The Ventilation-Perfusion Relationship Flashcards
How do ventilation and perfusion change across the lungs?
Both decrease with height.
Perfusion decreases faster than ventilation.
Describe ventilation and perfusion at the base of the lung.
Ventilation < perfusion.
Arterial pressure > alveolar pressure.
This compresses the alveoli.
Describe ventilation and perfusion at the apex of the lung.
Perfusion is low.
Arterial pressure < alveolar pressure.
This compresses the arterioles.
How does the ratio between ventilation and perfusion change?
Increases from the base to the apex, owing to the effect of gravity.
Base - ratio > 1.
Apex - ratio < 1.
Most mismatch takes place at the apex, but is regulated to keep the ratio close to 1.
What is a shunt?
The passage of blood through areas of the lung that are poorly ventilated (ventilation < perfusion).
What is alveolar dead space?
Alveoli that are ventilated, but not perfused.
The opposite of a shunt.
What is anatomical dead space?
Air in the conducting zone of the respiratory tract that is unable to participate in gas exchange, as the walls of the airways are too thick.
What is physiologic dead space?
Alveolar dead space + anatomical dead space.
When does alveolar dead space occur?
At the apex of the lung (to a small extent).
In a pulmonary embolus.
Describe ventilation and perfusion in a shunt.
Ventilation > perfusion.
Alveolar PO2 falls - pulmonary vasoconstriction.
Alveolar PCO2 rises - bronchial dilation.
Describe ventilation and perfusion in the opposite of a shunt.
Ventilation > perfusion.
Alveolar PO2 rises - pulmonary vasodilation.
Alveolar PCO2 falls - bronchial constriction.
What should occur in optimal conditions, in terms of ventilation and perfusion?
Ventilation = perfusion.
When does autoregulation occur?
Ventilation < perfusion.
The shunt dilutes oxygenated blood from better ventilated areas.
What happens in respiratory sinus arrhythmia?
Synchronising HR with respiration.
Occurs due to an increased vagal activity during the expiratory phase.
Ensures ventilation : perfusion remains as close to 1 as possible.
What would happen if HR stayed constant and was not synchronised with respiration?
Inspiration - alveolar dead space would increase.
Expiration - shunt would increase.
