Respiratory System 3 Flashcards
What can a single tidal volume range in size from?
Residual volume to total lung capacity
What is functional residual capacity?
The amount of air in the lungs at the end of a normal (relaxed) expiration
What moves air?
A pressure gradient
When does air enter the lungs?
Only if alveolar pressure (Pa) is less that atmospheric (barometric) pressure (Pb)
When does alveolar pressure become sub-atmospheric?
When thoracic volume increases
When does thoracic volume increase?
By the descent of the diaphragm and elevation of the rib cage
What is the bulk rate of flow of air (ie the volume per unit time) of air entering or leaving the lungs proportional to?
The difference between atmospheric pressure and alveolar pressure
When does air leave the lungs (exhalation)?
If alveolar pressure exceeds atmospheric pressure
When does air enter the lungs (inhalation)?
If alveolar pressure is less than atmospheric pressure (provided the glottis is open)
What reduces the rate at which the lungs expand or deflate (ie the rate at which air enters or leaves the lungs)?
Any factor that increases resistance to air-flow (R)
What’s the airflow/resistance proportionality equation?
Rate of change of volume (bulk flow) = (Pb-Pa)/R
What happens to the highly elastic lungs?
They tend to collapse at zero volume which tends to separate the visceral and parietal pleura which reduces intra-pleural pressure (Pip) below atmospheric while simultaneously pulling the ribcage outwards
What is a balance between the forces (collapsing lungs and recoiling ribcage) achieved at?
Functional residual capacity (FRC)
What force causes the lungs to have a tendency to collapse and what does it arise from?
FL, which arises from the elastic recoil of stretches elastic fibres and the surface tension of wet alveolar cells
What force does the chest wall (rib cage and its musculature) experience that causes it to spring outwards, thereby increasing its volume, and where does it arise from?
Fcw, and it arises from stretched tissues in the sterno-costal and costo-vertebral joints
What does the collapsing tendency of the lungs to exactly counteract the expanding tendency of the rib-cage do?
Leaves the combined system in an equilibrium position
If the pressure in airways is zero, what does this mean?
This unique “neutral” position corresponds to FRC
What does the negative sign in the force equation?
Merely a formal statement that the forces operate in different directions
What happens when a penetrating injury of the chest wall creates a connection between the atmospheric and the intrapleural space?
The balance of forces which normally occurs in the lungs is destroyed on the affected side
What allows the lungs to collapse?
The introduction of air between the pleural membranes interrupts the adhesive forces between water molecules
What happens when the lung collapses?
The chest wall recoils outward (ie assumes a slightly increased volume)- exactly as expected from the sign of Fcw in the above equation (Fcw= -FL)
