Ventilation Flashcards
How does intrapleural pressure and alveolar pressure change in inspiration?
Reduced intrapleural pressure (-11cmH2O)
Reduce alveolar pressure by (5cmH2O)
What does ventilation allow?
Inspired air reaches alveoli and blood gas barrier, expired gases are removed from the alveoli
What is total ventilation?
Total rate of flow in and out of the lung during normal tidal breathing
Units for total ventilation
L/min
What do changes in rate and depth of ventilation cause?
Composition of alveolar gas, and therefore composition of gases entering/exiting blood
What is alveolar ventilation?
Volume of air that reaches the alveoli and is avaliable for gas exchange with blood (vol/min)
What is the resting O2 consumption and CO2 production?
Resting O2 consumption = 250ml/min
Resting CO2 production= 200ml/min
What zones do airways compromise and what is their role?
1) conducting airways: delivery of gas to alveoli
2) exchange zones: exchange to and from pulmonary circulation
What are the volumes of the conducting and exchange zones?
Conducting = 150ml Exchange = 3000ml
What is the anatomical dead space?
Volume of conducting airways
What is the physiological dead space?
Anatomical dead space plus the alveolar dead space (volume that doesn’t take part in gas exchange)
How many times do airways bifurcate to reach the alveolar ducts?
23 times
After how many divisions do we reach respiratory bronchioles?
17th is respiratory bronchioles
What are normal alveolar partial pressures of O2 and CO2?
PAO2 = 13.3.KPa (100mmHg)
PACO2= 5.3 KPa (40mmHg)
What happens as total cross sectional area increases going down the airways?
Velocity (of gas flow) decreases
Describe how cross sectional area changes across conducting zone and respiratory zone
Increases in conducting zone and then rapidly increases in respiratory zone
Describe how the mechanisms of convection and diffusion change going further into the lungs
In conducting zone, convection dominant
In respiratory zone convection slows (velocity decreases due to larger cross-sectional area) so diffusion becomes dominant
Describe the oxygen percentage: airway generation graph
From generation 1-16 oxygen percentage 20% (dead space)
Then from 16-17 large decrease in percentage to 13% and stays there for subsequent generations (exchange is occuring)
In exhalation the O2% remains at 13%
How are lung volumes measured?
Spirometer (subject breathes into a sealed container)
The changes in the spirometer are equal and opposite to the the changes in the lungs of the subject
Lung volumes vary with…
Sex, size and gender
What is functional residual capacity (FRC)?
Volume of gas in lungs after normal expiration
What is normal FRC value?
2.5-3 L
What is the tidal volume?
0.5 litre
What is the vital capacity
Amount of gas that can be inhaled by a maximal inspiratory effort following maximal expiration
Normal value for vital capacity
5L
What is intrapleural pressure, what is it normally?
Pressure of fluid within the pleural cavity. Normally negative (less than atmospheric)
What is the pleural cavity?
Fluid filled space between each pleura (visceral and pareital)
What generates the normally negative intrapleural pressure after expiration?
1) Ribcage has natural tendency to spring upwards
2) Lungs have intrinsic tendency to collapse
What happens to intrapleural pressure when pleura damaged?
Air introduced into pleural space, intrapleural pressure may exceed/equal atmospheric pressure leading to pneumothorax
What happens to lungs and chest wall in a pneumothorax?
Lungs collapse inwards, chest wall outwards
What is an assumption when we consider normal intrapleural pressure value?
Static mechanics (after normal expiration and no gas flow in/out) so pressure outside lung and in alveoli is the same
How does inspiration change intrapleural pressure and alveolar pressure (mechanism)?
Lift ribcage and increase intra-thoracic volume (stretch lungs)
Reduces intrapleural pressure (holds lungs in more stretched position)
Alveolar pressure is reduced and inspiraiton initiated
Describe how alveolar pressure compares to atmospheric pressure when no air flowing in
Equal
Describe the mechanism behind expiration in terms of pressures
Muscles of chest wall and diaphragm relax
Decrease intra-thoracic volume
Increased intrapleural pressure and alveolar pressure (above atmospheric)
Hence expiration initiated
Describe the mechanism for forced expiration in terms of pressures
Contraction of chest wall muscles results in even higher increase in intrapleural pressure, increasing expiratory rate further
Which part of the lung receives more ventilation per unit?
Lower part of the lung
Why does lower part of lung experience more ventilation?
Gravity (due to the weight of the lung) means increased pleural pressure at the base (making it less negative).
Reduce the alveolar volume so are smaller percentage of max volume (pre-inspiration).
This gives each alveoli as greater ventilation potential so more compliant and so capable of more oxygen exchange whereas the apex ventilates less efficiently since its compliance is lower and so smaller volumes are exchanged.
What happens to regional ventilation differences when person is supine?
Apical and basal ventilation become same
Posterior aspects of lung are best ventilated
How is FRC measured?
Can’t use spirometer
Use helium dilution
How does helium dilution work?
Helium gas used as indicator (known volume and concentration)
Use concentration of helium in expired air to work out difference
How can regional ventilation be detected?
Analyse distribution of inhaled radioactive gas (133Xe)
Single breath of 133Xe taken and breath held while counts taken at different levels of lung
133Xe breathed to and from until evenly mixed in lungs to estimate volume at each level
What is meant by compliance?
Measure of the pressure required to inflate lungs by a certain incremental volume
What are the units for compliance?
ml/cmH2O
Equation for compliance
delta V/delta P
Which forces work to deflate the lungs (acting against pressure to inflate lungs)?
Inherent elasticity of lungs
Forces which arise due to surface tension
What happens to compliance in obstructive diseases, why?
Increases
Poor elastic recoil