Before beginning any practical breathing exercises, it is useful to familiarize yourself with the three elements that make up a single breath and their relative importance in your breathing practice. These are inhalation, exhalation, and the transition between these two actions, known as retention. Exhalation is the focus of most breathing exercises.

Inhalation

As your lungs take in air, oxygen passes into your body, providing it with one of the essential ingredients of life. The inward flow of air into your lungs is more or less automatic: once you exhale, inhalation effortlessly follows. The in-breath is not emphasized in many of the yogic breathing exercises.

Exhalation

The outward flow of air from your lungs expels gaseous waste products, such as carbon dioxide, from your body. Your lungs therefore act as an organ of excretion. Yogis believe that exhalation also eliminates impurities from the mind, and that if you suffer from shortness of breath or cannot exhale completely, toxins will accumulate in your body which can negatively affect your mind. Exhaling also helps your mind and body adjust to change. For example, if you jump into a shower that is much colder than expected, you are likely to exhale sharply.

Retention

The absence of either inhalation or exhalation is known as retention, the transition between the two actions. When you inhale and hold your breath, the rate of gaseous in your lunch goes up as a result of the increase in pressure. this means that more oxygen passes from your lungs into your bloodstream. At the same time, more carbon dioxide and other gaseous waste products pass from your blood into your lungs, read to be eliminated with your exhalation. The pause made by your out-breath, as it stops for a moment en route to becoming your in-breath, is technically also a retention. This portion of your breath is rarely noticed but it is actually deeply calming.

Source: The Art of Breathing Well for Harmony, Happiness, and Health by Swami Saradananda