B 

Breath 

Breath dictates how we experience all that is.
When the inhale and exhale are equal in length, breath brings equilibrium to the nervous system.
A long exhale releases excess waste product buildup from the lungs and blood. 
An inhale longer than exhale brings in additional oxygen.

In daily life as well as during focused physical movement we need a certain depth of breath to release carbon dioxide buildup and prevent oxygen deficiency that occur if we "get behind" on our breathing. Think of when you suddenly sprint, run up some stairs, or move much more quickly than usual, and end up panting for breath. This is a healthy physical response -- your respiratory and vascular systems refreshing the elements they need to continue supporting your physical life.

Even breath, which supports calm thinking and serenity, can be achieved in this way:
Inhale (through the nose if you can) for 6 seconds, pause with full lungs for one second, exhaling through the nose for 6 seconds, and pausing with empty lungs for one second. Repeat. Notice any change in how you feel.
 
How to Breathe during Yoga Pose (Asana) Practice, in Ashtanga Vinyasa Yoga:
The breathing technique performed with vinyasa is called ujjayi [victorious breath] (Scott 20), which consists of puraka [inhalation] and rechaka [exhalation] ("Ashtanga Yoga"). "Both the inhale and exhale should be steady and even, the length of the inhale should be the same length as the exhale" ("Ashtanga Yoga"). Over time, the length and intensity of the inhalation and exhalation should increase, such that the increased stretching of the breath initiates the increased stretching of the body (Scott 21). Long, even breathing also increases the internal fire and strengthens and purifies the nervous system ("Ashtanga Yoga").

From "Ashtanga Yoga Background".
Works Therein Cited include:
Scott, John. Ashtanga Yoga: The Definitive Step-by-Step Guide to Dynamic Yoga. New York: Three Rivers Press, 2000.
"Ashtanga Yoga." Ashtanga Yoga Research Institute: Method. 2001. 11 June 2003 <http://www.ayri.org/method.html>.

Since breath is the most crucial focus of pose (asana) practice, here are further resources to peruse on the role of breath, breath practices (pranayama) and how breath affects health in yoga and beyond.

Further reading on Breath in Yoga:
How Yoga Affects Breathing 
How Yoga Affects Your Nervous System
What Yoga Therapists Should Know About the Anatomy of Breathing  
Nadi Shodana to Balance the Nervous System

Therapeutic Use:
Breathing, Chronic Obstructive Pulmonary Disease, and Yoga 
Yoga and Mild Asthma