Gas Exchange and V'/Q' Ratio

I can't be bothered thinking up an introduction for this post, so let's just get into it!

Recall the blood pressures in the pulmonary circulation.

Blood pressures in the pulmonary circulation are lower than those in the systemic circulation. The pressure in the pulmonary arteries is around 22/8. The capillary pressure is also around 10mmHg less than in the systemic capillaries.

Explain how the Starling equilibrium is altered in pulmonary capillaries.

Firstly, you might want to refresh your memory on the Starling equilibrium by looking here. In pulmonary capillaries, one of the outward driving pressures (blood pressure) is greatly reduced as compared to the systemic circulation. How, then, is this compensated for?

Alveoli affect the Starling equilibrium as well. Air pressure within the alveoli pushes outwards (i.e. towards the blood), whereas the surface tension pulls stuff inwards (i.e. out of the blood). Everything's usually all nicely balanced so that pulmonary oedema (fluid in the lungs) doesn't occur (though obviously this can change in disease states).

Define V'/Q' mismatch

First some quick definitions: V' (which is sometimes displayed as V with a dot on top) is airflow, whereas Q' (again, Q with a dot on top) is blood flow. Blood flow and airflow are not uniform throughout the lung- some areas have better blood flow than airflow, and some areas have the opposite problem. For optimal gas exchange, blood flow should equal airflow, but this isn't possible everywhere in the lung. All of this can be expressed as the Ventilation-Perfusion ratio, or V'_A/Q'.

Now I'll give a quick overview on the main factors controlling pulmonary blood vessel resistance and air pressure (both of which ultimately control flow, as flow is equal to (change in pressure)/resistance):

For blood flow, we need to look at how many pulmonary vessels are open or closed. At rest, many vessels are closed. When blood pressure increases, more blood vessels are open, and those that are open may distend. This causes a reduction in pulmonary resistance. When blood pressure decreases, more vessels close off, increasing pulmonary resistance.

For air flow, see my earlier post: Mechanics of Breathing. In particular, read the part about compliance and the LaPlace equation.

Explain how V'/Q' mismatch produces hypoxia

As I just mentioned, airflow and blood flow are not equal everywhere in the lung. To understand the consequences of this, let's consider the two gases separately.

Oxygen

In underventilated alveoli (i.e. alveoli with more blood flow than airflow), the blood will arrive and leave with a smaller than usual increase in O₂ due to the limited amount of airflow. In overventilated alveoli (i.e. alveoli with more airflow), there will be a larger than usual increase in O₂.

However, these do not cancel each other out! Remember, haemoglobin saturation plays a large role in oxygen content of the blood. When blood flow and air flow are well-matched, you'll get a normal increase in airflow, and haemoglobin saturation will go back up to 95-100% (after becoming unsaturated during oxygen transfer to the cells of the body). When there is a larger than usual increase in O₂, haemoglobin saturation won't change much (as you can't get more than 100% haemoglobin saturation), so you're really not changing the oxygen concentration much at all! Hence, V'/Q' mismatch produces hypoxia.

Carbon Dioxide

Carbon dioxide is the opposite to oxygen: underventilated alveoli will not remove as much CO₂ as an over-ventilated one. These do, however, cancel out, as saturation is not an issue here. Hence, V'/Q' mismatch will produce little (if any) hypercapnia.

V'/Q' mismatch is a very common cause of hypoxia in disease states. V'/Q' mismatch is particularly marked in disease states such as asthma, where air will flow into healthy alveoli, whereas blood will flow to affected alveoli.

Describe the cause of orthostatic V'/Q' mismatch

When we are standing, airflow goes to the bottom of the lung. This is because gravity causes alveoli to stretch and become less compliant, particularly those towards the top (apex) of the lung. Blood flow also goes to the bottom of the lung when we're standing, as the higher blood pressure at the base causes vasodilation (as explained above). That sounds great, right? Airflow and blood flow increase in the same places!

Not quite. It's true that gravity does have an effect on both blood flow and ventilation, but it has more of an effect on blood flow than on ventilation. Hence, there is still some V'/Q' mismatch in most places in the lung. Towards the base of the lung, there will be more perfusion than ventilation; towards the apex, there will be more ventilation than perfusion. Ventilation and perfusion are relatively even at around the third rib.

Explain how V'/Q' mismatch is minimized in the normal lung 

In this post, I described how local metabolites can affect vasoconstriction and vasodilation. I also mentioned that O₂ is a vasoconstrictor everywhere except for in the pulmonary circulation, where it is a vasodilator. Well, that's relevant again: well-ventilated alveoli will cause vasodilation, whereas poorly perfused alveoli are hypoxic and cause vasoconstriction. In severe hypoxia, all of the pulmonary vessels can constrict, increasing blood pressure in the pulmonary circulation. This causes pulmonary hypertension, which can lead to oedema, right heart failure and death.

At really low levels of CO₂ (like really low), the airways may constrict to prevent more CO₂ from leaving. This generally isn't very important, however: most of our V'/Q' mismatch compensation is done by the blood vessels, rather than by the airways.