Ventilation, lung mechanics and gas exchange Flashcards
What is the expiratory reserve volume and residual volume?
Expiratory reserve volume is difference between the lung volume after tidal expiration and after forced expiration. Residual volume is the volume left in the lungs after forced expiration.
What is the functional residual capacity?
the volume of air left in the lungs after pssive (tidal) expiration
What is the differnence between the inspiratory reserve volume and the inspiratory capacity?
Inspiratory reserve volume is the inspiratory capacity minus the tidal volume
What is the functional residual capacity determined by?
the balance between outward forces of chest wall wanting to expand and inward force of the lungs wanting to recoil (less recoli= less FRC)
What creates the negative pressure in the pleural space?
- Surface tension of the pleural fluid ‘sticks’ the parietal and visceral layers together
- the parietal layer is pulled outwards by chest wall
- the visceral layer is pulled inwards by the elastic recoil of the lungs
- therefor the two membranes are being pulled in opposite directions creating a vacuum- negative pressure
How does the pressure in the pleural space change on inspiration and expiration and why?
- Pressure becomes more negative inspiration and less negative on expiration
- It becomes more negative on inspiration because the diaphragm moves down, so the volume of the pleural space increases so the pressure decreases
State 3 accessory muscles to inspiration
- SCM
- Scalene
- pec major and minor
- trapezius
State 3 accesory muscles to expiration?
abdominal muscles
internal intercostals
What is lung compliance?
The relationship between pressure and volume- basically how easily does the volume change when the pressure changes
How is lung compliance calculated?
change in vol/ change in pressure
Elastin degenerates with age- what effect does this have on lung compliance and functional residual volume?
Compliance increases as less inward force resisting volume increases.
This means FRV increases because less elastic recoil= less lung emptying
What effects will empysem and fibrosis have on lung compliance?
Empyesema= less elastin= more slack lungs= more compliant
Fibrosis= stiff lungs= less compliant
What 3 factors affect lung compliance?
- elastic fibres
- fibrosis
- surface tensio (higher= less compliant)
What week gestation is surfant produced?
between 15-28 weeks
How does surfant make breathing easier?
it breaks up water molecules to reduce surface tension, this allows alveoli to inflate more easily
How does sufactant stop large alveoli engulfing small alveoli (la places law)
- Large ones normally engulf small ones because large have less surface area and so lower pressure than two equally sized bubbles
- But surfacant is spread more thinly over large alveoli so their surface tension is relativley higher than the smaller alvoeli, this means they have the same pressure so the structre of the lungs remains stable
How is minute pulmonary ventilation calculated?
tidal volume (normall 0.5L) x respiration rare (normal 15)
15x 0.5= 7.5 L/min
What are the 3 types of deadspace in the lungs?
- anatomical dead space (between lungs and bronchioles, where gas exchange of the air doesnt take place- 150ml)
- Alveolar deadspace (alveoli are ventilated but poorly/ not perfused so no gas exchange)
- Phyiological deadspace (combination of both)
Where does most airway resistance come from and why?
The upper resp tract because its cross sectional area is smaller than in the lower resp tract- therfor more air is in contact with the surface in the upper tract so there is more resistance
What 3 ways can airway resistance increase?
- increased mucus (narrows lumen)
- hypertrophy of smooth muscle and/ or odema (narrows lumen)
- loss of radial traction (break down of lung parenchyma, which normally helps hold the airways open on expiration- emphysema)
What happens to the intra and extrathoracic pressures on inspiration and what implications does this have?
- Extra thoracic pressure more positive causing lumen to narrow
- intrathoracic pressure more negative as air fills them and airspaces expand
What 3 factors determine rate of gas exchange?
- Surface area (not usually rate limiting as so massive)
- resistance to diffusion
- graident of partial pressures
What does O2 have to pass through to get from alveolar space to Hb?
- alveolar epithelial cell
- intersitial fluid
- capillaru endothelium
- plasma
- RBC plasma membrane
Why does fibrosis tend to affect oxygen diffusion much more than carbon dioxide diffusion?
carbon dioxide is much more soluable than oxygen, so diffuses much more easily
Give three diseases causing diffusion defects
Pulmonary odema- the O2 has to get through fluid as well
Interstitial lung disease- fibrosis and so thickening of alveolar walls
Emphysema- reduces SA for diffusion because alveolar walls are destructed resulting in large airspaces
What can cause intersitial lung disease?
