heat, temperature and thermometry. overview 1.why measure body temperature? 2.what is temperature?...

Heat, Temperature and Thermometry

Overview

1. Why measure body temperature?

2. What is temperature?

3. The Celsius scale.

4. Body temperature.

5. What is heat?

6. The effects of heat on matter.

7. Heat transfer

7. ThermometryMercury

Chemical

Thermistor

Infrared Tympanic

إستعراض

لماذا تقاس درجة حرارة الجسم ؟1.

ماهي درجة الحرارة ؟2.

مقياس سلسيوس المئوي3.

درجة حرارة الجسم4.

ماهي كمية الحرارة5.

تأثيرات كمية الحرارة على المادة6.

التحوالت الحرارية)تحول كمية الحرارة(7.

قياس درجة الحرارة :7.الزئبق

المواد الكيميائية

الترميستور )مقاوم حراري(

القياس باألشعة تحت الحمراء

Why measure body temperature?

Homeostasis – the tendancy of the body to maintain the stability of its internal environment.

Homeostasis توازن أجزاء الجسم الداخلي :

Stress – is any condition likely to disturb the homeostasis of the body. The stress can be phsiological or psychological.

الضغط و هي حالة تؤدي إلرباك التوازن الداخلي للجسم وقد يكون الضغط جسديا أو نفسيا.


Metabolism ensures:

Heat production = heat loss الحرارةالمفقودة= الحرارة المتولدة

Energy is released in the cells by oxidation. األكسدة من تتولد

i. Used immediately for work or heat يستخدموالحرارة للشغل

ii. Stored as adenosine triphosphate )ATP( شكل في يخزن Heat production depends on the type and

amount of food eaten. نوع على تعتمد الحرارة كميةالغذاء وكمية


90% of the energy released by oxidation of food appears in the body as heat.90% من المتولدة الحرارة من

الجسم في كحرارة يظهر األكسدة

This oxidation of food occurs in all cells in the body but particularly in secreting tissues and muscles. اكسدة

الخاليا في تتم الطعام

The heat is spread quickly and carried throughout the body by blood to the tissue fluids. الجسم في الحرارة تنتسر

الخاليا سوائل الى الدم يواسطة


High Temperature

Infections can result in an increase in body temperature or fever. الحرارة درجة من تزيد االلتهابات

Fever can damage important proteins and enzymes which are temperature sensitive, in extreme cases this can lead to organ failure. الحمى

العضو فشل الى تؤدي وزيادنها واالنزيمات البروتينات يحطم


Low Temperature

Patients in circulatory shock suffer from

reduced blood flow to the periphery. المريض

الدم تدفق في انخفاض من يعاني الدورة بصدمة الصاب

الجسم محيط الى

A drop in temperature of the extremities is

a good early indicator of shock. درجة انخفاض

للصدمة مؤشر الحرارة


For some surgical procedures hypothermia is induced to reduce metabolic processes and blood circulation.

Anaesthesia can result in a drop in body temperature due to repression of normal thermo-regulatory activity.

Temperature is one of the most commonly made physiological measurements.

What is temperature?

Temperature is a measure of how hot something is.

To use temperature as a measure of hotness or coldness we need to construct a Temperature Scale.

We use the Celsius scale to measure body temperature.

Celsius Temperature Scale

0o

100o

0o The freezing point of

water

Temperatures lower than the

freezing point of water are

negative

100o The boiling point of

water

L divided into 100 degrees

L

L 360C-380C

Body temperature

The normal range of body temperature lies in the range 360C to 380C.

Body temperature increases during the day reaching a maximum in the evening and then drops to a minimum in the early morning. The difference is between 0.50C and 0.10C.

Body temperature is not fixed but is responsive to cyclical changes, called circadian rythms, and menstrual cycles.

Body temperature

The hypothalamus in the central area of the brain regulates many processes including body temperature, metabolism, heart rate and blood pressure.

In a similar way that a central heating thermostat switches on and off a heating system periodically to maintain a set temperature, so the hypothalamus switches on and off the bodies heating and cooling systems to maintain a constant temperature

Body temperature

The body can try to increase its temperature when it detects it to be too low by activating mechanisms such as 1( Vasoconstriction within the skin by which blood flow to the skin is reduced and loss of heat by convection is reduced.

2( Shivering – contractions of the skeletal muscles occur at a rapid rate but the energy is converted into heat as well as mechanical energy.

3( Sympathetic stimulation of metabolism - the body’s cells increase their rate of heat production.

4( Piloerection – hair standing on end captures a layer of air that can act as an insulator.

Body temperature

The mechanisms that are brought into operation when the body’s temperature is too great are 1( Vasodilatation – an increase in the diameter of the blood vessels in the skin can increase the rate of heat transfer to the skin by up to eight times.

