32 Physiology of respiratory system. External breathing

Physiology of respiratory
system. External breathing
General functions of respiratory
The respiratory system comprises of the nose, mouth,
throat, larynx, trachea, bronchi and lungs. The function
of the respiratory system is to facilitate gaseous
exchange to take place in the lungs and tissue cells of
the body.
Oxygen is required by cells in the body to allow
various metabolic reactions to take place and to
produce energy and is therefore essential to life. The
respiratory system may be defined as the organs and
tissues through which air is passed into and out of the
body to allow the necessary gaseous exchanges to take
Respiratory System - a complex of
structures that participate in gas
exchange, and mechanisms for their
The activity of respiratory system is
best evaluated by oxygen consumption
(OC) in 1 min. In the adult resting OC
is about 3.5 ml (min / kg).
“Respiration” includes three
separate but related functions:
 (1) ventilation (external breathing);
 (2) gas exchange, which occurs
between the air and blood in the
lungs and between the blood and
other tissues of the body;
 (3) oxygen utilization by the tissues
in the energy – internal breathing
Stages of exchange of gases
} ventilation
} diffusion
1. exchange of gases between
atmospheric air and
intraalveolar air
2. exchange of gases between
intraalveolar air and blood
3. transport of gases
4. exchange of gases between
blood and tissue
5. internal (tissue) respiration
} perfusion
Respiratory muscles
Main muscles for
diaphragm and
external intercostal,
Muscles for
expiration: internal
pectoral, abdominal
Muscles of Inspiration:
Most Important Muscle Of Inspiration
Responsible for 75% of inspiratory effort
Thin dome-shaped muscle attached to the lower ribs, xiphoid
process, lumbar vertebra
Innervated by Phrenic nerve (Cervical segments 3,4,5)
During contraction of diaphragm
Abdominal contents forced downward & forward causing increase in
vertical dimension of chest cavity
Rib margins are lifted & moved outward causing increase in the
transverse diameter of thorax
Diaphragm moves down 1cm during normal inspiration
During forced inspiration diaphragm can move down 10cm
Paradoxical movement of diaphragm when paralyzed
Upward movement with inspiratory drop of intrathoracic pressure
Occurs when the diaphragm muscle is denervated
The dome of the diaphragm
flattens, ribs elevate
In the rest, 4/5 of
inhalational work is done by
Pressure in the alveoli
reduces below atmospheric,
the air moves under pressure
gradient into the lungs
Movement of Thorax During
Breathing Cycle
Biomechanism of breathing
Breathing is an active process - requiring the contraction of
skeletal muscles. The primary muscles of respiration include
the external intercostal muscles (located between the ribs)
and the diaphragm (a sheet of muscle located between the
thoracic & abdominal cavities).
 The external intercostals plus the diaphragm contract to bring
about inspiration:
 Contraction of external intercostal muscles > elevation of ribs
& sternum > increased front- to-back dimension of thoracic
cavity > lowers air pressure in lungs > air moves into lungs
 Contraction of diaphragm > diaphragm moves downward >
increases vertical dimension of thoracic cavity > lowers air
pressure in lungs > air moves into lungs:
To exhale:
 relaxation of external intercostal muscles & diaphragm >
return of diaphragm, ribs, & sternum to resting position >
restores thoracic cavity to preinspiratory volume > increases
pressure in lungs > air is exhaled
Normally is a passive
process. After relaxation
of muscles, due to the
elastic tension of thorasic
tissues air is removed
(Becoming active during
bronchial obstruction)
Effect of Rib and Sternum
Movement on Thoracic Volume
The Pleura Space
Two parts of the pleural membrane
Pleura space enclosed by a continuous membrane
The two pleural membranes slide against each other
The pleural membranes are difficult to separate apart
Separated by a thin layer of serous fluid ( a large amount would be a
pleural effusion as seen in CHF, CA, infection)
Pleura sac
Visceral pleura is a thin serosal membrane that envelopes the lobes of
the lungs
Parietal pleura lines the inner surface of the chest wall, lateral
mediastinum, and most of the diaphragm
The continuous membranes fold to create a sac inferiorly
Both pleura line this potential space inclosing a small amount of fluid
Pleural fluid
Functions as a lubricant between the membranes, prevents frictional
Causes the visceral & parietal pleura to adhere together, maintains
surface tension
The pressure
of the
fluid in the space between
the lung pleura (visceria) & chest wall pleura
(parietal), always negative
Normally at rest suction creates a negative
pressure at beginning of inspiration (-5cmH20)
This suction holds the lungs open at rest
Pressure becomes more negative during
inspiration moving to -7.5cmH20 allowing for
negative pressure respiration
If pleural pressure becomes positive the lung
will collapse: Pneumothorax, Hemothorax,
Pressure in the lungs and
intrapleural pressure
Intrapleural pressure is always
lower than the alveolar one:
First: chest is a sealed
Second, the lungs are
characterized by elastic tension,
which is due to these factors:
1. presence of ellastic fibers,
which make 1 / 3 of elastic
2. surface tension of the liquid
layer on the inner surface of
alveoli, which makes 2 / 3 of
the elastic tension of the lungs.
Thirdly, “negative” pressure in
the pleural cavity is maintained
by the large absorbtion capacity
of pleural leaves.
Pressure in lungs
As the external intercostals & diaphragm contract, the
lungs expand. The expansion of the lungs causes the
pressure in the lungs (and alveoli) to become slightly
negative relative to atmospheric pressure.
As a result, air moves from an area of higher pressure
(the air) to an area of lower pressure (our lungs &
During expiration, the respiration muscles relax & lung
volume descreases. This causes pressure in the lungs
(and alveoli) to become slight positive relative to
atmospheric pressure. As a result, air leaves the lungs.
Physiological dead space
Pulmonary Volumes &
Lung Volumes,
and Lung
Lung capacity
1. The total maximum lung capacity - the maximum volume of
air that fits in the lungs or the sum of all lung volumes. Normally
is 4,5-6,5 liters.
2.Vital lung capacity - the largest amount of air you can exhale
after maximum inhalation or the amount of the first three
volumes. Normally it is: women - 3,0-3,5 l; in men - 3,5-5,0
3. Inspiratory capacity - the maximum amount of air you can
breathe after calm exhalation, or the amount of the first two
volumes. Normally it - 1,8-2,8 liters.
4. Functional residual capacity - the amount of air contained in
the lungs after calm exhalation or the amount of the last two
volumes. Normally - 2.5-3.5 liters.
REMEMBER: Spirometry cannot
measure Residual Volume (RV) thus
Functional Residual Capacity (FRC) and
Total Lung Capacity (TLC) cannot be
determined using spirometry alone.
FRC and TLC can be determined by 1)
Helium dilution, 2) Nitrogen washout,
or 3) body plethysmography