We all understand breathing involves taking in oxygen, but is that its only role? Interestingly the answer is no.
Removing carbon dioxide
Our breathing rate is determined by our need to remove carbon dioxide from our bodies not our need for oxygen. Our blood always contains excess oxygen such that we only use a small proportion during our daily activities. However the accumulation of waste gases produced by our cells (e.g. carbon dioxide) is something we can not tolerate as it makes our blood more acidic. Breathing removes carbon dioxide thus our breathing rate is dependent on how much we need to remove.
Returning blood back to our hearts
Interestingly breathing is the second most important reason (the first is obviously the heart) for our blood flow. If it was not for breathing our blood would form pools in our legs! The heart pushes the blood around the body, whilst breathing sucks the blood back up. Breathing achieves this by creating pressure differentials in the body, in a similar fashion to how we suck fluid up through a straw.
Strengthening heart beats
The heart is securely fastened within the ribcage via various attachments to the surrounding skeletal and muscular structures. One of these structures is the diaphragm. When the diaphragm moves (i.e. descends during breathing) it pulls the bottom part of the heart with it. Therefore the chambers of the heart get stretched which makes the contraction stronger.
Mobilising our organs
Directly beneath our diaphragm are our abdominal organs (e.g. stomach, liver, etc). Therefore if our diaphragm descends and rises at a set frequency (i.e. breathing) it creates a rhythmical wave which travels through our organs. This motion maintains our organs mobility and hence health.
Behavioural breathing is how our breathing responds to changes in our emotions, e.g. when we are nervous, angry, etc. The reverse is also true, i.e. breathing can alter our emotions. A good example is when parents instruct their upset children to take deep breaths in an attempt to calm them down. It is this link between breathing and emotion which the parents are using.