- inhaled substances- dust, asbestos ect
- drugs- some antibiotics
- many auto immune diseases (RA, lupus)
- infections- TB, pneumonia
- malignancies
How can diffusion resistance be measured?
- Get them to take one large breath of 0.1% CO.
- Then measure amount of CO in blod- if high then good diffusion, if low then you know something is wrong
How can blood oxygen be measured?
- Oxygen saturation using pulse oxymeter which uses light absorbtion to determin what % of the Hb is saturated with oxygen
- arterial blood gas analysis can tell you the pO2, pCO2 ect
What is boyles law?
For a fixed amount of an ideal gas kept at a fixed temperature, pressure and volume are inversely proportional.
More volume= lower pressure when quanity of gas is the same
What is partial pressure?
The pressure that would be exerted by one of the gasses in a mixture if it occupied the same volume on its own
If a space has a pressure of 50kPa, and the gas in the space was made up of 40% oxygen and 60% nitrogen, what would be the partial pressure of oxygen in that space?
50 x 0.4= 20kPa
Water will evapourate into a gas until it is saturated with water. At 37 degrees, what is the partial pressure of H20 when a gas is fully saturated with H20?
6.28kPa
Why does the partial pressure of O2 and N2 decrease on entry to the upper respiratory tract?
They decrease, because the air become saturated with water, but the air is still in communication with the atmosphere so the pressure of the air doesnt increase
What does the amount of a gas dissolved in a solution depend on?
- partial pressure (higher= more being dissolved to reach an equilibrium)
- Soluability coefficient
Why is Hb needed? why can O2 not just be carried to tissues as dissolved in plasma?
Because the partial pressure oxygen in alveoli is only around 13kPa, which would dissolve in and make 13kPa in blood/ a concentration of 1.3mmol/L. This is not enough to meet demands of tissues.
If it binds to Hb, more can be carried and then dissolve into the solution. This allows an extra 8/9 mmol/L of O2 to be carried in blood.
Why is kPa of O2 in alveoli 13.9kPa but almost 20kPa in the amosphere?
- its decreased by water vapour saturating the air
- its also decreased by blood flow removing it
therefor pO2 of alveoli depends on not only rate of veniilation but blood flow (high flow= low pO2, low flow= higher pO2)
How does alveolar pO2 change at altitude?
- atmospheric pO2 is lower (6.5 kPa at everest)
- so alveolar pO2 also much lower, too low for life
What causes nitrogen narcosis when deep sea diving?
Pressure is much greater due to weight of the water, this means pN2 is also greater, meaning pN2 in blood is much greater, and can causes drunken like symptoms (nitrogen narcosis)
Why should divers ascend slowly from dives?
- fast ascent doesnt allow time for the high levels of nitrogen which as been built up to be blown off, fast ascent can cause this nitrogen to form bubbles in blood vessels, which can be painful and embolise
- also on ascent the pressure decreases, meaning volume in cavities (sinuses, ear, lungs) will increase which can rupture alveoli and also cause air bubbles to from in blood vessels
What investigation is used to test lung function?
Spirometery
What is FEV1?
the amount of air expired in the first 1 second of inspiration
What spirometery findings are found in obstructive diseases?
FVC is nearly normal, but FEV1 is low so FEV1/ FVC ratio is <70% of normal.
Also flow volume loop is scooped
What defects to flow volume loops will be found in restrictive defects?
Low FVC and low FEV1
slight scooping on expiration
Give 3 examples of diseases causing restrictive patterns on spirometry?
- scoliosis
- marked obesity
- interstitial lung disease/ fibrosis
Give 3 examples of obstructive lung diseases?
COPD
Asthma
Bronchiectasis
How can you use spirometery to differentiate between asthma and COPD?
COPD shows little- no improvement when given bronchidilatiors
Asthma will get better when given bronchidilators
How can you measure residual volume?
helium dilution test
How can you measure dead space?
Nitrogen washout- give 100% O2 to fill conducting airspaces with O2. Then measure N2 and CO2 on breath out. Volume at the start that was purely O2 is the volume of dead space
What % of chest expansion is diaphragm movement responsible for?
70%
How is amount of O2 (mmol/l) calculated from the pKa and the soluability coeffieicient
pKa x soluability coefficient
What cells produce surfactant?
type 2 pneumocytes
