Lung Ventilation (M2)

Question 1

LO’s

Answer 1

• 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

Question 2

What is the ventilation-perfusion ratio?

Answer 2

• 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

Question 3

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

Answer 3

1.34ml of O2 in 1g HB

so times by 15

20.1 ml of 02

Question 4

What happens when ventilation is obstructed in an alveolus, and when does it happen?

Answer 4

• 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)

Question 5

What happens when blood flow is obstructed, and when does it happen?

Answer 5

• 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.

Question 6

What happens to the V/Q ratio in emphysema?

Answer 6

• 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

Question 7

What mechanisms come into play when there is low alveolar pO2?

Answer 7

• 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.

Question 8

What mechanisms come into play when there is low alveolar pCO2?

Answer 8

• 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.

Question 9

What is the last out/ first in principle?

Answer 9

1. A mechanisms to overcome mismatch of gas concentrations in different alveoli.
2. Air will leave out the side with the high V/Q, as doesn’t have obstruction.
3. When expire, the air from the low V/Q alveolus will go to the anatomical dead space, just above the alveoli.
4. This means, that when you breathe in next, the air in the anatomical deadspace will go into the unobstructed alveolus.
5. This helps balance the pO2 and pCO2 in the alveoli.

Question 10

What are the regional differences in ventilation and perfusion?

Answer 10

• 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”

Question 11

What does successful gas exchange depend on?

Answer 11

1. Efficient diffusion.
2. Good ventilation (getting the air from the atmosphere to the lungs.
3. Perfusion (getting the blood moving the dissolved gases to and from the lung.

Question 12

What are the percentages of each gas in inspired, alveolar and expired air?

Answer 12

• 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%

Question 13

What are the partial pressures in the pulmonar artery (before diffusion has happened) compared to the pulmonary vein (after diffusion has happened)?

Answer 13

• PA:
  
  • pO2: 5.3
  • PCO2: 6.1
• PV:
  
  • pO2: 13.7 (more!)
  • PCO2: 5.2 (less!)

Question 14

How long does a capillary have for diffusion to happen around an alveolus?

Answer 14

0.75s

Question 15

What can cause hypoxemia (low O2 in arterial blood)?

Answer 15

• 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.

Question 16

What is hypoventilation?

Answer 16

• Reduced alveolar ventilation
• This means that there is decreased pA O2 (A= Alveolar), causing decreased pa CO2 (arterial partial pressure)= HYPOXIA
• This means that there is increased pA CO2, causing increased pa CO2 = Hypercapnia

Question 17

What is the difference between hypoxia and hypoxaemia?

Answer 17

• Hypoxia: low oxygen to tissues
• Hypoxaemia: low oxygen in arterial blood
• These conditions are treated differently.

Question 18

What can cause hypoventilation (more difficult to ventilate)?

Answer 18

1) Increased airways resistance e.g. Asthma and COPD
2) Drugs e.g. morphine and barbituates
3) Paralysis of respiratory muscles

Question 19

How is alveolar ventilation calculated?

Answer 19

• Volume of air breathed in per minute.
• (Tidal volume- dead space) x Respiratory rate.
• Have to take away dead space.

Question 20

What is dead space?

Answer 20

• Dead space is the volume of air which is inhaled that does not take part in the gas exchange
• Either because it remains in the conducting airways, or reaches alveoli that are not perfused or poorly perfused.

Question 21

What is a normal/ideal V/Q ratio in just one alveolus?

Answer 21

• One: ventilation and perfusion are equally matched.