MECHANICS OF
BREATHING
Lecture-2
Dr. Zahoor Ali Shaikh
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Mechanics Of Breathing
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1.
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Inspiratory Muscles
1. Diaphragm [major muscle contributes 75%]
2. External Intercostals contribute 25%
Diaphragm
When Diaphragm contracts [on stimulation of phrenic
nerve C3, C4, C5], it descends down and increases
vertical diameter of the chest .
During quiet breathing, Diaphragm descends about
1cm but during deep inspiration it may descend up to
10cm.
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Inspiratory Muscles
2. External Intercostals
 Contraction of these muscles, whose fibers run
downward and forward between the adjacent ribs,
enlarge the thoracic cavity in lateral [side to side]
and anterio-posterior [front to back] dimensions.
 When external intercostal muscle contracts, they
elevate the ribs and the sternum upward and
outward.
 Intercostal nerves activate these intercostal muscles.
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Inspiration
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Before Inspiration, intra-alveolar pressure is equal
to atmospheric pressure (760mmHg) so no air is
flowing into or out of lungs.
As the thoracic cavity enlarges [due to contraction
Inspiratory muscles] the lungs also expand to fill the
large thoracic cavity.
As the lungs enlarge, the intra-alveolar pressure
drops by 1mmHg to 759mmHg, therefore, air flows
inside, down the pressure gradient from higher
atmospheric pressure to lower pressure.
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Inspiration
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Air enters the lungs till pressure equalizes
[atmospheric and alveolar].
Note : Air flows into the lungs because of fall in the
intra-alveolar pressure brought about by lung
expansion due to enlargement of thoracic cavity due
to contraction of inspiratory muscles.
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Intra-pleural Pressure during
Inspiration
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During Inspiration, the intra-pleural pressure falls to
754mmHg [-6mmHg], as a result of expansion of the
thorax.
Please note – Intra-pleural pressure before
inspiration is 756mmHg [-4mmHg].
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Accessory Muscles Of Inspiration
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Forceful or deep inspiration can be brought about
by contraction of accessory muscles of inspiration.
They work with diaphragm and external-intercostal.
Accessory Inspiratory Muscles Are:
- Sternocleidomastoid
- Scalenus
- Trapizus
- Pectoralis major
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Accessory Muscles Of Inspiration
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Contraction of Accessory muscles causes lifting of
sternum and elevation of first two ribs, enlarging
the upper portion of thoracic cavity.
As the thoracic cavity increases even further, lungs
expand more, dropping intra-alveolar pressure
further.
Therefore, more inward flow of air occurs.
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Expiration
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Expiration is passive process.
It occurs due to the relaxation of Inspiratory muscles
i.e. diaphragm and external-intercostal muscle.
As diaphragm and external-intercostal muscle
relax, the chest wall and stretched lungs recoil to
their original [pre-Inspiratory] size, due to their
elastic properties.
As the lungs recoil and become smaller in size, the
intra-alveolar pressure rises by 1mmHg
[761mmHg].
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Expiration
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As intra-alveolar pressure increases, air leaves the
lungs down its pressure gradient from high intraalveolar pressure to lower atmospheric pressure.
Air flow stops when intra-alveolar pressure becomes
equal to atmospheric pressure.
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Muscles of Forced Expiration
Muscles of forced [active] expiration are:
- Internal-intercostal muscles
- Abdominal muscles [rectus abdominas, transverse abdominas ,external oblique, internal oblique]
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NOTE
During quiet breathing, expiration is passive process. It
occurs due to relaxing of inspiratory muscles.
Forceful expiration which is active process, we need
contraction of forced expiratory muscles.
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Muscles of Forced Expiration
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Contraction of abdominal muscles decrease the
vertical diameter of thorax.
Contraction of internal-intercostal muscles pull the
ribs downward and inward, flattening the chest wall
and decreasing the size of thoracic cavity [this
action is opposite of external-intercostal muscles
which pull the chest upward and outward].
