1. MECHANICS OF BREATHING Lecture-2 Dr. Zahoor Ali Shaikh 1

2. Mechanics Of Breathing 2  Inspiratory Muscles 1. Diaphragm [major muscle contributes 75%] 2. External Intercostals contribute 25% IMPORTANT– Normal inspiration is ACTIVE process 1. 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.

3. Inspiratory Muscles 3 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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5. Inspiration 5  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.

6. Inspiration 6  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.

7. Intra-pleural Pressure during Inspiration 7  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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10. Accessory Muscles Of Inspiration 10  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

11. Accessory Muscles Of Inspiration 11  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.

12. Expiration 12  Normal 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].

13. Expiration 13  As intra-alveolar pressure increases, air leaves the lungs down its pressure gradient from high intra- alveolar pressure to lower atmospheric pressure.  Air flow stops when intra-alveolar pressure becomes equal to atmospheric pressure.

14. Muscles of Forced Expiration 14  Muscles of forced [active] expiration are: - Internal-intercostal muscles - Abdominal muscles [rectus abdominas, transverse - abdominas external oblique, internal oblique] 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.

15. Muscles of Forced Expiration 15  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].

16. Muscles of Forced Expiration 16  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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18. Summary of Inspiration & Expiration 18  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 – Active process we use accessory muscles of inspiration also e.g. sternomastoid, sclene etc.  Forceful expiration – Active process, we use accessory muscles of expiration e.g. internal-intercostal and abdominal muscles.

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20. Clinical Application 20  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.

21. Air way resistance affects Air flow 21  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.

22. Air way resistance 22  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.

23. Clinical Application 23  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

24. Chronic-Bronchitis 24  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.

25. Bronchial Asthma 25  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.

26. Emphysema 26  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.

27. What You Should Know From This Lecture 27  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.

28. Thank you 28