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Structure of Airways

Physiology and Pharmacology

QuestionAnswer
Functional structure of airways Cast of airways shows that they bifurcate to produce smaller and smaller airways Idealised model suggests there are 23 generations Split into 2 types - conducting zone (no gas exchange) and respiratory zone (gas exchange) Roundly 70m2 area
Generations of airways 0 - larynx, pharynx and trachea 1-2 - bronchi 3-15 - bronchioles 16 - terminal bronchioles 17-19 - respiratory bronchioles 20-22 - alveolar ducts 23 - alveolar sacs
Cross sectional area Total increases massively as you go further into the lungs Rapid increase around transition between conducting and respiratory zones As gas enters deeper into the lung, velocity slows down greatly At 16/17 diffusion becomes dominant
Effects of velocity on oxygen concentration In convection dominant area - conc of oxygen varies in respiratory cycle as air in this point is not involved in exchange in diffusion dominant area - conc of oxygen does not vary as new air is mixed with existing air by diffusion
Lung volumes Vital capacity - ERV + TV + IRV ERV - expiratory reserve volume TV - tidal volume IRV - inspiratory reserve volume FRC - functional residual capacity = ERV + RV RV - residual volume
Measurement of lung volume Measured using a spirometer Changes in volume of spirometer are equal and opposite to changes in the lung Volumes vary with age, sex and disease Breathing causes movement of a pen, which plots volume changes during normal and forced respiration
Measurement of residual volume Cannot be measured directly as cannot be breathed out Done using helium dilution - known conc and vol of gas added to spirometer After a while, conc in spirometer measured and used with known values to calculate volume of lung
Static mechanics When no airflow and open glottis - must be same pressure in alveoli and atmosphere Negative intrapleural pressure -6cmH2O At FRC rib cage springs up and lungs collapse creating this negative value
Intrapleural pressure in breathing Breathing lifts ribcage and lowers diaphragm More negative pressure in intrapleural space Holds the lung in a more stretched position
Pressure gradient in intrapleural space Due to gravity and weight of the lungs Pressure change from top to bottom is roughly 8cmH2O Bottom of lung is more squashed that the top and has a higher pressure - inflates unevenly
Regional ventilation Gas breathed in does not go evenly to the whole lung Some parts inflate more Typically top inflates less than bottom by 2x
Determination of regional ventilation Determined experimentally by taking a single breath of 133Xe and holding while count s taken Then breathed to and fro until evenly mixed to estimate volume Divide first set by second set to give ventilation per unit volume of lung
Compliance Change in volume achieved by a given change in pressure Can be experimented by attaching someone to a spirometer and compressing to to alter pressure At FRC no pressure needs to be applied as at natural volume
Relationship between compliance of lung, chest wall and total For a given change in volume total pressure change = pressure change in lung + pressure change in chest Divide by volume 1/total compliance = 1/lung compliance + 1/chest wall compliance Typically 200ml/cmH2O
Elastic nature of the lung Lungs can be inflated much more easily with liquid than with gas Suggests surface tension is important in elasticity of the lung Interactions between water molecules increases tendency of lung to collapse Surface tension at air-water interface
Laplaces equation P = 2T/r At air water surface T = 70mN/m
Surfactant To not collapse lungs need a lower surface tension (20mN/m) Surfactant helps reduce this, by acting to reduce interactions between water and by helping dry out the epithelium Premature babies can lack this, so cannot inflate their lungs
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