MECHANICS OF BREATHING Prof Sultan Ayoub Meo MBBS

MECHANICS OF BREATHING Prof. Sultan Ayoub Meo MBBS, M. Phil, Ph. D (Pak), PG Dip Med Ed, M Med Ed (Scotland) FRCP (London), FRCP (Dublin), FRCP (Glasgow), FRCP (Edinburgh) Professor and Consultant, Department of Physiology, College of Medicine, King Saud University, Riyadh, Saudi Arabia

MECHANICS OF BREATHING

MECHANICS OF BREATHING q Mechanics means science that deals with forces q Breathing mechanics is interplay of force generated by pressure, volume and flow changes occurring during the breathing cycle q Role of respiratory muscles q Alveolar, Pleural and Trans pulmonary pressures q Compliance of lungs, chest wall & together q Lung recoil due to elastic & collagen fibers q Work of breathing.

MUSCLES OF RESPIRATION Muscles of inspiration q Principal muscles • Diaphragm • External intercostals q Accessory muscles • Sternocleidomastoid • Scaleni • Serratus anterior • Pectoralis minor Muscles of normal expiration • Normal expiration is due to elastic recoil of lungs and associated structures Muscles of forced expiration • Internal intercostal • External oblique abdominis • Internal oblique abdominis • Rectus abdominis • Transversus abdominis

MUSCLES OF RESPIRATION

FUNCTIONS OF THE RESPIRATORY MUSCLES

ALVEOLAR PRESSURE q Intra Alveolar pressure (Alveolar pressure): Pressure inside the alveoli. The process of inspiration and expiration depends upon this pressure. q Normal alveolar pressure is atmospheric pressure=760 mm Hg. equal to the

ALVEOLAR PRESSURE q Alveolar pressure during inspiration: The alveolar pressure is lower than the atmospheric pressure during inspiration (760 to 758 mm Hg) so air moves from the atmosphere towards the lung alveoli. q Alveolar pressure during expiration: Alveolar pressure is increased during the process of expiration and is slightly more with respect to the atmospheric pressure (763 mm Hg) so air moves from lungs to the atmosphere.

ALVEOLAR PRESSURE

PLEURAL AND TRANSPLEURAL PRESSURE q Intrapleural pressure or pleural pressure: Pressure in the narrow space between the lung pleura and chest wall pleura. Negative pressure in the pleural space prevent the collapse of the lungs. -5 cm H 2 O at the beginning of inspiration -7. 5 cm H 2 O at the peak of inspiration q Transpulmonary pressure: Pressure difference between the alveolar and pleural pressure is called transpulmonary pressure.

CHANGING ALVEOLAR VOLUME

NORMAL BREATHING CYCLE

ROLE OF NEGATIVE INTRAPLEURAL PRESSURE q At rest the intra pleural pressure is negative (Sub- atmospheric) More negative at apex (-10 cms of water) as compared to base (-2. 5 cms of water) q It can become more negative during deep Inspiration and becomes substantially more positive during Forced expiration q Main function is that it prevents the lungs from collapsing and the chest wall from going out. 19

PULMONARY VENTILATION q Pulmonary ventilation (Pulmo=lungs, ventilation=breathing) q Pulmonary ventilation is the inspiration (inflow) and expiration (outflow) of air between the atmosphere and lungs q In ventilation important factor called the pressure gradient exists. q Air moves into the lungs when the pressure inside the lungs is less than that of the atmospheric pressure. q Air moves from the lungs to the atmosphere, when the pressure in the lungs is greater than the atmospheric pressure.

PULMONARY VENTILATION [INSPRATION AND EXPIRATION]

PULMONARY VENTILATION [INSPIRATION] n Movement of air into and out of lungs n Air moves from area of higher pressure to area of lower pressure n Pressure is inversely related to volume

PULMONARY VENTILATION [INSPIRATION]

PULMONARY VENTILATION [INSPRATION]

THORACIC VOLUME

PULMONARY VENTILATION [EXPIRATION] q Relaxation of inspiratory muscle q Decreased vertical and antero-posterior diameter of the chest cavity q Increased intra-thoracic pressure q Size of lungs decreases q Increased alveolar pressure from 760 -763 mm Hg q Air moves from lung alveoli towards atmosphere q Expiration

PULMONARY VENTILATION [EXPIRATION]

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