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Mechanism of Breathing

Mechanism of Breathing. Barasa Ambrose. Mechanical Factors in Breathing. P atmos. Air flows from region of high pressure to region of low pressure Flow = (P 1 – P 2 )/R 1/R = k Flow = k(P 1 – P 2 ). P atmos = P alv No air flow. P alv. P atmos. P atmos > P alv Inspiration. P alv.

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Mechanism of Breathing

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  1. Mechanism of Breathing Barasa Ambrose

  2. Mechanical Factors in Breathing Patmos • Air flows from region of high pressure to region of low pressure • Flow = (P1 – P2)/R • 1/R = k • Flow = k(P1 – P2) Patmos = Palv No air flow Palv Patmos Patmos > Palv Inspiration Palv Patmos Patmos < Palv Expiration Palv Respiratory Movements

  3. Mechanical Factors in Breathing Patmos • Two ways of producing the necessary pressure differences • Alveolar pressure can be lowered below atmospheric pressure • Natural negative pressure breathing Patmos = Palv No air flow Palv Patmos Patmos > Palv Inspiration Palv Patmos Patmos < Palv Expiration Palv Respiratory Movements

  4. Mechanical Factors in Breathing Patmos • Atmospheric pressure can be increased above alveolar pressure • Positive pressure breathing Patmos = Palv No air flow Palv Patmos Patmos > Palv Inspiration Palv Patmos Patmos < Palv Expiration Palv Respiratory Movements

  5. Natural Breathing • Accomplished by • Active contraction of inspiratory muscles • Thoracic volume increases • Intrathoracic pressure decreases • Pulls on the lungs • Enlarges the alveoli Increase in thoracic volume decrease intrathoracic pressure Respiratory Movements

  6. Natural Breathing • Expands alveolar gas • Decreases its pressure below atmospheric pressure • Air at atmospheric pressure • Flows into lungs Increase in thoracic volume decrease intrathoracic pressure Respiratory Movements

  7. Respiratory Muscles • Inspiratory muscles • Diaphragm,external intercostals • Others • Scaleni, sternocleidomastoid, pectoralis minor • Expiratory muscles • Internal intercostals • Abdominal recti Respiratory Movements

  8. Respiratory Muscles • Have no inherent rhythm • Do not contract if they do not receive motor impulses • Motor impulses originate from • Higher centers, respiratory centers, spinal cord Respiratory Movements

  9. Muscles of Inspiration • Diaphragm • Most important muscle of inspiration • In quite breathing • May be the only active inspiratory muscle • Its motor nerve leaves the spinal cord C3,4,5 Diaphragm Abdominal content Respiratory Movements

  10. Muscles of Inspiration • When the diaphragm move down • Abdominal contents are forced downward • Increase the vertical dimension of the thorax Diaphragm Abdominal content Respiratory Movements

  11. Muscles of Inspiration • In quite breathing • Diaphragm moves down by about 10mm (1 cm) • In forceful inspiration • It can move down by 10 cm Diaphragm Abdominal content Respiratory Movements

  12. Muscles of Inspiration • The area of the diaphragm • About 250 cm2 • During normal tidal breathing • It increases the thoracic volume by • 250 x 1 = 250 cm3 Diaphragm Abdominal content Respiratory Movements

  13. Muscles of Inspiration • During forceful inspiration • It increases the thoracic volume by • 250 x 10 = 2500 cm3 Diaphragm Abdominal content Respiratory Movements

  14. Muscles of Inspiration • External intercostals • Connect adjacent ribs • Slope downwards & forwards • When they contract • Ribs are lifted upwards • Causing an increase in AP diameter • “Pump handle” External intercostals Lift sternum upwards and forwards AP diameter Diaphragm Abdominal content Respiratory Movements

  15. Muscles of Inspiration • When the external intercostals contract • Ribs are lifted upwards • In addition they are also moved outwards • “Bucket handle” effect • This increases the transverse diameter (From Hassen Taha Sherrif ) Textbook of Physiology CD Respiratory Movements

