L2 - Respiration - Ventilation and Perfusion

Question 1

What is ventilation?

Answer 1

Ventilation is the volume of air moved out of the lungs per unit time

Question 2

How do you calculate ventilation?

Answer 2

Tidal volume - 0.5 litres
Breathing frequency - 12 breaths per minute
Total ventilation = 0.5 x 12 = 6 litres/min

Question 3

Why does not all the fresh air reach the alveoli in the respiratory zone?

Answer 3

First portion is stale air from conducting zone (anatomical dead space) - 0.15 litres

Question 4

How do you calculate alveolar ventilation?

Answer 4

Alveolar ventilation = total ventilation – dead space ventilation
= 6 litres/min - (0.15 x12) litres/min
= 4.2 litres/min

Question 5

How does ventilation rate have an impact on alveolar and arterial gas composition?

Answer 5

As alveolar ventilation (l/min) increases alveolar partial pressure of

• Oxygen increases
• Carbon dioxide decreases

Question 6

How does lung ventilation differ in the different areas of the lung?

Answer 6

Ventilation is greater at the base compared to the apex – linked to starting volume of the alveoli

• At apex - larger starting volume - low compliance
• At base - smaller starting volume - high compliance (small change in pressure gives big change in volume)

Question 7

When is pulmonary resistance at its lowest?

Answer 7

When lungs are at functional residual capacity

Total resistance is sum of alveolar and extra-alveolar resistances

Question 8

Pulmonary resistance in alveolar vessels?

Answer 8

Capillaries and slightly larger vessels surrounded by alveoli
Resistance linked to transmural pressure and lung volume

Question 9

Pulmonary resistance in extra-alveolar vessels?

Answer 9

Vessels not surrounded by alveoli

Sensitive to changes in intra-pleural pressure

Question 10

What is the importance of capillary recruitment?

Answer 10

Pulmonary circulation can accommodate increases in pulmonary pressure and flow by recruitment
- This reduces the system resistance

Question 11

What are the 3 different states capillary vessels can be found in during capillary recruitment?

Answer 11

Collapsed
Open but do not conduct blood
Open and conduct blood

Question 12

Capillary recruitment method

Answer 12

In first stage of recruitment
- Previously collapsed vessels now become patent but still do not conduct blood
- Previously open but non-conducting vessels now conduct blood
- Distension widens vessels that previously had been open and conducting
Later during recruitment the previously collapsed vessels now conduct blood
All vessels now dilate causing resistance to fall

Question 13

What impacts lung perfusion?

Answer 13

Perfusion varies according to posture and gravity

Perfusion is greater at the base compared to the apex

Question 14

What is the reference point for pulmonary circulation pressures?

Answer 14

Outside the heart at the left atrium level

Question 15

What is Ppa?

Answer 15

Pressure in pulmonary arterioles

Mean of about 15mmHg (20 cm H2O)

Question 16

What is Ppv?

Answer 16

Pressure in pulmonary venules

Mean of about 8mmHg (10 cmH2O)

Question 17

How do Ppa and Ppv change when moved above or below the left atrium?

Answer 17

Fall by about 1 cm H2O for every 1cm above the left atrium

Increase by 1 cm H2O for every 1cm below the left atrium

Question 18

What is Pa?

Answer 18

Alveolar pressure
Is relative to atmospheric pressure
Model considers the point when there is no movement of air
- At this time, no difference between atmospheric pressure and PA so PA = 0 cmH2O.

Question 19

What is Ptm?

Answer 19

Transmural pressure gradient across the wall of the vessel

Question 20

How many zones does the lung have?

Answer 20

4

Question 21

What factors alter perfusion?

Answer 21

Dilators 
- Increase in alveolar oxygen 
- Decrease in alveolar carbon dioxide 
- Increased pH
Constrictors 
- Decrease in alveolar oxygen 
- Increase in alveolar carbon dioxide 
- Decreased pH

Question 22

Ratio of the rate of alveola ventilation and pulmonary blood flow influences?

Answer 22

Gas composition

Question 23

If alveoli are not ventilated the ventilation/perfusion ration becomes?

Answer 23

0
Gas composition will become the same as that of mixed venous blood
40mmHg O2 and 46 mmHg CO2

Question 24

If alveoli are not perfused the ventilation/perfusion ration becomes?

Answer 24

Infinity
Gas composition will become the same as that of inspired humified air
149mmHg O2 and 0mmHg CO2

Question 25

How does the ventilation/perfusion ratio vary down the lung?

Answer 25

Apex – 3.3
Base – 0.6
Overall – 0.84

Question 26

Ventilation/perfusion mismatch - Alveolar dead space ventilation

Answer 26

Local reduction of perfusion e.g. pulmonary embolism
No exchange means gas composition in affected alveoli becomes like moist inspired air
Ventilation/perfusion ratio becomes ∞
Compensation
- Blood redirected to other areas
- Bronchiolar constriction
- Reduction in surfactant production

Question 27

Ventilation/perfusion mismatch - shunt

Answer 27

Local reduction of ventilation e.g. tumor or foreign body
Gas can’t be removed so gas composition becomes like mixed venous blood
Compensation
- Air redirected to other areas of the lung
- Hypoxic pulmonary vasoconstriction