Heat in medicine
Prof. Dr. Moustafa Moustafa Mohamed Ahmed
Vice Dean
Faculty of Allied Medical Scince
Pharos University
By ;dr ;mervat mostafa
• By the end of the lecture, the students will
Learn the basic concepts of heat, heat
transfer, units of heat energy and latent
heat
Heat
• Heat is thermal energy transferred from system
to another
• Heat is measured in calories.
• The amount of heat required to raise one gram
of water 1° Celsius is one calorie.
• When heat energy flows into a substance, the
temperature of the substance usually rises.
No heat energy can be
extracted.
Temperature Scales
• Degree Centigrade Scale
• The Celsius scale is determined by fixing
the temperature span between the
freezing and boiling points of water to be
100°C, and by defining the freezing point
of water to be 0°C.
• Fahrenheit scale
• The Fahrenheit scale uses 180°F to span
between the same two physical points,
and uses 32°F as the freezing point of
water. These two temperature scales are
simply related to each other (as you
should verify) by:
• KELVIN is another unit of temperature
that is used for many scientific
calculations. It begins at absolute zero
and therefore has no negative
numbers.
Example
Find the general relation between the
Fahrenheit and Kelvin temperature scales
and determine absolute zero in °F.
Solution:
We can find the general relation by
substituting Equation (12.2) for TC into
Equation (1) for TF. After substitution
we find that
• The amount of heat energy needed to
raise the temperature of a substance ”Q”
is proportional to the temperature change
“DT” and the mass of the substance “m”
Q= m c DT = C DT
• where C is heat capacity = mc and c is the
specific heat of the substance
• Specific heat capacity is the amount of
heat required to raise the temperature of
one gram of that material 1° C.
Amount of Heat Gain Or Loss
• The amount of heat energy needed to
raise the temperature of a substance ”Q”
is proportional to the temperature change
“DT” and the mass of the substance “m”
Q= m c DT = C DT
• where C is heat capacity = mc and c is the
specific heat of the substance
• Specific heat capacity is the amount of
heat required to raise the temperature of
one gram of that material 1° C.
Unit of heat energy
Calorie (Unit of heat energy) define as the
amount of heat energy needed to raise the
temperature of one gram of water one Celsius
degree
1 Calorie = 4.18 Joule
where Joule is the unit of energy in SI unit
Example
• Ex: How much heat is needed to raise the
temperature of 3 kg of copper by 20o
(c=0.886 kJ/kg)
• Solution:
• Q =mc DT
= 3.0 x 0.886 x 20
= 53.16 kJ
Calorimetry
1. The specific heat of an object can be
measured by:
1. heating the object to some temperature,
2. placing it in water bath that of known mass and
temperature, and
3. measuring the final temperature
2. If the system is isolated from the surrounding,
then the heal leaving the object equals the
heat entering the water and its container.
3. This procedure is called Calorimetry and the
insulated water container is called Calorimeter.
• Heat Loss = Heat Gain
• Heat loss from the object
• Qout = m c ( Tio –Tf)
– M= mass of the object,,
– c= specific heat of the object
– Tio= initial temperature of the object
– Tf= final temperature
• Amount of heat gained by water and
container
• Qin = mwCw(Tf – Tiw) + mccc (Tf –Tiw))
Example
• Calculate specific heat for lead given:
–
–
–
–
–
–
–
–
Mass of lead mpb = 0.6 kg
Mass of water mw = 500 g
Mass of calorimeter Mc = 200 g
Initial temperature of lead Tio =100
Initial temperature of water Tiw = 17.3
Final temperature Tf =20 oC
Specific heat capacitance of calorimeter Cc = 0.900 kJ/kg.ok
Specific heat capacitance of water = 4.18 kJ/kg.ok
Latent Heat
• When heat is added to ice at 0 oC the temperature of ice
does not change. Instead the ice melts.
