Physiology Flashcards
Define lung compliance
The volume displaced within the lung per unit pressure change i.e. the gradient of the pressure-volume curve
Specific lung compliance is the same, but indexed to total lung volume
Compliance is higher in expiration
Describe the action of surfactant
Surfactant is a phospholipid that opposes the normal attraction between surface fluid molecules, and so reduces surface tension.
This opposes Laplace’s law, which would encourage alveolar collapse as Pressure ∝ Radius, and hence would cause smaller spheres (alveoli) to empty into bigger ones. Surfactant action means the pressures are more similar between alveoli of different sizes.
At lower lung volumes the surfactant molecules are brought closer together and repel each other, which prevents alveoli from collapsing in end expiration, and also reduces compliance at this stage.
What forces are balanced at the end of expiration?
The elastic recoil of the lungs (causes passive expiration) and the elastic recoil of the chest wall (opposes the lung’s collapse)
Define hysteresis in the context of the pressure-volume loop of the lungs
The difference in lung pressure at a given volume between inspiration and expiration
Give factors that increase and decrease lung compliance
Compliance is increased with age and emphysema
Compliance is decreased by high pulmonary venous pressures, fibrotic lung disease, and alveolar oedema
What is a Reynold’s number?
The ratio between inertial and viscous forces, and a number which can therefore be used to predict the switch from laminar to turbulent flow
At low rates of flow, viscous forces predominate i.e. a gas has an inherent resistance to deformation at a given rate. At higher rates of flow this inherent resistance is overcome, so inertial forces predominate, which leads to more complex fluid dynamics with eddies etc. which is turbulent flow.
A Reynolds number describes the balance of these forces within a given liquid or gas, and so can be used to predict the change in flows.
In which airways is turbulent flow more likely?
Larger airways, or those with bifurcations/ changes in diameter.
Larger airways have higher flow (Hagen-Pouiseulle equation) so inertial forces will trump viscous ones (i.e. the Reynolds number will be higher)
What effect would xenon have on flow through an airway?
More laminar flow
Xenon has a very low density, and density is proportional to the Reynolds number. Since turbulent flow begins at a threshold Reynolds number, the lower the density of gas, the more laminar flow.
When is density important in gas flow through airways?
In turbulent flow
In laminar flow, the Hagen-Poiseulle equation governs, and it is not affected by density but viscosity
Explain closing capacity
At any time, patency of the smallest airways of the lungs depends on the elastic stretch within the lung. This decreases as the lungs contract, and at a certain point the stretching forces will not be sufficient to keep the smallest airways open.
Closing capacity is the volume at which this collapse first occurs. A higher capacity will be caused by anything that reduces these elastic forces, or adds extra pressure that leads to small airway collapse e.g. obesity, advanced age, smoking. Young healthy lungs exert enough elastic force at end expiration that airways don’t collapse, and this is expressed as them having a low closing capacity
Define:
Dead space
Physiological dead space
Dead space is the volume of inspired air that does not take part in gas exchange (~30%)
Dead space consists of apparatus, anatomical, and alveolar
Physiological dead space discounts the apparatus
How does Fowler’s method work?
A vital capacity breath of pure O2 is taken, after which the forced expiration is analysed for nitrogen. The volume exhaled between the start, and halfway up the slope of nitrogen detection is the anatomical dead space, as there was nitrogen present in the lung but pure oxygen in the anatomical dead space.
Give an overview of perfusion vs. ventilation in a healthy lung
Zone 1/ Apex: higher resting volume due to less compression secondary to gravity, which means it doesn’t ventilate that well as it can’t expand that much more (lower compliance). Comparatively poorly perfused due to gravity, in cases of IPPV or hypovolaemia, intrapleural pressure max exceed capillary pressure and splint them shut.
Zone 2/ Mid: well balanced, capillaries aren’t splinted shut, venous pressure isn’t high enough to affect perfusion, and ventilation is okay.
Zone 3/ Base: lower resting volume due to compression from the above lung/ surrounding tissue, so ventilation is better because it has further capacity to expand than the apex (higher compliance). Higher capillary pressures because of the advantage of gravity, so perfusion is good, and even outstrips ventilation. Venous pressure may exceed alveolar pressure.
Summarise body plethysmography, helium dilution, and nitrogen washout
Body plethysmography uses an airtight box and measures change in pressure within the box to extrapolate volumes during respiration
Nitrogen washout measures FRC - the subject breathes pure oxygen on a closed circuit and de-nitrogenates their lungs. The total amount of nitrogen washed out is then measured which gives the FRC, assuming the FRC was full of atmospheric air at the start (79% nitrogen)
What is the most influential driver of ventilation?
PaCO2 stimulating Central chemoceptors by making CSF more acidic (CO2 + H2O -> HCO3 + H), this accounts for 80% of response to high CO2, though peripheral chemoceptors generate a faster response.
It’s unknown how the chemoceptors detect pH changes