2( Sweating – sweat is water containing dissolved salts and is a very effective cooling agent because a small amount of water takes a large amount of heat to evaporate.

Body temperature

Thermal Equilibrium

To make a temperature measurement of an object you place the thermometer in contact with the object.

Example: To measure the temperature of a cup of tea.

Place the thermometer in the tea.

Thermometer becomes hotter.

Tea becomes )very( slightly colder.

When the thermometer has reached a steady state temperature you take a reading.

The system of tea and thermometer has reached Thermal Equilibrium.

What is heat?

As we have said, temperature is a measure of how hot something is and is measured in degrees celsius.

Heat is energy.

Heat is measured in Joules which is the unit of all forms of energy; electrical, heat.

Because heat is a form of energy it can do work.

What is heat?

It is important to remember that a measure of the temperature of a body does not tell you the amount of heat it contains.

We can illustrate this with a simple example:

It takes a much greater amount of heat to bring a kettle full of water to 1000C )i.e. to boil it( than an eggcup full of water.

What is heat?

In a clinical setting we only measure temperature and do not calculate the total heat.

This is because when the body is healthy it regulates its temperature to lie between 360C and 380C independent of its mass.

To detect disease we look for temperature change or failure of the body to regulate its internal environment.

The Effects of Heat on Matter

We start by looking at the atoms in a solid.

In a solid, atoms are arranged in a very regular tight pattern.

A solid has a defined shape because its atoms have very little room to move.

All they are allowed is a slight vibration toward and then away from each other.


Now let us add some heat energy to the solid and see what happens.

Now the atoms are much more energetic and are vibrating much more violently, they are creating more space between them.

But they must still stay in position, so the solid still retains its defined shape.

The vibration of the atoms is so small we cannot see it.

To our eyes the object has increased in size – it has expanded.

Thermal Expansion

Most materials expand when heated.

Bridges and railway tracks are always constructed with expansion gaps to allow for an increase in length on very hot days of the track or road without buckling.

A jam jar lid can be loosened by running hot water over it.

These are examples of thermal expansion.


Heat

We return to our expanded solid and give it some more heat energy.

Now the atoms have enough energy to break free from the regular pattern that has been restricting them.

The atoms still remain in contact with each other but now they can move and flow over and around each other.

They move to take the shape of whatever container surrounds them.

Solid

Liquid


The solid has melted. It has become a liquid.

We call this a change of state. The atoms have changed from the solid state to the liquid state.

It takes a huge amount of energy for a substance to change state.

Liquid

Changes of state

Adding heat to ice at 0oC - the temperature of the ice DOES NOT increase.

Instead some of it melts to form liquid.

If the heat is added slowly the temperature will remain at 0oC until all the ice has melted.


Heat

Liquid

Gas

Now we add even more heat energy to our liquid.

At first like the solid did, it will expand.

Then, when enough heat has been added, the atoms gain enough energy to break completely from each other.

They are completely free and move randomly, bumping into each other and they comtainer they occupy.

Gas


The liquid has boiled and become a gas.

This is another change of state, and it too takes a huge amount of energy to achieve.

Changes of State

Water

Solid state Ice

Liquid state Water

Gaseous state Steam

‘Change of state’ – a transition from one phase to another.


Heat and Electricity

Some special metals have the unusual property that they conduct electricity better the hotter they get.

These metals are called semiconductors.

When used to make a thermometer these devices are called thermistors.


Imagine you have two wires, made of different metals.

We will join an end of one with an end of the other.

We make sure the unconnected ends are kept at the same temperature; our reference temperature.

The place where the two wires are joined is heated; our probe.

A tiny electrical voltage appears between the wires.

Probe


The voltage produced is proportional to the difference between the temperature of the junction and the reference temperature.

This device is called a Thermocouple.

The amount of electricity produced by one thermocouple is tiny, so if we use these to measure temperature its usual to join lots of them together.

Such a device is called a Thermopile.

1. Conduction

2. Convection

3. Radiation

How Heat Travels

How Heat Travels

If you hold one end of a metal rod and place the other end in a flame, the end you are holding gets hotter and hotter even though it is not in direct contact with the flame.

Heat reaches the other end by conduction through the material.

1. Conduction

How Heat Travels…conduction

Looking at the molecules again, those in the flame have more energy than those in your hand.

The hotter molecules strike their neighbouring molecules and pass on some of their energy in the process.

These in turn bump into their neighbours and this process continues through the material until the energy is spread throughout.

Substances that allow heat to be passed through them easily are called good conductors.

Substances that prevent the movement of heat through them are called good insulators.

Heat transfer only occurs between regions of different temperatures and always from the higher to the lower temperature.

