Ventilation-Perfusion Relationships Flashcards
What are the maximum and minimum partial pressures of oxygen in the alveolus that are possible at sea level?
Max = 150 mm Hg Min = 40 mm Hg
What would have to happen to get the arterial oxygen pressure to match that of the alveoli?
The ventilation to perfusion ratio would have to increase
What would the ventilation to perfusion ratio be if the pressure of oxygen in the arteries were 40 mm Hg?
Zero; this is a reflection of perfusion to a physiologic dead space
What are the two main reasons why ventilation and perfusion never perfectly match?
1) Ventilation is not homogenous
2) Perfusion is not homogenous
Where in the lung is blood flow the greatest?
The base
What are the West zones?
Even though the alveolar pressure is largely homogenous in the lungs at inspiration, the blood vessels at various portions have various radii, so the resistance and pressure of those vessels varies.
What is the state of lung tissue in West zone 1?
Blood flow tends to be collapsed and perfusion is minimal. The alveolar pressure is greater than that of the vessel
What is the state of lung tissue in West zone 2?
There is partial perfusion in the blood vessels and they will be in various states of open/shut. As a general rule, the arterial pressure is higher than that of the alveolar pressure, but the alveolar pressure is higher than the venous pressure
What is the state of the lung tissue in West zone 3?
There is high perfusion in the capillaries—arterial and venous pressure tend to be higher than the pressure of the alveolus.
Where is the transpulmonary pressure highest and why?
In the apex-gravity pulls the lung downward and makes the pleural pressure more negative.
Where are alveoli best ventilated and why?
At the based of the lung because they have higher compliance
What are the consequences of having a low V/Q ratio?
A low arterial oxygen pressure
What happens to the arterial oxygen pressure when blood from an alveolus with good V/Q ratios is mixed with blood from an alveolus with a bad V/Q ratio?
The overall partial pressure of oxygen will reflect the more poor V/Q ratio,
How does a lung mitigate poor V/Q ratios?
In hypoxic conditions, the lung will initiate vasoconstriction of the poorly ventilated alveoli in favor of those with good ventilation
What is a consequence of having a high V/Q ratio?
hypercapnea
What is the origin of the bronchial arteries and what is their typical pressure?
The bronchial arteries arise from the aorta and typically exhibit systemic pressure
Why would pulmonary arterial pressure need to be low?
Low pressures permit high rate of flow to be maintained and also minimize the amount of fluid that can escape into the pneumatic tissue
How is pulmonary vascular resistance calculated?
80*(Mean pulmonary arterial pressure-Pulmonary capillary wedge pressure)/Cardiac Output
How does the PVR change in exercise?
It drops to accommodate greater flow
What are the two mechanisms by which the lung can decrease the PVR?
It can encourage distension in capillaries that are already opened or it can recruit previously collapsed vessels (especially in the apexes)
What happens to alveolar blood vessels on inspiration?
Due to their large compliance, they tend to collapse, especially when the lung volume increases
How does the lung compensate for the restriction of alveolar blood vessels?
Blood vessels elsewhere within the respiratory network dilate to keep overall resistance and pressure normal
At what point is the resistance within the pulmonary network the lowest?
At the functional reserve capacity
How might the lungs control their own vasoconstrictive forces in hypoxia? (i.e. the mechanism)
Low O2 seems to affect smooth muscle cells to close their K channels, and depolarization occurs permitting Ca++ to enter the cell and trigger smooth muscle contractions