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Muscles of Forced Expiration
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Contraction of expiratory muscles decreases the
volume of thoracic cavity, lung volume decreases,
intra-alveolar pressure increases, therefore, air
leave the lung.
Lungs are emptied more during forceful expiration
[during active expiration as compared to passive
expiration].
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Summary of Inspiration & Expiration
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Normal Inspiration is active process due to contraction
of diaphragm and external-intercostal muscles.
Normal Expiration is passive process due to relaxation
of diaphragm and external-intercostal muscles.
Forceful inspiration - we use accessory muscles of
inspiration e.g. sternomastoid, sclene etc.
Forceful expiration – we use accessory muscles of
expiration e.g. internal-intercostal and abdominal
muscles.
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Clinical Application
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Inspiratory muscles
Diaphragm which contributes to 75%, if paralyzed
due to nerve damage or muscle disorder, it leads to
severe respiratory impairment.
External-intercostal which contribute to 25%, if
paralyzed, it does not seriously affect the
breathing.
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Air way resistance affects Air flow
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Air flow depends on the resistance offered by air ways
(bronchi and bronchioles).
Resistance will increase if the radius of conducting air
ways (bronchi) gets smaller.
Air ways are supplied by Autonomic Nervous System
[ANS].
Parasympathetic ANS – causes broncho-constriction
(decreases the radius of bronchioles) therefore
increases the air way resistance.
Sympathetic ANS and Epinephrine
They cause broncho-dilation (increase the radius of
bronchioles) therefore decrease the air way resistance.
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Air way resistance
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Nowadays new drugs e.g. ventolin which act on β2
receptors which are present in bronchi (air ways)
cause broncho-dilation.
They are commonly used for Bronchial Asthma.
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Clinical Application
Chronic Obstructive Pulmonary Disease [COPD]
 In COPD, there is increased air way resistance due
to narrowing of lumen of lower air ways.
 Patient with COPD have difficulty in breathing.
 COPD applies to 3 diseases:
1. Chronic-Bronchitis
2. Bronchial Asthma
3. Emphysema
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Chronic-Bronchitis
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Affects lower respiratory airways.
It is triggered and ppt by cigarette smoking,
polluted air and allergens.
Due to chronic irritation – air way is narrowed due
to thickening of its lining, cilia lining the airways are
also affected, therefore, mucus can not be removed
satisfactorily.
Pulmonary Bacterial Infection are common.
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Bronchial Asthma
Air way obstruction occurs due to
- thickening of airway walls due to inflammation.
- increased secretion of mucus.
- airway constriction [spasm of smooth muscle in walls
of airways] due to exposure to dust, pollen,
cigarette smoking, infection, severe exercise.
- increased airway resistance causes more difficulty
during expiration than inspiration, causing wheeze.
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Emphysema
Emphysema is characterized by
- collapse of small airways
- breakdown of alveolar walls
- increased airway resistance
 Emphysema is irreversible condition.
 Emphysema can occur due to chronic exposure to
irritants e.g. cigarette smoking due to release of
enzyme trypsin released from alveolar macrophages.
 This enzyme trypsin causes destruction of lung tissue.
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What You Should Know From This
Lecture
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Muscles of Inspiration [Diaphragm and External-intercostal] and their
function.
Muscles of Forced Inspiration [Accessory Muscles].
Normal Expiration is passive process.
Muscles of Forced Expiration [Accessory Muscles]
Atmospheric Pressure, Intra-Alveolar Pressure, Intra-Pleural Pressure.
Airway Resistance.
Effect of Parasympathetic ANS and sympathetic ANS on airways
[bronchi and bronchioles].
Effect of ventolin on airways.
Chronic Obstructive Pulmonary [Lung] Diseases.
- Chronic-Bronchitis, Bronchial Asthma, Emphysema.
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THANK YOU
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