Mechanics of Pulmonary Ventilation PDF

Nikolai P Pace MD To supply the body with oxygen and dispose of carbon dioxide For which body process do we need oxygen? Electron transport system of cellular respiration What is the source of carbon dioxide? Krebs cycle of cellular respiration...

Nikolai P Pace MD To supply the body with oxygen and dispose of carbon dioxide For which body process do we need oxygen? Electron transport system of cellular respiration What is the source of carbon dioxide? Krebs cycle of cellular respiration 2  The process of gas exchange between the atmosphere and the alveoli - breathing.  The alternate contraction and relaxation of respiratory muscles generates a pressure difference between the atmosphere and the lungs that drives air flow.  Breathing in  inspiration  Breathing out  expiration  For air to enter the lungs, the pressure inside the alveoli must be lower than atmospheric pressure i.e. (760mmHg or 1 atm)  Lowering intraalveolar pressure is achieved by increasing lung volume.  Boyle’s law: Inverse relationship between the pressure of a gas in a closed container and its volume  Increasing lung volume by lung expansion lowers the pressure to below 1 atm.  The first step involves contraction of the diaphragm. This floors the thoracic cavity.  Contraction innervated by the phrenic nerve [C3/4/5]  This flattens upon contraction and increases the vertical dimensions of the thoracic cavity.  As the overall size of the thoracic cavity increases, the pressure within the alveoli drops [intaalveolar pressure] below 1 atm  This allows air flow from the atmosphere into the alveoli along a high-low pressure gradient.  In normal quiet respiration, the diaphragm is the main muscle that drives ventilation.  In deep forceful respiration, other muscles participate in expiration  The external intercostals elevate ribs  Sternocleidomastoids elevate the sternum  Scalene muscles elevate the first 2 ribs  Pectoralis minor elevates the ribs 3 , 4, 5 Left to right 1) Pectoralis Minor 2) Sternocleidomastoid 3) Scalene Anterior and Posterior  The pressure between the 2 pleural layers is about 4mmHg less than atmospheric during normal breathing.  The expansion of the thoracic cavity volume lowers this intrapleural pressure even more  The two layers are adherent due to the subatmospheric pressure and surface tension between the two layers  Therefore expansion of the thoracic cavity results in expansion of the two pleural layers and the attached lungs.  Similar to inspiration but with a reversed pressure gradient  Pressure inside alveoli > 1 atm  Normal expiration during quiet breathing is a passive process – i.e. does not involve active muscle contraction.  Quiet expiration is driven by the elastic recoil of the chest wall and lungs – which have an inherent elastic properties.  In forceful expiration – other muscles contract –  The abdominal wall musculature and the internal intercostals increase pressure in the abdomen and thorax and push the diaphragm upwards  Contraction of the internal intercostals pulls the ribs inferiorally.  Healthy adult respiratory rate is 10 – 12 breaths/ minute  Resting ventilation moves around 500ml of air in and out of the lungs  Tidal Volume= the volume of air exchanged in one breath  Only around 70% of the tidal volume reaches the respiratory portion of the respiratory system and is therefore available for gas exchange  The remaining volume of air remains in the conducting airways – from nose, pharynx, larynx , trachea, bronchi, bronchioles and terminal bronchioles.  These conducting airways constitute the anatomic dead space  The air in these conducting airways is not available for gas exchange.  To supply the body with oxygen and dispose of carbon dioxide four distinct processes must occur Pulmonary ventilation – moving air into and out of the lung Alveolar ventilation – air that actually reaches the alveolar membranes External respiration – gas exchange between the lungs and the blood stream Internal respiration – gas exchange between systemic blood vessels and tissues 21  Pulmonary ventilation = tidal volume (500 ml)  Approximately 150 ml of air does not make it to the alveolar tissues  Alveolar ventilation is the 350 ml of air that reaches alveolar tissues  Only 20% of the air we breathe is oxygen; 79% is nitrogen  350 ml of air x 20% oxygen = 70 ml of oxygen reaches alveoli for respiration 22  Component Atmospheric Air Expired Air Difference (%) (%) (%) N2 (plus inert 78.62 74.9 -3.72 gases O2 20.85 15.3 -5.55 CO2 0.03 3.6 +3.57 H2O 0.5 6.2 +5.7 100.0% 100.0%  Inspiratory reserve volume (IRV) is the additional volume of air that can be forcibly inhaled following a normal inspiration.  It can be accessed simply by inspiring maximally, to the maximal inspiratory level.  Approximately 3.1L  Expiratory reserve volume (ERV) is the additional volume of air that can be forcibly exhaled following a normal expiration. It can be accessed simply by expiring maximally to the maximal expiratory level.  Around 1.2L  Vital capacity (VC) is the maximal volume of air that can be forcibly exhaled after a maximal inspiration.  VC = TV + IRV + ERV  Residual volume (RV) is that volume of air remaining in the lungs after a maximal expiration. It cannot be expired no matter how vigorous or long the effort.  RV = FRC - ERV.  Functional residual capacity (FRC) is the volume of air remaining in the lungs at the end of a normal expiration.  FRC = RV + ERV.  Total lung capacity (TLC) is the volume of air in the lungs at the end of a maximal inspiration.  TLC = FRC + TV + IRV = VC + RV