Topic 1: Lung Volumes and Gas Transport Tidal Volume: volume of air inspired and expired with a normal breath Vt Residual volume: volume of air remaining after a maximal expiration effort Total Lung Capacity: volume of gas contained within the lungs at the end of a max inspiration Total Dead Space: volume of gas that does not eliminate CO2 and is made up of: - Anatomical Dead Space: volume of the conducting airway - Alveolar Dead Space: volume in the alveoli that are not or poorly perfused Alveolar Volume: volume of gas per unit of time that reaches the alveoli, the respiration portions of the lungs where gas exchange occurs. Minute Volume: amount of gas expired per minute Inspiratory Reserve Volume: extra volume of air that can be inspired after a normal Vt inspiration Expiratory Reserve Volume: extra volume of air that can be expired after a normal Vt expiration Vital Capacity: volume of air that can be expired following a max inspiration Functional Residual Capacity: volume of gas remaining in the lungs at the end on a normal expiration Inspiratory capacity: the maximum volume of gas that can be inspired from resting end-expiratory level Topic 2: Functions of the Lung and Lung Mechanism Basic functions of the lung 1. Gas exchange: simple diffusion of oxygen from atmosphere into bloodstream and CO2 from bloodstream into atmosphere: high partial pressure to low partial pressure 2. Defence: protective function, able to deal with particles and microorganism 3. Reservoir for blood 4. Filtering of blood 5. Metabolism Lung Mechanism – Chest Wall The properties of the rib cage and abdomen (elastic walls) determine the mechanical characteristics of the chest wall as a whole. A number of factors must be overcome to enable air to move into/out of lungs: 1. Elastic recoil of chest wall and lungs 2. The friction resistance of the lungs and chest wall tissue and the frictional resistance of the airways to the flow of air 3. The inertia Lung Mechanism – The Respiratory Muscles Respiration is dynamic Normal quiet breathing: diaphragm major inspiratory muscle. It acts on the rib cage by changes in pleural or abdominal pressure. The inspiratory muscles are responsible for the anterio-posterior diameter of the rib cage and stabilize the chest wall. Diaphragm contracts to enlarge thoracic cavity: reduce intrathoracic pressure so that the lungs may expand to fill their alveoli. Expiration: passive, as the lung and chest wall are elastic and have a tendency to return to their equilibrium position. Abdominal wall muscles most expiratory important. Contraction causes intra-abdominal pressure to increase, and the diaphragm is pushed upwards. Lung Mechanism – Elastic Properties of the Lungs As pressure increases, lung volume increases. The lung distends easily at long lung volumes, however at high lung volumes, the dispensable components of alveolar walls already have been stretched out and large increases in pressure produces only small increases in volume. Compliance: the ease with which something can be stretched or distorted Elastance: the tendency for something to oppose stretch or distortion, as well as to its ability to return to its original configuration Lung Mechanism – Pressures during respiratory cycle Airflow occurs as a result of change in pressure 1. Intrapleural pressure: pressure between the visceral and parietal pleura (negative pressure) 2. Alveolar pressure: pressure inside the alveoli 3. Transmural pressure: pressure difference across the airway or across the lung wall Transpulmonary pressure: difference in pressure between inside and outside of the lungs: always positive during normal breathing Transairway pressure: pressure difference between alveolar pressure and intrapleural pressure: important to keep the airways open during expiration At rest: alveolar pressure = atmospheric pressure (no airflow) Alveolar pressure > intrapleural pressure (no airflow) Transmural P = alveolar p (0 cmH2O) – Intrapleural pressure (-5cm H2O) = + 5cm H2O Inspiration: intrapleural pressure becomes more negative Alveolar pressure < atmospheric pressure = increase in transmural pressure Transmural P = alveolar P (-1 cmH2O) – (-8 cmH2O) = +7 cmH2O Interdependence: factor of alveoli: if an alveolus starts to collapse, the surrounding alveoli are stretches and then recoil exerting expanding forces in the collapsing alveolus to open it. Important for alveoli stabilization. Each alveolus supports each other. Factors responsible for keeping the alveoli open: 1. Surfactant 2. Interdependence 3. Functional residual capacity Surfactant - Reduces muscular effort of breathing - Reduces elastic recoil of the lungs at low volume - Maintains the equality of size of alveoli during inflation/deflation - Lowers surface tension during deflation Topic 4: