Your vital capacity is a measurement of how much air you can breathe out after a maximum inhalation.
It is thought to be the most important indicator of your true physiological age as opposed to your chronological age and thus has long been used as a predictor of longevity.
Lung capacity naturally decreases as you get older but you can use breathing techniques to reverse this. If you think of breathing as the mechanism for your oxygen / CO2 exchange, it makes perfect sense that optimising your oxygen uptake is going to have positive repercussions on your health.
Now, breathing is the most automatic, unconscious thing we do, so improving our breathing quality has to be done with some conscious awareness, it won’t just naturally happen on its own. Luckily that couldn’t be simpler, just by paying attention to our breathing and trying to do it a little more slowly and deeply will begin to tone and stretch our lungs, improving our potential vital capacity.
But as all breathing experts know, there is another tool in the breathing toolbox that can really help make a difference, especially in our busy, stressful lives. Most people run on slightly anxious energy and find they often shallow breathe high up in their lungs. By exhaling, even a few counts longer than your inhale, you will not only be improving the quality of your breathing, but also calm the nervous system and become more focused.
This is because this deeper out-breath forces you to breath lower into your diaphragm, massaging your vagus nerve that sends a signal to your brain to turn up your parasympathetic nervous system (your rest, relax, digest response) and turn down the sympathetic nervous system (fight or flight response).
This is beneficial, as it stops the release of stress hormones which, although great if you are being chased by a lion, can actually be harmful when circulating in your blood unnecessarily every day because you are anxious you’ll be late for work or you've forgotten your lunch.
So there are many exercises worth looking at, my personal favourite is the 4-7-8 breathing technique, pioneered by Dr. Andrew Weil, which I find very useful for sleep and also when I feel my anxiety levels rising https://www.drweil.com/health-wellness/body-mind-spirit/stress-anxiety/breathing-three-exercises/
But keeping it really simple, why not try the fibonacci breathing technique where you take the fibonacci sequence 0,1,1,2,3,5,8,13 and choose any sequential numbers and breath in for the first and out for the second. A good place to start is 3 in and 5 out, and then you can pick higher numbers as you get more practiced. I tend to suggest 4 rounds of the breath and then breathe normally and try and do this a few times every day.
Don't worry if you find it difficult at first, you are exercising muscles that may not have been stretched for a while!
Have fun oxygenating!