Practitioners of yoga, especially within Hinduism, believe that vital currents are generated by the human body, and that these currents control all biological processes. The generic term for these currents is Prana, and they are categorised under the following five (functional) divisions :

1. Prana : Responsible for the beating of the heart and breathing. Prana enters the body through the breath and is sent to every cell through the circulatory system.
2. Apana : Responsible for the elimination of waste products from the body through the lungs and excretory systems.
3. Udana : Responsible for producing sounds through the vocal apparatus, as in speaking, singing, laughing, and crying. Also it represents the conscious energy required to produce the
   
   
   
   vocal sounds corresponding to the intent of the being. Hence Samyama on udana gives the higher centers total control over the body.
4. Samana : Responsible for the digestion of food and cell metabolism (ie. the repair and manufacture of new cells and growth). Samana also includes the heat regulating processes of the body. Auras are projections of this current. By meditational practices one can see auras of light around every being. Yogis who do special practise on samana can produce a blazing aura at will.
5. Vyana : Responsible for the expansion and contraction processes of the body, eg. the voluntary muscular system.

Source : Adapted from Rammurti S. Mishra Yoga Sutras : The Textbook of Yoga Psychology.