So, let us discuss on the Ventilation (purposely ignoring Perfusion)
Ventilation increases when we move from the apex to the base of lungs when a man stands upright
Why? GRAVITY FORCE
1. We are agreed that the intrapleural pressure is almost always negative (let us think that the lung is not inhaling nor exhaling)
2. But it must be clear that the distribution of the negativity is unequal throughout the lungs
3. Due to the force of gravity, the muscle mass of lungs are pushed towards the bottom of the pleural cavity
4. This makes the apex of the pleural cavity is literally empty making it even more negative ~10mm/H2O
5. Then, the base is the other way round, the base is now packed with muscle mass of lungs making it to be less negative ~2.5mm/H2O
6. So, when the lung is relax (no inspiration nor expiration) the alveolar at the apex of the lung is become super inflated in response to higher negativity of the interpleural pressure exerted at the apex. Thus making the alveoli at the apex of lungs less compliance since it is already in the superinflated form even before inspiration
7. What happens at the base then? Simple, the alveoli are under inflated cause the intrapleural cavity is less negative.
8. This makes the base is better ventilated compared to the apex of lungs
9. This explains why, in almost all Tuberculosis infections, caseous necrosis happens at the apex of lungs, not because of its ventilation but due to less perfusion at that particular area makes it possible for Mycobacterium Tuberculosis(Aerobic bacteria) to infect.
So, here comes Perfusion
We are all agreed that perfusion increases when we move from apex to the base of lungs
There are few explanations on how this happens
So, to make thing easier, I classified it into two
1. The higher the area away from the heart the lesser the blood pressure
2. The collapse of the capillary beds at the apex due to super inflated state of the alveoli
First
1. It must be clear that, when blood is pushed upward away from the heart, blood pressure reduces (corresponding to the gravity force that it needs to against with). Different books have different value, but let us take the average one, 1cm increases in height, there will be 1mmHg reduce in blood pressure
2. If we move from the right ventricle towards the apex of lungs, there are ~15cm gaps between them. So, there is a significant decrease in blood pressure at the apex of the lungs
3. But if we move downward, the blood pressure increases accordingly (it is now nearer to the heart). So, the recruitment of capillary beds becomes greater, so more and more blood is pooled towards the base of lungs.
4. This explains why the base of lungs is better perfused compared to that of the apex
Second
1. We are now clear that at the apex of lungs, the alveoli is at the superinflated state due to the more negative pressure at the apex.
2. And we also clear that the blood pressure exerted by the heart to the apex of lungs is lesser due to height difference
3. So, let us put it this way, the arterial blood pressure of the pulmonary system (right ventricle) is too weak to pass through the capillary beds of the apex’s alveoli (which already in the superinflated state) making the capillary beds seem to be crushed by the alveoli. So, less perfusion at the apex
3. So, you do your own judgment on the base of lungs, it would be interesting to know it in details =)
Then, now we may conclude our knowledge on it
Eventhough we knew that perfusion and ventilation are lower at the apex of lungs compared to its base, but ventilation is significantly higher than perfusion at the apex.
This maybe explains how much blood pressure has decreased in response to its height above the heart. That is why V/P ratio is highest at the apex
But in the base of lungs, perfusion is slightly higher compared to its ventilation making its V/P ratio is lowest at the base.
Moving downward from apex to base of lungs - ventilation and perfusion are increasing but Ventilation/Perfusion Ratio is DECREASED
Virtual Medic 