Apana

(Sanskrit) The vital energy that carries downward unassimilated food and drink. An aspect of prana. 

"The Apana dwells in the organs of excretion and generation... The goddess of the earth attracts Apana downwards." —Prasna Upanishad

"Prana is one; but it has many functions to do. Hence it assumes five names according to the different functions it performs, viz., Prana, Apana, Samana, Udana and Vyana. According to the different functions they perform, they occupy certain places in the body." —Swami Sivananda

"The chief aim of Pranayama is to unite the Prana with the Apana and take the united Pranapana slowly towards the head... The seat of Prana is the heart; of Apana, the anus... Apana does excretion." —Swami Sivananda

"Q. Will you be kind enough to elucidate ‘Apana Vayu’? We inhale air, thereby oxygen is absorbed by blood-cells as well as plasma. But from that oxygen, how Apana is formed? In what part it resides? What is the nature? How and where Prana and Apana unite? Kindly explain scientifically mentioning parts affected thereby.

"A. Apana is not formed from oxygen. Apana is energy. Apana resides in the lower part of the abdomen, in the Muladhara, in the descending colon, rectum and anus. Its nature is downward motion. Its function is ejection of urine, gas and excreta." —Swami Sivananda