and heat loss, there has to be a system

to keep this in balance. There are several components

to this system. One of which is that it has to have a

thermoreceptor (both in the skin and in the brain) that

responds to changes in temperature. And then you

should have an afferent pathway which is projected

towards your central integrator in the brain. From the

central integrator, you should have an efferent pathway

that goes towards your target organs (muscles, blood

vessels and sweat glands) which help in the control of

heat exchange.

A lot of patients are subjected to surgery, in surgery,

they are subjected to anesthesia. Anesthesized patients

have a special need as far as thermal regulation is

concerned so they have to be watched because if their

normal thermoregulatory apparatus is dysfunctional,

they can go into a lot of trouble. They can develop a

malignant hypothermia in reaction to anesthetic.

Temperature Receptor Reaction

Freezing cold Cold

nociceptor

Pain withdrawal reflex

before cold block ensues

15-38°C Cold

receptor

Thermoregulatory reflex

30-43°C Warm

receptor

45-49°C Cold

receptor

Paradoxical cold

sensation

Heat

receptor

Burning hot Heatnociceptor

Pain withdrawal reflex

Afferent fibers

Cold afferents: Type Aδ and Type C

Warm afferents: Type C

Number of receptors

Cold receptor > warm receptor

In most areas of the skin such as the lips, the

fingers etc.

These receptors are stimulated dependent upon the

temperature

Aside form the skin thermoreceptors, you also have

receptors in the central nervous system (in the

hypothalamus). When you apply an electrical impulse

(heat) in the hypothalamus you can activate these warm

sensitive neurons which are found in the anterior

hypothalamus and preoptic area. This will result in the

activation of the heat loss center. This will then activate

mechanisms in the periphery that will result in the lossof heat such as increase blood flow in skin, activate

sweat gland. Lesions at the hypothalamus and the

preoptic area will result to hyperthermia.

On the other hand, when the hypothalamus is sent cold

stimulus, then there are also cold sensitive neurons

(cooling center) found in the anterior hypothalamus and



preoptoc area which in turns activates the posterior

hypothalamus (heat conservation center), inducing heat

conservation in the body (shivering, muscle contraction,

vasoconstrictionless blood flow to skin less loss of

heat).

Hypothalamic area Effects of

LocalWarming

these nuclei

Effects of

LocalCooling

these nucleiAnterior

Hypothalamus and

Pre-optic Area of

Hypothalamus(gene

rally contain cold-

and warm-sensitive

neurons – respond to

local cooling and

local warming

respectively).Majority are warm-

sensitive. Two

thermoregulatory

fcns:

1. Central

thermostat-

hypothalamic

thermostatic

detector ,

2. cooling center

Local warming

activates

thermo-

regulatory

cooling

mechanisms .

What do you call

this center ?

Effects ofLesion ?

Local cooling

of this region

activates cold-

sensitive

neurons

which in turn

may activate

neurons in the

posterior

hypothalamus

Posterior

Hypothalamus

(generally contain

neurons which

respond to signals

from distant or

peripheral

thermosensors, and

are not responsive to

local changes in

temperature ).

Majority respond tocold signals from the

periphery. Two

thermoregulatory

fcns:

1. Integrates

peripheral and

central temperature

2. Heat production

and conservation

center)

Localized

warming has no

clearcut effect.

Localized

cooling has no

clearcut

effect.

However,

cooling

anterior

hypothalamic

neurons-

preoptic area

can activate

this region –to

induce heat

production

and heat

conservation

mechanism.

Lesion

effects ?

There are other thermo-sensitive structures in thebrainstem and spinal cord, which also mediatethermoregulatory reflexes.

Other CNS Effects of Local Effects of

areas Warming of these

structures

Local Cooling of

these structures

Lower

Brainstem

thermo-

sensitive

structures

(midbrain,

medulla

oblongata)

Enhanced heat loss

(lesser degree

compared to

hypothalamic

sensitivity)

Spinal cord

thermosensitiv

e structures

(extremely

thermosensitiv

e)

Local warming

Activate heat loss

mechanisms such as

(a) panting in dogs

and other

animals

(b) vasodilation

(c ) inhibition of

thermogenesis

Local

cooling activates

shivering

thermogenesis

.

Lateral



As much as 30% of cardiac output can go to the skin in an

effort to dissipate heat.

alteration of behavior:

when hot, there is a tendency for the person to go to an

airconditioned room.

Dysfunction of temperature control

One of the most common changes as far as the body

temperature is concern is FEVER. Fever is the shifting of set

point for body temperature which results from increased

heat production by shivering and increased vasoconstriction.

During the recovery from fever, there is an increase in sweat

secretion, vasodilation reduction in body temperature.

Posterior hypothalamus (cooling center) is activated as fever

develops.Fever can be caused by several things. When we are dealing

with bacterial infection, this triggers the release of cytokines

(IL-1, IL-6 ). In malignancies, you can develop fever without

infection, this is due to the production of cytokines through

the TNFa (tumor necrosis factor).

Sometimes, the IL-1 cannot penetrate the blood brain barrier.

How can these cytokines induce fever?

They act on the Organum

vasculosum of the lamina

terminalis (OVLT). These

structure has fenestrated

capillaries. These OVLT serves

as a chemoreceptors for

cytokines and thereby activate

the preoptic area of the

hypothalamus. This in turn will

induce the production of prostaglandin E (PGE2) which

will in turn act with the cells in

the hypothalamus thereby

resetting the set point of

temperature regulation. This

will be set in a higher level.

This will activate the posterior

hypothalamus as a result the

body will produce more heat

shivering, increase in

metabolic rate and promote

the concentration of heat by

vasoconstriction FEVER. In

infants, this will activate the

sympathetic nervous system which will metabolize brown fats

which will induce heat production.

Why do we develop fever during infection? At a higher

temperature, the phagocytes act to combat the infection.

temperature, blood flow, phagocytosis (due to

metabolism), lymphocyte proliferation. In viral infection,

fever can stimulate the production of interferon which can

limit viral damage.

Heat stoke – condition wherein you can induce brain damage

as a result of very high body temperature (41°C) for a long

period of time. This can cause nerve degeneration, protein

denaturation, cerebral edema, disorientation, delirium,

convulsion.

Heat stroke above 43°C can lead to death.

Antipyretics

Example: aspirin (NSAID), paracetamol

These act on cyclooxygenase pathway which is the one

responsible for the production of PGE2.Its basic mechanism is to prevent prostaglandin production to

prevent resetting the temperature regulation at the

hypothalamus.

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it’s the time of year again when all we can think about are the

trips that we’re going to go this summer, the warm waters

where we’re going to be dipped in, and people who we

missed this past school year. But there are still a few more

blows of evaluations on our way! So let’s give our best to

these last set of exams so we can enjoy our summers to the

fullest!!!

Good luck batch 2014 we started this year together, we are

going to end this TOGETHER!!!!



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PHYSIOLOGY :THERMOREGULATION

physio_thermoregulation

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