We all breathe automatically, but when we're focused on tasks at work we aren't necessarily breathing deeply and slowly. You don't have to wait for a stressful situation to focus on deep breathing — do it regularly throughout your working day and it will help keep you calm.
With so much going on already in your mind, give yourself a helping hand by setting a regular reminder (on your phone, or whatever works for you) to breathe deeply. If you're sitting down for most of the day, take the opportunity to get up and stretch while you're at it.