Ventilation Perfusion Relationships in the Lungs

Question 1

Define Pulmonary Ventilation

Answer 1

The movement of air in and out of the lungs

Question 2

Define Pulmonary Gas Exchange

Answer 2

The Diffusion of oxygen from the lungs into the blood and the diffusion of CO₂ from the blood to the lungs

Question 3

Define Pulmonary Perfusion

Answer 3

The flow of blood through the pulmonary capillaries surrounding the alveoli (expressed as Q ml/min)

Question 4

What is alveolar ventilation?

Answer 4

The amount of air reaching the alveoli in a given amount of time

(Functional TV - anatomical dead space) x Resp Rate

Typically expressed as V

Question 5

What is the normal V/Q ratio of the lungs?

Answer 5

1

Question 6

In order to maintain a match between V/Q, how must perfusion be controlled?

Answer 6

Alveoli with increased ventilation get increased perfusion

Alveoli with decreased ventilation get decreased perfusion

Question 7

How is blood diverted away/ to perfused alveoli?

Answer 7

• Pulmonary Capillary Low P_aO₂ causes vasoconstriction of pulmonary arterioles → blood diverted to better ventilated alveoli
• Alveolar P_AO₂ is low causes bronchoconstruction

Question 8

What is the consequence of inadequate ventilation for perfusion of an alveolar unit

Answer 8

V/Q ratio falls <1

• P_ACO₂ rises
• P_AO₂ falls

Question 9

What are some of the causes of a V/Q <1?

Answer 9

• Asthma - airway narrowing
• COPD (early stages)
• Pneumonia- acute inflammatory exudate in affected alveoli
• RDS in newborn
• Pulmonary Oedema

Question 10

What happens to the PO₂ of mixed blood from a normal alveoli mixing with an alveoli of V/Q <1

Answer 10

There is only a minor/ no fall due to most O₂ in blood being bound to Hb which saturates at partial pressure ~8kPa

Therefore in a drop in pO₂ the binding is unaffected

Question 11

What happens to the PCO2 of mixed blood from a normal alveoli mixing with an alveoli of V/Q <1

Answer 11

PCO₂ is still high but has proportionally decreased, it is an equal mix between the 2 PaCO₂ from affected and unaffected

Question 12

In hypoxia from V/Q <1, hyperventilation occurs. Why does the hyperventilation not correct the hypoxia?

Answer 12

Hyperventilation will increase the ventilation of both affected and unaffected alveoli so there is still a mismatch

affected lung = V/Q still <1

unaffected lung = V/Q >1