Work is measured in joules and is done when a force causes an object to move. The work of breathing involves overcoming resistance to airflow and expanding the lungs and chest wall. This results in some mechanical energy being stored as potential energy in elastic tissues. Power is the rate of working and is measured in watts. The work and efficiency of both respiration and cardiac contraction are discussed. Hyperventilation increases the power needs of breathing while factors like hypertension increase the workload of the heart.

1. Work, Energy and Power
MOHAMED ANWER RIFKY
kinetic
energy is the
energy that is produced
when something moves

2. 1-WORK: Work is done and energy is
expended--
One joule of work is done when a force of one newton moves its point of application one metre in the direction
of the force.
A-An apple which has a mass of 102 grams is 1 newton>> raised 1 metre vertically against the force of
gravity, 1 joule of work is performed as
illustrated.
B-The mechanical energy of the apple is converted >>potential energy represented by additional height of the
apple.
Shortening of the muscle multiplied by the mean force
exerted>work.
2-WORK OF A VENTILATOR DURING INSPIRATION:
A constant-pressure generator type of
ventilator:
Work done = F x D Consequently the work done = PA x V/A (F = PA & ).
Work done = (0.6 x 103 ) Pa x (0.5 x 10-3)
m3
= 300 mJ= 0.3J indicates the work of the ventilator for one inspiration is
still valid.
The shaded area enclosed by the
loop
3-ENERGY CHANGES:
Simple syringe model
Energy cannot be lost,in inspiration half of the mechanical energy is stored ( potential energy) in the elastic
tissues of the lung and the chest wall.This energy can be divided into two parts; 1-(stretching the elastic band), and
2-(in overcoming resistance) to airflow into the syringe model. a constant-pressure generator attached to the
patient's lungs are represented by a syringe in which movement of the plunger is opposed by the stretching of an
elastic band. In this model the plunger represents the diaphragm and the elastic band represents the elasticity of
the lung tissues and chest wall.Airway resistance and the pressure within the syringe is indicated by PL.The end-
inspiratory pressure in the syring ereaches the ( PM ),pressure generated by the elastic band giveS a tracing C to
D, the slope of which represents ( compliance), stippled section(energy expended in moving the gas against the
airway resistance,PM is 0.6 kPa,with a volume of 0.5 litre ,and the energy expended in moving the gas is 1/2PV.
4-WORK OF EXPIRATION:The cycling device is open to the atmosphere >>pressure within the syringe
then falls linearly to zero if plotted against volume expired.The work expended is consequently 1/2 PV and is used
in overcoming the airway resistance in the model.
5-WORK OF INSPIRATION IN SPONTANEOUS BREATHING:
a- inspiration >>desent of the diaphragm 15 mm.(in the model).
b-syringe now has two plungers with fluid between them (intrapleural)-right-hand plunger (diaphragm)and the left-
hand
plunger (lung tissues).
c-A plot of volume against intrapleural pressure for a typical inspiration is shown. d-The stippled area on the graph
gives the
work expended in overcoming airway resistance and the triangular shaded area gives the stored elastic energy
which is used during expiration.
e- In this model 0.5 litre of air is inspired when pressure is 0.6 kPa, then 300 mJ of work is done in inspiration.
f-Chest expansion (not shown in the graph), also occurs(10 to 40%)of the inspired tidal volume in the respiratory
ms.,10% of chemical E. >>mechanical E.,and 90%>>heat.
syringe
model
6-MEASURING THE WORK OF INSPIRATION:
The total work of inspiration is more difficult to
measure in a spontaneously breathing
patient.the volume inspired may be measured
by a pneumotachograph.Intrapleural pressure
changes>>
in he lower oesophagus (a catheter with a
balloon at its
tip).The greater the expiratory resistance the
greater the bowing of this line to the right(
figure).Oesophageal pressure readings can’t
give absolute accuracy(only one site and it
varies accordingly).

3. 7-POWER OF BREATHING:
Power is the rate of working and is measured in watts, 1 watt being 1
joule per second
8-EFFICIENCY OF RESPIRATORY
MUSCLES:
Are only 10% efficient in producing mechanical energy, the rest >>heat.
Thus, the actual energy requirements in the example above would be
ten times greater than 80 Mw(i.e. about 800 mW). As the normal
metabolic rate (80 W) so,energy requirements for breathing would be
1% of the total .
9-THE EFFECT OF THE TYPE OF FLOW:
The above calculations are simplified in that they do not allow for
the
kinetic energy in the flowing fluid, but in the flow normally present
in ther espiratory and circulatory systems the kinetic energy is
very small.
.
Wher
e
: 9-EFFECT OF HYPERVENTILATION IN PATIENTS:
The power of breathing increases rapidly at a rate approaching the
cube of the gas flow>> increase O2 need,and try to avoid
resp.stimulation.
10-LUNG COMPLIANCE AND AIRWAY RESISTANCE:
The time-constant for the lungs =compliance X resistance (Patients with a low time-constant >>rapid respiratory rate .whereas,with a high constant
>>slower rate.
11-WORK OF MYOCARDIAL
CONTRACTION:
A-left ventricular volume increased 60 ml during diastolic
B-The pressure increased from 0 to 16 kPa (0-120 mmHg) during
isovolumetric contraction.
.
C-Work done = (16 x 10 ) Pa x (60 x 10 ) m3
= 960mJ
D-If the heart rate is 60, then the power of the left ventricle of the heart is about 60 joules per minute, i.e.
one watt.
E-Continuous ventricular pressure>>intraventricular cardiac catheter, and volume
measurements may be deduced >> ultrasonic flow measurement or estimated from
echocardiography ,cineangiography or other imaging techniques.
12-POWER OF THE HEART: power as a product of pressure difference and fluid flow
may also be applied to the cardiac output.
C-The right heart can be similarly calculated>> = 0.2 W
D-Total power of heart = 1.2
W
E-If the efficiency of the heart were 15% in this instance, then the total energy requirements of 8 W would be 10% of a typical basal metabolism of 80W.
F-Hypertension results in additional energy requirements even if the cardiac output is
A-
B-
Thyrotoxicosis, anaemia or exercise increases the work load of the hea
if maintained, may lead to heart failure.
3 -6