Lung Ventilation (M2) Flashcards
1
Q
LO’s
A
- Describe how ventilation/perfusion (V/Q) ratio is applied to the whole lung and to individual alveoli, and why the V/Q ratio is clinically important
- Describe the mechanisms tending to reduce V/Q differences between individual alveoli
- Describe regional differences in ventilation and perfusion in the healthy lung and the effect this has on V/Q ratio
- Describe how regional differences in gas exchange can be predicted from the PO2/PCO2 diagram
- Describe the effect of V/Q mismatch on overall gas exchange
2
Q
What is the ventilation-perfusion ratio?
A
- V/Q ratio is a way of assessing gas exchange efficiency as a mismatch between ventilation and perfusion can be an indicator of pulmonary disease.
- Alveolar Ventilation/ Cardiac Output.
- V= air going into all alveoli.
- Q= blood flow in the capillaries around the alveoli.
- Resting alveolar ventilation = 4 L/min
- Resting cardiac output for whole lung = 5 L/min
- Therefore V/Q ratio of lung = 4/5 = 0.8
- Therefore V/Q ratio of lung = 4/5 = 0.8
3
Q
How much oxygen is in 100ml of blood?
Be aware:
- Heamoglobin has an oxygen-binding capacity of 1.34 mL O2 per gram
Assume:
- There is about 15g of haemoglobin in 100ml of blood
A
1.34ml of O2 in 1g HB
so times by 15
20.1 ml of 02
4
Q
What happens when ventilation is obstructed in an alveolus, and when does it happen?
A
- There is no oxygen getting to the alveoli, so pAO2 will fall.
- CO2 is not being removed out, as there is an obstruction, so pACO2 will rise.
- These alveoli are said to be overperfused (as they have more blood flow than they need).
- The blood leaving from ventiltion obstructions are said to be undersaturated.
- Airway limitation (asthma & COPD), lung collapse, loss of elastic tissue (emphysema)
5
Q
What happens when blood flow is obstructed, and when does it happen?
A
- Ventilation unchanged, perfusion obstructed.
- Some ventilation is wasted and contributes to dead space.
- pAO2 will rise, pACO2 will fall.
- No more O2 will be taken up by these overventilated alveoli as haemoglobin fully saturated and extra CO2 will be blown off.
- Pulmonary embolism, necrosis or fibrosis of capillary bed.
6
Q
What happens to the V/Q ratio in emphysema?
A
- Have destruction of alveoli: the inner walls of the air sacs weaken and rupture — creating larger air spaces instead of many small ones. This destroys blood capillaries too. This means diffusion is less efficient.
- Destruction of alveoli leads to underventilation (less oxygen reaching the alveoli, as there are not as many left) = low V/Q ratio
- But, strangely, loss of capillaries leads to alveoli which are underperfused = high V/Q ratio
- Decreases the effectiveness of gas exchange to 1/10 of normal
7
Q
What mechanisms come into play when there is low alveolar pO2?
A
- Low alveolar pO2 means that there is not enough O2 from the airways.
- This makes the pulmonary arterioles and capillaries constrict, so the perfusion is less, matching the less o2 available.
8
Q
What mechanisms come into play when there is low alveolar pCO2?
A
- If there is low alveolar CO2, this means that the perfusion is low.
- To match the low perfusion, the alveolar ducts constrict.
- This matches low perfusion to now, low ventilation.
9
Q
What is the last out/ first in principle?
A
- A mechanisms to overcome mismatch of gas concentrations in different alveoli.
- Air will leave out the side with the high V/Q, as doesn’t have obstruction.
- When expire, the air from the low V/Q alveolus will go to the anatomical dead space, just above the alveoli.
- This means, that when you breathe in next, the air in the anatomical deadspace will go into the unobstructed alveolus.
- This helps balance the pO2 and pCO2 in the alveoli.
10
Q
What are the regional differences in ventilation and perfusion?
A
- Ventilation: greatest at the apex.
- Perfusion: greatest at the base, as gravity pull blood down to the base.
- Overall, have a high V/Q at the apex, and low at the base.
- Blood from base: Low pO2 and High pCO2 than “ideal”
- Blood from apex: High pO2 and Low pCO2 than “ideal”
11
Q
What does successful gas exchange depend on?
A
- Efficient diffusion.
- Good ventilation (getting the air from the atmosphere to the lungs.
- Perfusion (getting the blood moving the dissolved gases to and from the lung.
12
Q
What are the percentages of each gas in inspired, alveolar and expired air?
A
-
Inspired air
- O2: 20%
- CO2: 0%
- N2: 80%
- H2O: 0%
-
Alveolar air
- O2: 14%
- CO2: 5%
- N2: 76%
- H20: 6%
-
Expired air
- pO2: 16%
- pCO2: 3%
- N2: 76%
- H20: 6%
13
Q
What are the partial pressures in the pulmonar artery (before diffusion has happened) compared to the pulmonary vein (after diffusion has happened)?
A
- PA:
- pO2: 5.3
- PCO2: 6.1
- PV:
- pO2: 13.7 (more!)
- PCO2: 5.2 (less!)
14
Q
How long does a capillary have for diffusion to happen around an alveolus?
A
0.75s
15
Q
What can cause hypoxemia (low O2 in arterial blood)?
A
- Hypoventilation
- Shunt: Blood supplying the lung tissues via the bronchial arteries is being returned via the pulmonary veins without undergoing gas exchange. This causes a mixing of oxygenated and deoxygenated blood.
