Zone 2 is the middle part of the lung.
- Zone two tends to be the zone where the blood vessels and the alveoli are at their maximal sizes.
- As a consequence the ventilation and perfusion will be balanced.
Zone 3 is the base of the lung, or the lowest part in the upright patient.
- The blood supply is very distended in zone 3 because of the effects of gravity. They are below the heart, so are subject to a slightly higher pressure than those above them.
- This increased venous pressure will also make the alveoli smaller as it acts on them, relative to those above.
- As a consequence the perfusion will be greater than the ventilation.
A very important point to know here is that at any given time over half of our lung is not being utilised. Potentially our lungs can hold up to 6 litres of air, but normally is only holding just under 3 litres.
This means that a large proportion of out alveoli are collapsed. If we were perfusing all of those collapsed alveoli we would have a lot of our lung blood volume not being effectively ventilated. So how does the body compensate for this?
The body has a process called Hypoxic Vasoconstriction. Anytime a region is poorly ventilated and has low oxygen levels the muscles around the blood vessels in the area concerned constrict, thereby limiting the blood supply to the region.
So the blood flow to Zone 3 is actually reduced compared to Zone 2. This means that Zone 3 is neither perfused or ventilated well under normal circumstances. When we take a deep breath then we will utilise Zone 3 more fully.