WORK OF BREATHING
Work of breathing is the
work done by respiratory muscles during breathing to overcome the resistance
in thorax and respiratory
tract.
WORK DONE BY
RESPIRATORY MUSCLES
During respiratory processes,
inspiration is active process and the expiration is a passive process. So,
during quiet breathing,
respiratory muscles perform the work only during inspiration and not during
expiration.
UTILIZATION OF ENERGY
During the work of breathing,
the energy is utilized to overcome three types of resistance:
1. Airway resistance
2. Elastic resistance of
lungs and thorax
3. Nonelastic viscous
resistance.
1. Airway
Resistance
Airway resistance is the
resistance offered to the passage of air through respiratory tract. Resistance
increases during
bronchiolar constriction, which in creases the work done by the muscles during breathing.
Work done to
overcome the airway resistance is called airway resistance work.
2. Elastic
Resistance of Lungs and Thorax
Energy is required to
expand lungs and thorax against the elastic force. Work done to overcome this
elastic resistance is called compliance work.
3. Non-elastic Viscous Resistance
Energy is also required to
overcome the viscosity of lung tissues and tissues of thoracic cage. Work done
to overcome this viscous
resistance is called tissue resistance work.
Above factors are
explained by the curve that shows the relation between lung volume and pleural
Pressure.
Increase in Compliance
Compliance increases due
to loss of elastic property of lung tissues, which occurs both in physiological
and pathological conditions:
1. Physiological
condition: Old age
2. Pathological condition:
Emphysema.
Decrease in
Compliance
Compliance decreases in several
pathological conditions such as:
1. Deformities of thorax
like kyphosis and scoliosis
2.
Fibrotic pleurisy (inflammation of pleura resulting in fibrosis)
3. Paralysis of
respiratory muscles
4. Pleural effusion
5. Abnormal thorax such as
pneumothorax, hydrothorax, hemothorax and pyothorax
INTRA-ALVEOLAR
PRESSURE
Definition
Intraalveolar pressure is
the pressure existing in the alveoli of the lungs. It is also known as intrapulmonary pressure.
Normal Values
Normally, intraalveolar pressure
is equal to the atmospheric pressure, which is 760 mm Hg. It becomes negative during
inspiration and positive during expiration.
Normal values are:
1. During normal
inspiration:
–1 mm Hg (760 – 1 = 759 mm
Hg)
2. During normal expiration:
+1 mm Hg (760 + 1 = 761 mm
Hg)
3. At the end of
inspiration and expiration:
Equal to atmospheric
pressure (760 mm Hg)
4. During forced
inspiration with closed glottis
(Müller maneuver): –80 mm
Hg
5. During forced
expiration with closed glottis
(Valsalva maneuver): +100
mm Hg.
Measurement
Intraalveolar pressure is
measured by using plethysmograph.
Significance of Intra-alveolar Pressure
1. Intraalveolar pressure causes flow of air in and
out of alveoli. During inspiration, the
Intraalveolar pressure
becomes negative, so the atmospheric air enters the alveoli. During expiration,
intraalveolar pressure becomes positive. So, air is expelled out of alveoli.
2. Intraalveolar pressure
also helps in exchange of gases between the alveolar air and the blood.
Transpulmonary
Pressure
Transpulmonary pressure is
the pressure difference between intraalveolar
pressure and intrapleural pressure.
It is the measure of elastic forces in lungs, which is responsible for
collapsing tendency of lungs.
0 Comments