Physiology of Heat and
Cold

CONTENTS
Objectives
Introduction
Principles of heat and cold
Energy transfer
Physical effects of heat
Latent heat
Thermoregulatory mechanism and thermo
receptors
Thermoregulation
Physiological effects of heat and cold
Therapeutic effects of heat and cold
conclusion

Objectives
1. To know about the principles of heat
and cold
2. To know about the energy transfer
mechanism related to therapeutic
modalities
3. To know about thermoregulatory
mechanism
4. To know about physiological and
therapeutic effects of heat and cold

INTRODUCTION
Using a sunbath to relax acing
muscle or splashing cool water
when overheated.
 At present, both heat and cold
are widely used physical therapy
modalities.
 Often thermal modalities are
used as adjunct to other physical
therapy treatments.

PRINCIPLES
Heat
Closely packed atoms or
molecules move vibrate about their
equilibrium position and it is these
movement kinetic energy which are
recognized as heat. As the
temperature of a substance increases
the motion of the molecules
increases. Thus increased molecular
motion produces heat.

Cold
When ice is applied to the skin, heat is
conducted from the skin to the ice in order
to melt it. to change its state, ice requires
considerable energy (latent heat of fusion
) to raise the temperature of 1 g of ice at
0
0
to 1gm of water at 37
0
C requires 491 J,
whereas to raise 1g of water at 0
0
C to
37
0
C requires only 155 J.

Physical Effects of Heat
Expansion
•increased kinetic energy
A change of state
•cohesive force
•kinetic force
An increased state of chemical reaction
• Van’t Hoff’s law

Production of electromagnetic radiation
By movement of electrons within the
atom. If energy is added to an atom, e.g. by
heat this can cause an electron to move out
to a higher energy electron shell. (Excited
stage) when the electron returns to its
normal level energy is released as a pulse
of electro magnetic energy

A potential difference between dissimilar metals
•If the junction of two dissimilar metals is heated a potential
difference is produced between their free ends.
Thermionic emission
•The heating of molecules of some materials e.g.
tungsten.
Reduced viscosity in fluids
•. Heating increases the kinetic movement of these molecules and
reduces their cohesive mutual attraction. This made the fluid less
viscous

Energy transfer
Conduction
Conduction is a method of heat transfer
from one place to another by successive
molecular collision. Heat transfer by
conduction is a slow process. When two
objects of different temperature come in
contact

Convection
Convection is a method of heat
transfer in which heated
molecules move from one place
to another. This method heat
transfer is more rapid than
conduction and occurs in liquids
and gases.

Radiation
All objects can give off or take on thermal
energy through the process of radiation

Latent heat
A specific amount of energy is required to change the solid
form of a particular substance into liquid, or the liquid into gas.
This energy is called latent heat
The freezing point of water is 0
0
C
The boiling point of water is 100
0
C

Thermoregulatory mechanism
Major heat is from metabolism which is
vastly increased during vigorous exercise.
When the body is exposed to cold the loss
of heat can be much reduced by
vasoconstriction
The temperature gradient between the core and surface
can be expressed by the isothermal lines

Cutaneous receptors
Pain nerve endings discharge at the extremes
of temperature range where the temperature
stimulus becomes merged with pain beyond
45
0
c for heating and 15
0
c for cooling.

• These endings are sometimes described
as warm pain and cold pain endings
respectively. The more numerous cold
receptors discharge over a wide range,
different neurons have different ranges
and different peak discharges, many
around 25
0
C. the warm receptors also
vary greatly many with peaks around 40
0
c

Thermal regulation
Metabolic control
Vasomotor control
Hidrotic control
Behavioural control

Physiological effects
Metabolic activity
Viscosity
Collagenous tissue changes
Nerve stimulation
Blood vessel changes
Blood and the tissue fluid

Therapeutic effects
Heat
►Healing.
► Control of infection
►Relief of pains
►Muscle spasm
►Sedative effect
►ROM
►Pressure sores
►Oedema
►skin diseases

Cold
Recent injuries
Pain and ms spasm
Spasticity
Ms strength

References
Clayton’s Electrotherapy – Angela forster,
Nigel Palastanga
Electrotherapy Explained – John low & Ann
Reed
Physical Agents – Bernadette Hecox, Tsega
Andemicael Mehreteab, Joseph Weisberg
E – Medicine Superficial Heat and Cold by
Milton J Klien 2006 Feb 13.