Pulmonary Mechanics

Gases and Liquids are both fluids.

Pressure = Force / Area = (Force x Distance) / (Area x Distance) = Work / Volume = Energy / Volume = Energy Density. Fluids move from high pressure aka energy density to low pressure aka energy density.

Gauge pressure = referenced to atmospheric pressure.

Pressure transducer = deformable membrane between two fluids - deformation indicated difference in pressure. This is basis for our measurements. "Zeroed" = referenced to atmosphere

1 mmHg (more dense, pressure = density * g * height of column) = 1.36 cmH2O

A "vacuum" = absolute pressure less than P_barometric, aka negative gauge pressure. (though, obviously, there are no absolute negative pressures).

Why do elephants not have pleural spaces? (but whales do). Consider that elephants snouts function like going submerged and breathing through a hose while under water... https://journals.physiology.org/doi/full/10.1152/nips.01374.2001

Starling Resistors

Applies to : -venous return in tension pneumothorax / tamponade or RA pressure below atmospheric. -expiration flow: forced or copd -inspiratory flow during snoring -pulmonary blood flow in West Zone 2 (which is increased by CPAP). -blood flow in tissues as compartment syndrome begins to develop (once tissue pressure is above venule pressure but below arterial pressure).

At the beginning of an exhaled breath, there is an effort dependent portion (trying harder = can exhale faster). However, as the breath continues - the rate of exhalation becomes effort independent. Why?

Starling resistors describe when a fluid is moving through a collapsible circuit (in contrast to Ohmic resistors = rigid conduits): In addition to flow being dependent on P_inlet -> P_outlet and a fixed resistance (as in a rigid conduit), in a starling resistor the conduit collapses when the transmural pressure becomes positive (p_outside > p_inside).

In forced exhalation, P_pleural becomes positive. As P_alveoli gets the contribution from the pleural pressure but P_atmosphere (at mouth) does not, there will be a point where P_pleural = P_outside is equal to inside = P_airway - this is the equal pressure point. Beyond this, the airway will collapse - limiting flow.

Thus, more pleural pressure = EPP moves toward mouth and exhalation slows.

Convective Acceleration

Frictional lateral pressure loss = large when flow is turbulent, small when flow is laminar.

If tube narrows, the velocity must increase since flow in series has to stay the same = convective acceleration. Because the total energy (lateral pressure + kinetic energy + potential height energy) has to remain the same, the lateral pressure will drop (Bournulli effect).

In lung, cross sectional area is larger in the peripheral airways than the central airways. Central airways = faster, peripheral = slower.

This is going to cause a Starling resistor to collapse earlier the larger the convective acceleration (and earlier than if only friction mediated pressure loss were at play).

Ohmic Resistors / Ohms Law

Rigid conductor (or a flexible conductor in which the outside pressure is below the outlet pressure): flow is proportional to the flow-resistive pressure drop. Resistance = flow resistive pressure drop / by flow.

Analogous to West Zone 3 with respect to blood flow (note: West 2 = starling, West 1 = closed.