The Forms of Prāṇa

There are five forms of prāṇa, all having different names according to the bodily functions with which they correspond. These forms of prāṇa are:
*udāna-vāyu, corresponding to the throat region and the function of speech
*prāṇa-vāyu, corresponding to the chest region
*samāna-vāyu, corresponding to the central region of the body and the function of digestion
*apāna-vāyu, corresponding to the region of the lower abdomen and the function of elimination
*vyāna-vāyu, corresponding to the distribution of energy into all areas of the body

We will concern ourselves with two forms: prāṇa-vāyu and apāna-vāyu.
That which enters the body is called prāṇa and that which leaves it is called apāna. The term apāna also refers to the region of the lower abdomen and all the activities that take place there. Apāna describes that part of prāṇa that has the function of elimination and provides the energy for it, and it also refers to the lower belly and the rubbish that collects there when the power of prāṇa is not in a state of equilibrium. When a person is slow and heavy we sometimes say that he has too much apāna. Apāna as prāṇic energy is something we need, but apāna as refuse left from activating this energy actually prevents prāṇa from developing within. All forms of prāṇa are necessary, but to be effective they must be in a state of balance with each other. If someone has a lot of rubbish in the region of the lower abdomen then he or she consumes too much energy there, and this imbalance should be addressed. The goal is to reduce apāna to an efficient minimum.
Apāna as waste matter accumulates because of many factors, some of which lie within our control. The practice of yoga aims to reduce these impurities. People who are short of breath, cannot hold their breath, or cannot exhale slowly are seen as having more apāna, whereas those who have good breath control are considered to have less apāna. An overabundance of apāna leads to problems in all areas of the body. We have to reduce the apāna so that we can bring more prāṇa into the body.
When we inhale, prāṇa from outside the body is brought within. During inhalation, prāṇa meets apāna. During exhalation, the apāna within the body moves toward the prāṇa. Prāṇāyāma is the movement of the prāṇa toward the apāna and the movement of the apāna toward the prāṇa. Similarly, holding the breath after inhalation moves the prāṇa toward the apāna and holds it there. Holding the breath after exhalation moves the apāna toward the prāṇa.

TKV Desikachar - The Heart of Yoga