Respiratory Physiology Lecture 5 Part 23: More on the Ventilation-perfusion Relationship Flashcards
What is the V/Q ratio of the lung?
There are regional differences in both lung perfusion and lung ventilation
What determines local local alveolar PO2 and PCO2?
The local ventilation-perfusion ratio (VA/Q) determines the local alveolar PO2 and PCO2
What causes regional differences in lung perfusion?
- The weight of the lungs increases pressure in regions near bottom (Makes PIP less negative) therefore less pressure pulling it open than regions at top of lung
- Since alveoli at bottom are starting more deflated, they are able to expand more. Thus, bottom regions of lung receive a larger portion of the inspired air.
Where is the lowest ventilation and greatest ventilation in the lung?
How can regional differences in lung perfusion be measured?
Inject patient with 133Xe into vein, which then travels to right heart and pulmonary circulation
- If the patient holds breath as soon as Xe is injected we can measure radioactivity in different regions of the lungs. Perfusion is higher at base of the lung per unit volume and falls toward the apex. Associated with gravity and posture so if you change postures it will effect ventilation and perfusion
Where is the basal and apex of the lung?
- basal → bottom/ base of lung
- Apex → top of lung
What does perfusion depend on?
- gravity
- posture
Ideal VA/Q of apical and basal of lung
- Basal VA/Q = 0.6x ideal VA/Q
- Apical VA/Q = 3x ideal VA/Q
Ventilation-perfusion relationship diagram
Both decrease as we move from bottom to top of lung but slope is different which means that if we calculate the ventilator to perfusion ratio there will be lower volume at bottom of lung and higher at top
What helps to keep ventilation and perfusion matched?
Homeostatic mechanisms exist to limit the mismatch
What homeostatic mechanisms exist to limit the mismatch of ventilation-perfusion?
Most important is the unique response of pulmonary capillaries to low O2: pulmonary hypoxic vasoconstriction
Hypoxic pulmonary vasoconstriction
HPV
is a homeostatic mechanism that is intrinsic to the pulmonary vasculature. Intrapulmonary arteries constrict in response to alveolar hypoxia, diverting blood to better-oxygenated lung segments, thereby optimizing ventilation/perfusion matching and systemic oxygen delivery.
Hypoxia
Hypoxia is a state in which oxygen is not available in sufficient amounts at the tissue level to maintain adequate homeostasis; this can result from inadequate oxygen delivery to the tissues either due to low blood supply or low oxygen content in the blood (hypoxemia)
What happens with decreased airflow to regions of the lung?
⇣ PO2
⇡ PCO2
What happens with decreased bloodflow to regions of the lung?
⇡ PO2
⇣PCO2