  16. Respiratory Movements

  17. Overall Effects • Inspiratory muscles • Increase the thoracic volume • Increase lung volumes • Decrease in intrapulmonary pressure • Cause influx of air • From region of high pressure • To region of low pressure From Textbook of Work Physiology by Astrand, Rodahl, Dahl & Stromme Respiratory Movements

  18. Expiration • During quite breathing • Expiration is Passive • After inspiratory muscles relax • Elastic recoil of lungs and chest wall • Cause movement of air from lungs to atmosphere Respiratory Movements

  19. Expiration • During exercise • Expiration is by active process • Contraction of expiratory muscles • Internal intercostal muscles • Assist active expiration by • Pulling ribs downwards and inwards Respiratory Movements

  20. Mechanics of Breathing From: Exercise Physiology by McArdle, Katch & Katch Respiratory Movements

  21. Pressure Changes in the Lungs and Thorax • Lungs are separated from the rib cage by • Parietal & visceral pleura • Between these there is • Pleural fluid • Lubricant film 20 m thick Respiratory Movements

  22. Pressure Changes in the Lungs and Thorax • The thoracic cage • Has a tendency to expand • The lungs • Have a tendency to collapse • Held together by the action of pleural fluid Respiratory Movements

  23. Pressure Changes in the Lungs and Thorax • Intrathoracic (intra pleural) pressure • Normally = -5 mm Hg • At the end of expiration during quiet breathing • During inspiration it is = -8 to –10 mm Hg • It is a measure of elastic recoil of the stretched lungs and the compressed thoracic cage Respiratory Movements

  24. Pressure Changes in the Lungs and Thorax • Alveolar pressure • Pressure of the air inside the lung alveoli • When glottis is open & no air flowing into or out of the lung • This pressure is equal to atmospheric pressure P atmos P alv Alveolus P alv = P atmos Respiratory Movements

  25. Pressure Changes in the Lungs and Thorax • To cause inward flow of air into alveoli during inspiration • Pressure falls to values below atmospheric (-1 cm of water) • This is enough to cause 0.5 liters of air move into lungs P atmos P alv Alveolus P alv < P atmos Respiratory Movements

  26. Pressure Changes in the Lungs and Thorax • During expiration • Alveolar pressure increases (+1 cm of water) • Enough to cause movement of 0.5 liters of air out of the lung P atmos P alv Alveolus P alv > P atmos Respiratory Movements

  27. Pressure Changes in the Lungs and Thorax • Trans-pulmonary pressure • Pressure difference between alveolar pressure and pleural pressure • It is a measure of elastic forces in the lungs that tend to collapse the lungs • Recoil pressure Inspiration Expiration Alveolar pressure +2 0 Trans-pulmonary pressure -2 -4 -6 Pleural pressure -8 Respiratory Movements

  28. Elastic Resistance • Lung tissue is elastic • Natural un-stretched volume • Elastic element neither stretched nor compressed • Is 1 liter Vol of lung 1 lt 2.5 lt Thoracic cavity & lung 5 lt Thorax Respiratory Movements

  29. Elastic Resistance • Human lung at the end of expiration • Volume = 2.5 liters • Thus the elastic tissue is always under tension • Tends to oppose expansion of the lungs Vol of lung 1 lt 2.5 lt Thoracic cavity & lung 5 lt Thorax Respiratory Movements

  30. Elastic Resistance • The natural un-stretched thorax volume is 5 liters • At end of expiration • Volume of thorax is 2.5 liters • The elastic tissues of thorax are compressed Vol of lung 1 lt 2.5 lt Thoracic cavity & lung 5 lt Thorax Respiratory Movements

  31. Elastic Resistance • Thus • Lungs tend to contract • Thorax tends to expand • The lungs and thorax • Held together by the integrity of the pleural cavity Vol of lung 1 lt 2.5 lt Thoracic cavity & lung 5 lt Thorax Respiratory Movements

  32. Elastic Resistance • If a gas is introduced in the pleural space • Chest volume tends to expand • Lung volume tend to decrease (collapse of the lungs) Vol of lung 1 lt 2.5 lt Thoracic cavity & lung 5 lt Thorax Respiratory Movements

  33. Compliance • Compliance • Measure of the ability of the lung or chest cavity to be expanded • The degree to which • The lung volume can be changed • By imposed intrapulmonary pressure Increased compliance ΔP ΔV Volume in ml decreased compliance Pressure cm H2O Respiratory Movements