How Heat Travels…..conduction

How Heat Travels…..conduction

Materials in contact with the body conduct heat away from the skin.

So there is a constant temperature gradient between the body and the surrounding air in the room.

Air is not a good conductor of heat. Air trapped in the fibres of the clothing and also a layer of air between the clothing and the skin acts as a good insulator for the body. Several light items of clothing are more effective at insulating the body than one heavy item because a layer of air is trapped in between each.

How Heat Travels

2. Radiation

Heat transfer by radiation depends on electromagnetic waves like light, infrared and ultraviolet radiation.

Everyone is familiar with the warmth of the sun’s radiation and the intense heat from the glowing coals in a fireplace.

Heat from these very hot bodies does not reach you by conduction or convection of the air between but by radiation.

This heating would occur even if there were no air in between i.e. radiation can occur in a vacuum.

How Heat Travels…..Radiation

All objects emit energy in the form of electromagnetic radiation.

A premature baby in an incubator can be cooled dangerously if the walls of the incubator are cold even if the air inside the incubator is warm.

This is why some incubators regulate their temperature based on a measurement of the baby’s skin temperature.


Heat is radiated away from the skin.

60% of the heat lost by an uncovered body in normal temperature air is lost by radiation.


As stated above the heat energy travels in waves and does not need a substance in which to travel.

The amount of heat radiated form the skin depends on the dilation of the blood vessels in the skin; the more they are dilated the more heat will be lost by radiation.

There is another process by which a huge amount of heat can be taken from the body.

This process involves a change of state of a substance that we have discussed already. Water can remove heat from the body in order to evaporate.

An enormous quantity heat is transferred during evaporation because a large quantity of energy is required to change the state of a substance from a liquid to a gas. It takes 2,256,000J to turn 1kg of water at 100oC into steam at 100oC. Loss of heat in this way can occur by insensible water loss and by sweating.

Heat Loss from the body

By insensible water loss we mean that heat is being lost constantly by evaporation despite the fact that the sweat glands are inactive. Water diffuses upwards from the deeper layers of the skin to the surface of the body and evaporates into the air.

During sweating the sweat glands are working and secrete sweat onto the surface of the body, which evaporates and cools the body. Sweat is water containing dissolved salts and is a very effective cooling agent because as we said earlier a small amount of water takes a large amount of heat to evaporate.

Heat Loss from the body

Thermometry

Traditionally, contact thermometers measureed temperature in oral rectal or axilliary sites. This is due to the ease of access of these sites rather than accuracy.

Thermometry

The pulmonary artery is an ideal site at which to meaure core temperature because in the heart the blood is mixed from the viscera and the skin, the temperature of the mixed blood representing an average of all these different organs.

Other accessible core sites are the oesophagus, tympanic membrane and urinary bladder.

Thermometry

As the hypothalamus is the regulator of the body’s core temperature it makes sense that it is an accurate site from which to measure it.

The tympanic membrane is chosen because it is located in close proximity to the internal carotid artery, which supplies the hypothalamus.

Tympanic temperatures are better at following changes in core temperature than rectal thermometers, which can lag up to an hour behind.

The way in which tympanic temperatures can follow core and the fact that access is easy makes the tympanic membrane a popular choice of site at which to measure temperature.

However most tympanic thermometers are very dependent on user technique and great care must be taken to ensure the measurement recorded is an accurate one.

Thermometry

As we saw above a liquid expands linearly when heated.

A mercury thermometer consists of a bulb and a narrow capillary tube.

The bulb has a thin glass wall to allow heat to pass through more easily, and it contains the reservoir of mercury.

The narrow capillary tube leading from the bulb is made of thicker glass.

The tube is calibrated.

Mercury Thermometer

A constriction is placed just above the bulb in clinical mercury thermometers to prevent the mercury from receding straight back into the bulb when the thermometer is removed from the patient.

Mercury Thermometer

Why we like mercury thermometers:

1. Mercury thermometers are extremely accurate and until recently were the gold standard in thermometry.

2. Mercury thermometers are relatively cheap and so a new thermometer can be used for each new patient, which would help contain the spread of infection.

Mercury Thermometer

Mercury Thermometer

Why we don’t like mercury thermometers:

1. Mercury is toxic and the thermometers are quiet fragile.

2. They requires 2-3 minutes to reach thermal equilibrium.

3. Thermometers are unsuitable for placement into certain orifices i.e. the inner ear

.

Cholesteric liquid crystals are coiled stacks of lengthy molecules that twist and untwist as a result of changes in temperature.

Light of the same wavelengths as the intervals between these crystals is reflected preferentially.

Therefore the temperature of the liquid crystal decides its temperature.

