Sub-Pranas and Their Functions
Naga, Kurma, Krikara, Devadatta and Dhananjaya are the five sub-Pranas. The function of Prana is respiration; Apana does excretion; Samana performs digestion; Udana does deglutition (swallowing of the food). It takes the Jiva to sleep. It separates the astral body from the physical body at the time of death. Vyana performs circulation of blood.
Naga does eructation and hiccup. Kurma performs the function of opening the eyes. Krikara induces hunger and thirst. Devadatta does yawning. Dhananjaya causes decomposition of the body after death. That man is never reborn, whenever he may die, whose breath goes out of the head, after piercing the Brahmarandhra (Sahasra Chakra or Crown Chakra).
The Color of Pranas
Prana is said to be of the color of blood, red gem or coral. Apana which is in the middle, is of the color of Indragopa (an insect of white or red color). Samana is of the color between that of pure milk or crystal or of oily and shining color, i.e., of something between both Prana and Apana. Udana is of Apandura (pale white) color and that of Vyana, resembles the color of archil (or that of ray of light).
The Length of the Air-Currents
This body of Vayu is 96 digits (6 feet) in length as a standard. The ordinary length of the air-current, when exhaled is 12 digits (9 inches). In singing, its length becomes 16 digits (1 foot), in eating it comes to 20 digits (15 inches), in sleeping 30 digits (22 1/2 inches), in copulation 36 digits (27 inches) and in doing physical exercise it is much more than that. By decreasing the natural length of the expirer air-currents (from 9 inches), life is prolonged and by increasing the current, duration of life is decreased.
The Centering of the Prana
Inhaling the Prana from outside, filling the stomach with it, centre the Prana with the mind, in the middle of the navel, at the tip of the nose, and at the toes, during the ‘Sandhyas’ (sunrise and sunset) or at all times. Thus the Yogi is freed from all diseases and fatigues. By centering this Prana at the tip of the nose he obtains mastery over the elements of the air; by centering at the middle of his navel, all diseases are destroyed; by centering at the toes, his body becomes light. He who drinks air through the tongue destroys his fatigue, thirst and many other diseases. For him who drinks the air with his mouth, during the two Sandhyas and the last two hours of the night, within three months, the auspicious Sarasvati (Goddess of speech) is present in his Vak (speech), i.e., he becomes eloquent and learned. In six months he is free from all diseases. Drawing the air at the root of the tongue, the wise man thus drinking nectar enjoys all prosperity.
The Science of Pranayama by Sri Swami Sivananda