• The heat needed to melt a substance of mass “m” is
proportional to the mass of the substance m with no
change in its temperature
Qf = m Lf
• Where Qf is the quantity of heat needed to melt the ice,
m its mass and Lf the latent heat of fusion.
• Also latent heat of vaporization is given by:
Qv = m I v
• Where Iv is the latent heat of vaporization
Example
• Ex 1: How much heat do you need to heat 1.5 kg of
ice at 1 atmospheric pressure from -20 until al the ice
has been changed to vapor at 100 oC. Given:
–
–
–
–
Specific heat of ice cice = 2.05 kJ/kg
Latten fusion of ice Lf= 333.5 kJ/kg,
Specic heat of water Cw = 4.18 kJ/kg, and
Latten vaporization Lv = 2.26 MJ/kg
Questions
• Define: heat, heat capacity, specific heat
capacity and latent heat
• If 500 gram of cupper at 200 oC is added
to 500 gram of water at 20 oC what is the
final temperature (Specific heat of cupper
and water are Cw = c=0.386 kJ/kg and
4.18 and kJ/kg, respectively)
Heat Transfer
• Heat always travels from a hot object to
a cold object.
• So how does heat travel from one
object to another?
• heat travel from one object to another
through
• CONDUCTION,
• CONVECTION and
• RADIATION.
• Conduction is the main way for heat to
transfer in solid materials.
• All solid materials conduct heat, but some
do a better job than others. Generally,
metals are good conductors while porous
materials are not. Good conductors of
electricity are usually good conductors of
heat.
Liquids and gases do not conduct heat very well, but they can transfer heat by
convection. Look at the illustration above.
Water carries heat from a hot engine through a pump and delivers it to a radiator,
whose duty is to give up heat to the air.
Remember, with convection, heat is transferred from one place to another by
motion of the gas or liquid.
Convection
During the daytime, cool air over water moves to replace the air
rising up as the land warms the air over it. During the nighttime,
the directions change -- the surface of the water is sometimes
warmer and the land is cooler.
The easiest way to explain heat transfer by radiation is by the heat
we feel from the sun.
Even though the sun is 93 million miles away, we still feel its heat. It
travels to earth through the vacuum of space (no air) by way of rays!
If you hold your hand near a light bulb (but not too close!), much of
the heat that you feel is from radiation.
Thermal expansion
• Almost all substances expand when heated
and contract when cooled. This is true of
most liquids and solids as well as gases.
• Thermometer: Fluid in a tube. The height of
the fluid is a measure for the temperature.
• Change of length (1 D), area (2 D) and
volume (3 D) is related to temperature:
Thermal Expansion
Part two
Linear Expansion
• A solid rod of length L is found to expand
by an amount that is directly proportional
to the temperature increase and to its
length according to
2 D Area Expansion
• Calculation gives us for area of any shape
3 D Volume Expansion
• Calculation gives us for volume of any
shape
Heat and Life
• The rates of the metabolic processes necessary
for life, such as cell divisions and enzyme
reactions, depend on temperature
• For a given animal there is usually an optimum
rate for the various metabolic processes. Warmblooded animals (mammals and birds) have
evolved methods for maintaining their internal
body temperature at near constant levels. As a
result, warm-blooded animals are able to
function at an optimum level over a wide range
of external temperature.
Energy Requirement of
People
• All living systems need energy to function. In
animals, this energy is used to circulate blood,
obtain oxygen, and repair cells, and so on.
• The amount of energy consumed by a person
during a given activity divided by the surface
area of the person’s body is approximately the
same for most people.
• The energy consumed for various activities in
Cal/m2. Hr. is known as metabolic rate.
Metabolic Rate for Selected
Activities
Activity
Metabolic Rate
(Cal/m2.hr)
Sleeping
35
Lying awake
40
Sitting upright
50
standing
60
Waking (3 mph)
140
Moderate Physical Work
150
Bicycling
250
Running
600
Shivering
250
Total energy consumption per hour
• To obtain the total the total energy consumption
per hour, we multiply the metabolic rate by the
surface area of the person.
