BG Chap 8 - Karma and transmigration

Action pertaining to the development of the material bodies of the living entities is called karma. The creative force which gives rise to the bodies of men and others by combination of the subtle and gross elements is called karma. It is called karma because it is produced from actions. By performance of jyotiṣṭoma sacrifice, one goes to heavenly planets and enjoys there in the body of a devatā. The results of pious actions in the form of enjoyment are eventually depleted, and the jīva comes back to earth and gets a gross body in the form of human or otherwise.

In the Chandogya Upaniṣad it is described that five kinds of offerings namely faith, the enjoyer on the moon, rain, food and semen are made into five kinds of fire namely the heaven, clouds, earth, man and woman. The jīva, following the Vedas, performs sacrifice with faith. This offering with faith envelops the jīva and goes with him to heaven when he dies. The jīva gets a body as an enjoyer in the moon and enjoys the results of his actions. At the end of enjoyment, the jīva is offered into the fire of parjanya (clouds or the god of rain), and becomes rain. That rain comes down into earth, by the offering of the rain along with the jīva into the fire called earth by the devas and become food such as grains. The rice with the jīva is offered into the fire called man (man eats rice) and becomes semen. The semen along with the jīva is offered into the fire called woman (man impregnating woman), and becomes an embryo and transforms into a man. The cause of such states, the consequence of actions, and the results from actions is called karma.

The analogy above is given to show that the jīva takes his subtle body and senses and prāṇas with him, holding his karma. The ācāryas do not comment whether one should take literally the statement that the jīva falls back to earth in the rain, and then becomes food etc. In any case after exhausting puṇyas, the jīva falls back to earth and gets another human body.