Pranayama

The word prāṇāyāma consists of two parts: prāṇa and āyāma. Āyāma means “stretch” or “extend,” and describes the action of prāṇāyāma. Prāṇa refers to “that which is infinitely everywhere.” With reference to us humans prāṇa can be described as something that flows continuously from somewhere inside us, filling us and keeping us alive: it is vitality. In this image, the prāṇa streams out from the center through the whole body.
Ancient texts such as the Yoga Yājñavalkya (see appendix 1) tell us that someone who is troubled, restless, or confused has more prāṇa outside the body than within. The amount of prāṇa outside the body is greater when we feel unwell; at those times the quality of prāṇa and its density within the body is reduced. Too little prāṇa in the body can be expressed as a feeling of being stuck or restricted. It can also show as a lack of drive or motivation to do anything; we are listless or even depressed. We may suffer from physical ailments when prāṇa is lacking in the body. And finally the Yoga Sūtra mentions disturbances in the breath, which can take very different forms.1 On the other hand, the more peaceful and well-balanced we are, the less our prāṇa is dispersed outside the body. And if all the prāṇa is within the body, we are free of these symptoms.

If prāṇa does not find sufficient room in the body there can be only one reason: it is being forced out by something that really does not belong there— let’s call it rubbish. What we are trying to do when we practice prāṇāyāma is nothing more than reduce this rubbish and so concentrate more and more prāṇa within the body.
Our state of mind is closely linked to the quality of prāṇa within. Because we can influence the flow of prāṇa through the flow of our breath, the quality of our breath influences our state of mind and vice versa. In yoga we are trying to make use of these connections so that prāṇa concentrates and can freely flow within us

TKV Dasikachar - The Heart Of Yoga