Generating pressure gradients - Respiratory Cycle Flashcards
What is the relationship between pressure, flow and resistance?
Flow = pressure / resistance
Flow requires a pressure gradient large enough to overcome any resistance that slows the flow down
At rest what is the relationship between PA (alveola pressure)and PB (Barometric pressure)?
PA = PB so no air flow
During Inspiration what is the relationship between PA and PB?
PA < PB so air flows in
During Expiration what is the relationship between PA and PB?
PA > PB so air flows out
What is FRC?
Functional Residual capacity - the air volume within the lungs before inspiration (2.5L)
At what point do we have the fastest flow during inspiration / expiration?
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.
During resting breathing, which muscles are working the breathing mechanism?
Only the diaphragm will be involved during quiet breathing where the muscle moves approximately 1cm.
What are the muscles used for inspiration?
External intercostal muscles - move ribcage up and out
Scalene - lifts the top few ribs up
Sternocleidomastoid
What muscles are used in expiration?
Internal intercostal muscles - pull the ribcage down and in
Abdominal muscles - push against the diaphragm to reduce the size of the thoracic cavity
How is resting expiration passive?
The muscles used in inspiration simply recoil without recruiting the expiration muscles
How is the FRC produced/maintained
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
What is the equilibrium volume?
Volume of the structure with no external influences.
What is the Visceral pleura?
Membrane around the outside of the lungs
What is the parietal Pleura?
Membrane that is lining the chest wall.
What is the Pleural Fluid?
a small volume of blood filtrate that lies between the 2 pleural membranes
What are the functions of the pleural membranes and fluid?
create a low friction environment meaning that less energy is required for breathing to occur.
What is the main role of the Pleural Fluid?
Keeps the lungs and chest wall close together (coupled). also provides a low friction environment for easy breathing.
How does the Pleural fluid couple the lungs and the chest wall together?
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
How is the intrapleural pressure generated?
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.
What are the absolute pressures for the barometric and intrapleural pressure?
PB = 100kPa Pl = 99.5kPa
Intrapleural pressure is 0.5 kPa lower than the barometric pressure.
How do we calculate the distending pressure?
P(dist) = P(aleveolar) - P(intrapleural)
How do we inflate the lungs?
Inflation requires a positive distending pressure where the Pressure in the lungs is greater than the pressure out
What is the distending pressure at FRC?
P(dist) = 0 - -o.5
therefore there is a positive distending pressure of 0.5kPa at FRC so they are slightly inflated at FRC.
Explain how we carry out inspiration
- 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)
What is the conducting zone and what does it do?
Respiratory passages that carry air to the site of gas exchange.
Filters, humidifies and warms air.
What is the respiratory zone and what is it composed of?
Site of gas exchange Composed of: Respiratory bronchioles alveolar ducts alveolar sacs
What is minute ventilation?
Total air that flows in and out of respiratory system in a minute.
How can minute ventilation be calculated?
Tidal volume x respiration rate
What is alveolar ventilation?
a measure of the volume of fresh air that reaches the alveoli each minute
How can the alveolar ventilation be calculated?
Minute volume - the anatomical dead space (the combined volume of the non-exchanging airways)