Liquid Crystal Thermometer


Liquid crystal thermometers are strips of plastic containing patches of different liquid crystals.

The thermometer is placed on the skin and as the liquid crystals heat up, due to conduction of heat from the body, the colours of the different patches change.

Eventually when the liquid crystals reach thermal equilibrium with the body, you can tell the temperature by observing the colour of the different patches.

When the thermometer reaches thermal equilibrium, the temperature of the body is indicated by the patch which has turned green


Why we like liquid crystal thermometers:

Liquid Crystal thermometers are relatively cheap and so we can afford to have disposable thermometer for single use only. This helps contain reinfection or the spread of infection.

They are easy to read and unambiguous.

Why we don’t like liquid crystal thermometers:

There are doubts about the accuracy of liquid crystals as thermometers but most have been found to be satisfactory for clinical use. However they have too narrow a range for use where hypothermia or hyperthermia is being monitored.


Electrical Thermistor Probes… used with physiological

monitors

Thermistors are metals that change electrical

resistance as they gain or lose heat.

In the clinical setting the thermistor is placed at

the body site of interest.

The heat in the body travels to the thermisitor as

a result of conduction and the thermistor heats up.

As its temperature increases its resistance falls.

The thermistor resistance therefore is a measure

of the temperature and, by extension, the body

temperature.


monitors

The thermistor itself must be connected to an

appropriate physiological monitor by a cable.

Thermistors can be made very small.

This means they can be placed in small orifices or

indeed inserted into arteries.

Also, because they have such small mass, they

quickly reach thermal equilibrium with the body. As a

result they can accurately track changes in body

temperature.


monitors

Thermistors are currently used for temperature measurement in both theatre and ICU.

Pulmonary artery temperature measured with a thermistor catheter is at present the “Gold Standard” of core temperature measurement.

Alternatively, thermistors can be inserted into the rectum, nasopharynx, oesophagus, or bladder.


monitorsWhy we like Thermistor Probes:

They can be manufactured extremely small in size and therefore can be introduced almost anywhere in the body.

As they as permanently attached to the patient, they continuously monitor changes in the patient’s body temperature.

They reach thermal equilibrium rapidly.

Once in place, thermistors do not require any action in order to measure temperature.

Why we don’t like Thermistor Thermometers:

They require connection to an expensive monitor. The hospital cannot afford a monitor for every patient so they are limited to use in areas equipped with Physiological Monitoring.

As they are used in conjunction with the monitors they require an extra cable from monitor to patient adding to clutter in ICU and Theatre settings.


monitors

As they are foreign bodies that remain in place in contact with the patient for considerable periods of time they may cause irritation and or infection.

Calibration can be altered by exposure to extreme temperatures, e.g. sterilisation.


monitors

Tympanic Infrared Thermometers

Tympanic thermometers measure the infrared

radiation emitted by the tympanic membrane in the

ear and from this estimate the tympanic membrane

temperature.

The tympanic thermometer’s probe does not

have to make contact with the tympanic

membrane, since the infrared is radiated.


The sensor in the thermometer is a thermopile )a

group of thermocouples(.

The measuring junction of the thermopile is

exposed to the infrared radiation coming from the

tympanic membrane.

This heats up and the thermopile produces a

voltage proportional to the difference between the

reference junction and the measuring junction

temperatures.

A thermistor in the thermometer then measures the thermometers own temperature )which is the reference junction temperature(. When added together they give you an estimate of the absolute temperature of the tympanic membrane.

So the estimated tympanic membrane temperature is the sum of the thermistor reading )device temperature( and thermopile reading )increase in temperature due to exposure to Infrared from the tympanic membrane(.

Estimated T tympanic membrane = TDevice + TInfrared



Why we like Infrared Tympanic thermometers:

The tympanic membrane is easily accessed, providing a non-invasive location for temperature measurement.

As the probe cover is disposable and changed after each patient, the risk of cross infection is reduced.


Why we don’t like Infrared Tympanic thermometers:

It is very important that the operator direct the thermometer directly at the tympanic membrane. Otherwise the device will obtain mixed signals from the ear canal, resulting in inaccurate temperature measurement. As a result the thermometer will most likely, display a lower temperature than the true reading.

The infrared tympanic thermometers need regular recalibration, which is not a simple process.


The draw down effect - Repeatedly taking temperature from the same site )i.e. the same ear( will result in a temperature drop within the ear, due to energy being dissipated through the thermometer.

The thermometer must be kept at a constant temperature, as sudden changes in the temperature of the cold junction of the thermopile will effect the resulting temperature measurements. This restricts the movement of thermometers between different wards/rooms with slightly different air temperatures.

heat, temperature and thermometry. overview 1.why measure body temperature? 2.what is temperature?...

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