Raw Science Clinic
Raw Science 7: The Oxygen Cascade Step 2b
What we know is not much. What we do not know is immense.
Welcome to Basic Science Clinic Raw Science 7. As a prelude to deconstructing gas exchange we have been examining how humans, as tidal ventilators, replenish the composition of the gas in the functional residual capacity to provide a plentiful oxygen repository to buffer fluctuations in the oxygen content of blood leaving the lung with every beat of the heart.
Convective, pressure gradient driven, bulk gas volume displacement can only occur if the displacing force is greater than the forces that oppose gas flow. These oppositional forces are the physiological targets of pathological processes that affect the lung, that alter pulmonary mechanics, increase work of breathing eventually critically compromising respiratory function and indicating the need for respiratory support measures. To effectively manage organ system dysfunction it is vital to develop an intimate understanding of your enemy so today we will examine the oppositional forces to gas flow that are among the key perpetrators of respiratory failure.
In this pod we’ll cover:
What are the oppositional forces to gas flow?
What is elastance?
What is elastic recoil and what are its determinants?
How does the lung prevent surface tension induced alveolar instability?
What is the 2nd major oppositional force to gas flow?
How do these driving and oppositional forces relate to work of breathing?
Raw Science Factoids
During inspiration the alveolar radius increases from 0.05mm to 1mm which should require a distending pressure of 10 cmH2O but surfactant’s detergent action means that 1 cmH2O will suffice.
Normal resting VO2 is approximately 2-4 mls/kg/min. In terms of VO2max an average untrained healthy male would approximate 35-40 ml/kg/min and former multiple Tour de France champion and King of the Mountains Miguel Indurain hit 88ml/kg/min at his peak. Racing Siberian sled dogs can reach 240 ml/kg/min.
Elite rowers can escalate their total minute ventilation to 240 L/min by hitting respiratory rates of 60/min and tidal volumes of 4000 mls and inspite of this heroic effort still generate lactates of 15-18 mmol/L.
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Thanks for listening. Next up we’ll begin our examination of pulmonary gas exchange, also coming soon is the second Crit Think series, Doors of Deception, in which we will look at the labyrinthine and mendacious ways our decision making faculties can deceive us.