  34. Compliance • Compliance • Change in volume (liters)/change in pressure (cm H2O) • Compliance of • Adult male = 0.09 to 0.26 L/ cm H2O • Newborn = 0.005 l/cm H2O • At 10 yrs = 0.06 L/ cm H2O • Old age = ↓ compliance Increased compliance ΔP ΔV Volume in ml decreased compliance Pressure cm H2O Respiratory Movements

  35. The Airways Resistance • Resistance offered to air as it flows through the respiratory airways • Flow = (P1-P2)/R • Vol of air that flow in/out of alveolar • Directly proportional to pressure gradient • Indirectly proportional to resistance From: Nunn’s Applied Respiratory physiology; 5th Ed Respiratory Movements

  36. The Airways Resistance • Airway resistance • Frictional resistant offered by the walls of tracheobronchial tree • This is note evenly distributed From: Nunn’s Applied Respiratory physiology; 5th Ed Respiratory Movements

  37. The Airways Resistance • During quiet breathing with mouth closed • Nose offers 50% of total resistance • During mouth breathing • Pharynx offers 25% of overall resistance • This figure can increase up to 50% during exercise From: Nunn’s Applied Respiratory physiology; 5th Ed Respiratory Movements

  38. Airway Resistance • Within the chest • Trachea, lobar & segmental bronchi offer 80% of the remaining resistance • Small airways with diameter less than 2mm contribute 20% Airway resistance VS airway generations 0.08 0.06 Airway resist (cm H2O/L/S) 0.04 Segmental bronchi Terminal bronchi 0.02 5 10 15 20 Airway generations Respiratory Movements

  39. Airway Resistance • Cross section of individual peripheral airways are small • Their large numbers • Generate large overall cross section area • Lowers the resistance Airway resistance VS airway generations 0.08 0.06 Airway resist (cm H2O/L/S) 0.04 Segmental bronchi Terminal bronchi 0.02 5 10 15 20 Airway generations Respiratory Movements

  40. Determinants of Airway Resistance • Lung volumes • Greater tethering effect of lung parenchyma on airways • Produce an increase in cross section area of each airway • Results in reduced resistance Airway resistance VS lung volumes 4 3 Airway resist (cm H2O/L/S) 2 1 2 4 6 8 Lung volumes (L) Respiratory Movements

  41. Determinants of Airway Resistance • Others • Resistance is proportional to • Length of airway • Physical properties of the gas • Density, viscosity • Resistance is inversely proportional to • 4th power of radius of the airway Respiratory Movements

  42. Determinants of Airway Resistance • Under normal condition • Airways diameter “large” • Interaction between gas molecules negligible • Length of conducting tube relatively constant • Resistance is largely controlled by radius • Bronchial tree contain smooth muscle • Under the influence of autonomic nerves • Parasympathetic • Sympathetic Respiratory Movements

  43. Determinants of Airway Resistance • Parasympathetic activity causes • Constriction of smooth muscles • Reduction in cross section of airways • Increased resistance • Increased secretion of mucous glands • Sympathetic activity • Bronchodilatation • Inhibition of mucous glad secretion • Reduction in resistance Respiratory Movements

  44. Airway Resistance • Certain disease condition • Increase airway resistance • Asthma • Contraction of bronchial smooth muscles • Narrowing of airways • Increased airway resistance Respiratory Movements

  45. Airway Resistance • Chronic bronchitis • Oedema of bronchial mucosa • Excessive secretion by bronchial mucosa • Increase airway resistance • Intramural masses • Bronchogenic carcinoma • Occlude airways Respiratory Movements

  46. The Work of Breathing • Breathing involves • Application of force over distance • Work is performed by respiratory muscles • Stretching elastic tissues of chest wall & lungs • Elastic work, compliance work • Moving inelastic tissue (viscous resistance) • Tissue resistance work Respiratory Movements

  47. Work of Breathing • Work involved in moving air through the respiratory passages • To overcome airway resistance • Normally negligible • But can be marked • With increase in ventilation (turbulence) • In asthma Respiratory Movements

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