Ventilation and perfusion

Question 1

Other than gas exchange, what roles does the respiratory system have?

Answer 1

1) Reservoir for blood and oxygen
2) Metabolism of some circulating compounds - bradykinin, histamine, RAAS system
3) Filter blood - break down clots
4) Immune defence

Question 2

What does the upper respiratory tract consist of?

Answer 2

Mouth, nose, sinuses and pharynx

Question 3

What are the functions of the upper respiratory tract?

Answer 3

Conducts air to the larynx, the entrance to the lower respiratory tract.

1) Warms and humidifies air
2) Protects lower airways

Question 4

What happens in conducting airways?

Answer 4

NO gas exchange (anatomical dead space) - bulk flow

Question 5

What happens in respiratory airways?

Answer 5

Gas exchange - diffusion

Question 6

Describe the stages in ventilation?

Answer 6

1) Expansion of cage –> negative pressure within the pleural cavity.
2) This causes lungs to expand.
3) Increase in volume reduces the pressure in respiratory airways.
4) Gas flows down pressure gradient into lungs.
5) Relaxation of muscles leads to reversal of process.

Question 7

What is the difference between lung volumes and lung capacities?

Answer 7

Lung volumes change with breathing pattern, whereas lung capacities do not

Question 8

Define Total Lung Capacity; what is it a sum of?

Answer 8

Total volume of air present in lungs at end of maximal inspiration.
=IRV+TV+ERV+RV

Question 9

Define inspiratory capacity; what is it a sum of?

Answer 9

The biggest breath that can be taken from resting expiratory level (lung volume at the end of quiet expiration)
=IRV+TV

Question 10

Define vital capacity; what is it a sum of?

Answer 10

Measured from maximum inspiration to maximum expiration (biggest breath that can be taken) - Key in monitoring neuromuscular disease as gives easily measurable indicator of degree of disease and warning about pending respiratory failure.
=IRV+TV+ERV

Question 11

Define functional residual capacity; what is it a sum of?

Answer 11

The volume of air remaining in the lungs at the resting expiratory level - key in anaesthesia and intensive care.
=ERV+RV

Question 12

What is (total) minute ventilation?

Answer 12

The amount of air moved into and out of the lungs per minute.
MV = TV x RR
Typical minute ventilation is 6-8l/min

Question 13

What is serial dead space?

Answer 13

The volume of the conducting airways (anatomical dead space)

Question 14

What is distributive dead space?

Answer 14

The parts of the lung that are not airways but cannot support gas exchange eg. damaged alveoli or alveoli with poor perfusion (physiological dead space)

Question 15

How do you calculate the Alveolar Ventilation Rate (AVR)?

Answer 15

AVR = (Tidal volume - dead space volume) x RR

Question 16

How will a change in AVR affect the diffusion rate?

Answer 16

Diffusion rate α (A. D .(∆P))/T

Increasing AVR will remove CO2 from the alveoli, thus lowering alveolar pCO2 and increasing ∆P, thus increasing CO2 diffusion from the blood (and removal from the body)

Question 17

What happens if there is a physiological mismatch (V/Q mismatch) in healthy people?

Answer 17

Nothing, this is insignificant in normal conditions. V and Q vary in different areas of lung.

Question 18

What happens to the V/Q ratio if there’s no ventilation?

Answer 18

V/Q –> 0

ie. shunt

Question 19

What happens to the V/Q ratio if there’s no perfusion?

Answer 19

V/Q –> infinity

ie. deadspace

Question 20

How does perfusion vary across the lung?

Answer 20

In an upright patient, blood flow is greater at the bases due to the effect of gravity causing higher blood flow to more dependant areas of the lung.
Zone 1 - Perfusion is absent.
Zone 2 - Perfusion is sporadic.
Zone 3 - Perfusion is constant.

Question 21

How does ventilation vary across the lung?

Answer 21

Basal lung relatively compressed in its resting state – more potential for expansion than lung at the apex

Therefore more air moved in and out i.e. increased ventilation at bases

(Due to gravity)

Question 22

How does the V/Q ratio vary across the lung?

Answer 22

As a result the V/Q ratio decreases moving down the lung

The base is relatively over-perfused (low V/Q ratio) and thus will have lower oxygen and higher carbon dioxide gas tensions in both alveoli and arterial blood

Apical alveolar PO2 is relatively high due to the lower blood flow (high V/Q ratio)

Question 23

How can the V/Q ratio at the apex of the lung change with exercise?

Answer 23

Apical alveolar PO2 is relatively high due to the lower blood flow (Q), as oxygen uptake is perfusion dependent

Distensible apical blood vessels that are usually collapsed open during exercise to accommodate the increased cardiac output and this augmented perfusion increases the oxygen uptake at the top of the lung

Question 24

Give a clinical example of a reduction in ventilation

Answer 24

Reduction in ventilation (decreasing V/Q ratio or shunt) of alveoli occurs in pneumonia; blood perfusing these areas is less oxygenated

Question 25

Give a clinical example of a reduction in perfusion

Answer 25

Reduction in perfusion occurs in pulmonary embolus resulting in increase V/Q ratio or deadspace