• The following empirical formula yields a good
estimate for the surface area.
Area (m2) = 0.202 x W0.425 x H0.725
• Here W is the weight of the person in kilogram,
and H is the height of the person in meters.
• Example
The surface area of a 70 kg man of height
1.55 m is about 1.7 m2. His metabolic rate
at rest is therefore (40 Cal/m2. Hr) x 1.7 m2
= 68 Cal/hr, or about 70 cal/ hr. the
metabolic rate at rest is called the basal
metabolic rate.
Example
• For a woman, the energy requirement increase
somewhat during pregnancy due to the growth and
metabolism of the fetus. As the following calculation
indicates, the energy needed for the growth of fetus is
actually rather small. Let us assume that the weight gain
of the fetus during the 270 days of gestation is uniform. If
at birth the fetus weight 3 kg, each day it gain 11 g.
because 75% of tissue consists of water and inorganic
minerals, only 2.75 g of the daily mass increase is due to
organic materials, mainly protein. Therefore, the extra
Calories per day required for the growth of the fetus is
Heat Transfer
• Heat always travels from a hot object to a cold
object.
• So how does heat travel from one object to
another?
• heat travel from one object to another through
1. Conduction,
2. Convection
3. Radiation.
4. Evaporation.
Conduction
Conduction: heat transfer by direct contact with •
another medium. (hot or ice packs)
The total heat transferred depend on: •
The area of contact .1
The temperature difference .2
The time of contact .3
The thermal conductivity of the materials .4
• Hot baths, hot pack and hot paraffin when
applied to the skin heat transfer to the body by
conduction.
• Conductive heat transfer leads to local surface
heating, since the circulating blood effectively
removes heat that penetrate deep in the tissue.
Convection
• Convection means heat transfer by indirectly
through secondary conductive medium. (air or
liquid)
• Example is the air circulation which is established
by a room radiator.
• The air near the radiator is heated by conduction
making it less dense than the cooler air in the
room.
• Therefore, the warmer air rises and is replaced by
the cooler air down-wards around the radiator.
Radiation
• Radiation means heat transfer by or from its
source to surrounding environment in form of
waves or rays. (ultraviolet light)
• Radiation is electromagnetic energy that travels
through space with the speed of light (3x108
m/s).
• The easiest way to explain heat transfer by
radiation is by the heat we feel from the sun.
Thermotherapy
• Treatments using heat
• Increase the temperature of the body region
to cause vasodilatation;
–Increases blood flow to area
• Decreases pain and muscle spasms
• Increasing flexibility to tissues
• Comforting for most patients
Cryotherapy
• Treatments using Cold
• Decreases tissue temperature
• Skin Color change from white to red
• Decrease in total blood flow
• Decrease in nerve conduction
Cryotherapy
Cryotherapy is the local or general use of low temperatures in •
medical therapy. Cryotherapy is used to treat a variety of
benign and malignant lesions.The term "cryotherapy" comes
from the Greek cryo meaning cold, and therapy meaning cure.
Cryotherapy has been used as early as the seventeenth century.
Its goal is to decrease cellular metabolism, increase cellular •
survival, decrease inflammation, decrease pain and spasm,
promote vasoconstriction, and when using extreme
temperatures, to destroy cells by crystallizing the cytosol. The
most prominent use of the term refers to the surgical treatment,
specifically known as cryosurgery. Other therapies that use the
term are cryogenic chamber therapy and ice pack therapy.
Cryosurgery
Cryosurgery is the application of extreme cold to
destroy abnormal or diseased tissue. Cryotherapy is
used to treat a number of diseases and disorders,
most especially skin conditions like warts, moles,
skin tags and solar keratoses.
Liquid nitrogen is usually used to
freeze the tissues at the cellular level.
The procedure is used often because of
its efficacy and low rates of side effects.
Medical
Cryotherapy gun
Assignments
Assignments :
1-2-3(the tpoics as in the lecture).