In April 2011, Charl Schwartzel won the US Masters Golf tournament with one of the most extraordinary final round of golf.  I am not sure if he does yoga on the side but this is what he said:
“You’ve got to really try and force yourself to stay in the present, which is very difficult around here.  There’s so many people and so many roars that go up.  That’s the biggest thing.  You’ve got to breathe.  Sometimes you forget to breathe.”
Sometimes we forget to breathe, but most of the time we forget that we are breathing.  We can live for weeks without food, days without water, but only minutes without breath.  I’m sure there’s hardly any Yoga classes we attended whereby the instructor did not repeatedly tell us to breath.  In Ashtanga Vinyasa Yoga, the breath is the key.  In the Yoga theory, the length of life is not measured by the number of years but by the number of breaths.
In each breath that we take, the air that we breathe in travels into the external nose and  through the internal nasal cavity where the air is filtered.  Thereafter, it passes through the Pharynx (Throat), Larynx, Trachea, Primary Bronchi then into the Lungs to the Secondary Bronchi, Tertiary Bronchi, Bronchioles and finally to the Alveolar where the gas exchange takes place. 
The lungs absorb about 5% of the oxygen from the air into the blood plasma, most of which is bound to the protein hemoglobin in our red blood cells.  Our cardiovascular system transports the oxygen in the blood throughout the body for our cellular respiration. 
In addition to keeping us alive as described above, slow and deep breaths calm our mind, slow down the heart rate, reduce stress and tension and even relieves pain.  Focusing on the breath brings clarity, helps us stay in the present, makes us feel more peaceful and hence have longer and healthier lives.
So the next time you are faced with a stressful situation (and also when in an Ashtanga class), remember…. Breathe.  Most of the time, it does help.