Efficient gas exchange in the lungs depends on two separate things happening together at the same specific location: ventilation, air actually reaching a given region of lung tissue, and perfusion, blood actually flowing through the tiny capillaries surrounding that same region, ready to pick up oxygen and drop off carbon dioxide. A ventilation-perfusion mismatch, often written as a V/Q mismatch, happens whenever these two normally coordinated processes fall out of step with each other at a given location, and it's one of the single most common underlying mechanisms behind real, clinically significant breathing problems.
Two separate delivery systems that normally track each other closely
Air reaches lung tissue through the airways, branching progressively smaller passages carrying inhaled air down to the tiny air sacs called alveoli where gas exchange actually happens. Blood reaches that same tissue through an entirely separate branching network of blood vessels, ultimately forming a dense capillary bed surrounding each alveolus. In healthy lung tissue, these two separate delivery systems are normally well matched region by region — areas receiving plenty of fresh air are also receiving plenty of blood flow ready to exchange gas with it, and the body actively works, through local blood vessel constriction and dilation, to keep the two reasonably well matched even as breathing and blood flow patterns naturally vary somewhat from moment to moment and region to region.
A mismatch in either direction wastes the resource that did arrive
When ventilation and perfusion fall meaningfully out of alignment at a given location, gas exchange there becomes considerably less efficient, whichever direction the mismatch runs. A region receiving air but little to no blood flow, sometimes caused by a blood clot blocking flow to that area, wastes the ventilation, since no blood is present at that spot to actually pick up the oxygen that air delivered. A region receiving blood flow but little to no fresh air, sometimes caused by an airway blockage or collapsed lung tissue, wastes the perfusion instead, since blood passing through that region has no fresh air available there to exchange gas with, and effectively passes back into general circulation still carrying much of its original, unrefreshed gas content. Either direction of mismatch reduces how efficiently the lungs as a whole can actually oxygenate blood and remove carbon dioxide, and clinicians assessing lung function specifically evaluate for this kind of localised mismatch, not just for whether a patient's lungs are ventilating and perfusing adequately in some general overall sense.
What we're still unsure about
The basic mechanics of ventilation-perfusion matching, and the physiological consequences of a mismatch in either direction, are well-established, extensively documented respiratory physiology. What remains a genuinely active area of clinical research is developing more precise, practical methods for measuring and mapping the specific pattern and severity of ventilation-perfusion mismatch in an individual patient's real lungs, since the underlying cause and the most appropriate treatment can differ considerably depending on exactly where and how severely the mismatch is occurring — researchers and clinicians continue refining imaging and diagnostic techniques aimed at capturing this kind of detailed regional information more reliably than older, more general lung function tests could provide on their own.
This sits inside Pulmonary Physiology & Ventilation-Perfusion, one of seven topics in Physiology, one of four domains in Medicine, one of seventeen subjects the app can quiz you on.