Mechanics/Compliance/Airflow Resistance/WOB Compliance Work of breathing is determined by elastic recoil of the lungs and chest wall, and resistance to airflow Compliance is the ease of which the lung is expanded. > volume change per unit of pressure change Cl = lung compliance Ccw = chest compliance Increased compliance: - Floppy airways: inflate easily and does not take much change in pressure to get lots of volume - High compliance at Residual volume Decreased compliance: - Stiff airways: they are harder to inflate and need bigger changes in pressures to get big changes in volume - Furthermore, at low lung volumes, the lungs are relatively stiff, as well as at high volumes > ‘S-shaped curve’ - Decreased compliance at TLC: because the distensible components of alveolar walls have already been stretched and therefore the recoil pressure is high as the lungs want to spring back Factors affecting compliance: 1. Lung volume 2. Surfactant 3. Pulmonary blood flow 4. Age 5. Disease 1. Lung volume At high lung volumes, the compliance is decreased because more pressure is required to stretch the already stretched elastic tissues: elastic recoil is high At low lung volumes, the compliance is decreased - Closed airways - Collapsed alveoli 2. Surfactant Surfactant is the substance secreted by the cells lining the alveoli, which lowers the surface tension of the alveolar lining layer. This leads to: - Lower surface tension - Increased compliance - Work of expanding the lungs is reduced - Promotes stability of the alveoli - Keeps alveoli dry 3. Pulmonary Blood Flow Contributes to the stiffness of the lungs - Increased capillary blood flow leads to decreased compliance Need good blood flow around the alveoli to exchange O2 and CO2. However, engorgement of blood around the alveoli can create it to collapse, which will reduce lung volume back to the bottom of the S-shaped curve. > Decreased compliance due to collapsed alveoli 4. Age As you increase in age, your chest wall stiffens > reduces chest movement > decreased chest wall compliance With age, you lose elasticity > increase lung compliance Therefore, floppy airways with a stiff thorax 5. Disease All diseases that have increased compliance (floppy airways) and early airway closure: - Obstructive Lung Disease o Emphysema (destroys elasticity) o Chronic Bronchitis o Asthma o Bronchiectasis o Cystic Fibrosis All disease that have decreased compliance (stiff airways) and have a problem with expansion: - Restrictive Lung Disease o Interstitial lung disease > idiopathic pulmonary fibrosis o Scoliosis o Diabetes Effects of Altered Compliance 1. Lung Compliance affects diameter of airways - Increase Lung Compliance: • Less Elastic Support • Early airway closure a. Closing capacity is the lung volume at which some of the small airway begin to close. The closing volume = closing capacity – residual volume. Occurs at very low lung volumes in younger; occurs at higher lung volumes in elderly due to the loss of its elastic recoil (intrapleural pressure is less negative) • Reduced airway diameter • Increase resistance to flow • Obstructive 2. Lung compliance and Chest wall compliance affect airflow - Decrease in chest wall and lung compliance: • Decrease airflow • Preferential ventilation of compliant lung units: air will follow path of least resistance Airflow Resistance Airflow resistance is the resistance of the respiratory tract to airflow during inspiration and expiration Factors affecting airflow resistance: 1. Character of airways 2. Pattern of airflow 3. Density/viscosity of gas 4. Lung volume 1. Airway character - Increase length of tube > increase resistance - Decrease diameter of tube > increase resistance High resistance in trachea due to the sum of the diameters Low resistance in alveoli due to the sum of the diameter Progressively lower resistance in bronchioles from trachea down Causes of airway narrowing: 1. Inside the lumen: partial occlusion by secretions or foreign materials 2. Wall of lumen: hypertrophy of mucus glands, oedema of bronchial wall, contraction of bronchial smooth muscle 3. Outside airways: loss of radial traction caused by destruction of lung parenchyma, local compression, peri-bronchial oedema 2. Pattern of flow Laminal > minimal resistance Turbulent > greatest resistance (caused by sputum) Transitional > nose, glottis, carina 4. Lung Volume High lung volumes; airways more distended Low lung volumes: airway closure > increased resistance Effects of altered resistance - Increased resistance > decreased airflow - Airflow takes the path of least resistance Dynamic hyperinflation: is a compensatory mechanism: occurs when a new breath begins before the lungs have reached the static equilibrium volume. - Increase CO2 - Increase resistance > decrease expiratory