3. Ventilation : Perfusion Relationship Flashcards
What is ventilation linked with ?
Perfusion (local blood flow L/min)
What is distribution of blood flow in the lung influenced by ?
Hydrostatic pressure and alveolar pressure
What decreases with height across the lung ?
Both blood flow and ventilation
Why is blood flow at the base of the lungs higher than ventilation ?
Arterial pressure exceeds alveolar pressure and vascular resistance, this compresses alveoli
Why is blood flow at the apex of the lungs low ?
Arterial pressure is less than alveolar pressure, this compresses the arterioles and vascular resistance is increased
Both ventilation and perfusion are greater where ?
At the base of the lung than the apex, but blood flow exceeds ventilation at base and ventilation exceeds ventilation at apex
Where is there a ventilation-perfusion mismatch ?
At the base of the lung and the apex of the lung
Where are ventilation and perfusion precisely matched ?
At rib 3
Where is the biggest V/Q mismatch ?
ApexOver 75% of the height the healthy lung performs quite well in matching blood and air
What is the optimal condition for ventilation ?
Ventilation = perfusionRatio = 1
What is mismatch 1 (base) ?
Where ventilation < perfusionratio < 1 (Under oxygenated blood
What is mismatch 2 (apex) ?
Ventilation > perfusionratio >1( can happen with too much breathing taking place compared to the rate of blood flow)
What happens if ventilation decreases in a group of alveoli ?
PCO2 increases and PO2 decreases. Blood flowing past those alveoli does not get oxygenated
How does blood get oxygenated when ventilation is decreased ?
Oxygenated blood is ‘shunted’ from better ventilated areas to dilute blood
What does shunt mean ?
Passage of blood through areas of the lung that are poorly ventilated (Ventilation
What happens when perfusion > ventilation ?
alveolar PO2 falls—> pulmonary vasoconstriction Alveolar PCO2 rises—> bronchial dilation
What does decreased PO2 around under ventilated alveoli cause?
Constricts their arterioles, diverting their blood to better ventilated alveoliConstriction in response to hypoxia is is particular to pulmonary vessels (systemic vessels dilate)
What does increased PCO2 cause?
Mild bronchodilation
What is alveolar dead space ?
Alveoli that are ventilated but not perfused More air in alveoli than blood flow, so there is air not participating in gas exchange
What happens when ventilation > perfusion ?
Alveolar PO2 rises—> pulmonary vasodilationAlveolar PCO2 falls—> bronchial constriction
What is the opposite of shunt ?
Alveolar dead space
What is physiological dead space ?
Alveolar DS + anatomical DS
What is Respiratory sinus arrhythmia ?
Normal change in heart rate during the breathing cycle
