The urge to breathe is hard wired and we need little ecouragement to begin taking a breath, in fact it is automatic. We do not need to pay any attention to our breathing, this is done in the background. Sometimes the volume and rate of our breathing can get skewed and we end up mildly hyperventila ting, this can cause health problems, but this is another discussion.
The urge to breathe begins when carbon dioxide levels in the blood rise, this is monitored by our brain stem and receptors all over our bodies, but one area which seems to be important is the diaphragm. I found a very interesting article in The Guardian online which talks about the role of the diaphragm in triggering our urge to breath. You can read it here.