Generating pressure gradients - Respiratory Cycle

Flow = pressure / resistance Flow requires a pressure gradient large enough to overcome any resistance that slows the flow down

PA = PB so no air flow

PA < PB so air flows in

PA > PB so air flows out

Functional Residual capacity - the air volume within the lungs before inspiration (2.5L)

The point at which we are halfway through inspiration/ expiration we have the greatest pressure gradient and thus the air flow is fastest at this point.

Only the diaphragm will be involved during quiet breathing where the muscle moves approximately 1cm.

External intercostal muscles - move ribcage up and out Scalene - lifts the top few ribs up Sternocleidomastoid

Internal intercostal muscles - pull the ribcage down and in Abdominal muscles - push against the diaphragm to reduce the size of the thoracic cavity

The muscles used in inspiration simply recoil without recruiting the expiration muscles

Lungs will collapse to small equilibrium volume and the chest wall will expand to a large equilibrium volume. Therefore when the volumes are equal and opposite, recoil sets the FRC

Volume of the structure with no external influences.

Membrane around the outside of the lungs

Membrane that is lining the chest wall.

a small volume of blood filtrate that lies between the 2 pleural membranes

create a low friction environment meaning that less energy is required for breathing to occur.

Keeps the lungs and chest wall close together (coupled). also provides a low friction environment for easy breathing.

Lungs equilibrium volume is small at FRC lungs are stretched away from the equilibrium volume (outwards) this generates an inward elastic recoil pressure Chest wall equilibrium volume is large at FRC the chest wall compressed away from the equilibrium volume (inwards) this generates an outward elastic recoil pressure

By increasing the volume of the intrapleural space and the fluid causes the pressure to fail due to the lung and chest wall elastic recoil. The intrapleural pressure is more negative than the recoil pressures of the lungs and chest wall (sub atmospheric) meaning that the two structures are pulled together.

PB = 100kPa Pl = 99.5kPa Intrapleural pressure is 0.5 kPa lower than the barometric pressure.

P(dist) = P(aleveolar) - P(intrapleural)

Inflation requires a positive distending pressure where the Pressure in the lungs is greater than the pressure out

P(dist) = 0 - -o.5 therefore there is a positive distending pressure of 0.5kPa at FRC so they are slightly inflated at FRC.

• inspiratory muscles contract increasing thoracic cavity volume • intrapleural pressure becomes more negative • Lung distending pressure increases Pdist = 0 – -1 = +1kPa • lungs inflate • increase in lung volume causes PA to fall below PB • air flows into the lungs until PA = PB at end inspiration (in quiet breathing then expiration is simply the reverse of this)