The vital force of a living entity includes the five kinds of air working within the body, which are known as prāṇa, apāna, vyāna, samāna and udāna. The vital force is compared to a serpent because a serpent can live by simply drinking air. The vital force carried by the air is described as the pratīhāra, or the bodyguard. Without the vital force one cannot live for a moment. Indeed, all the senses are working under the protection of the vital force.
(Srimad Bhagavatam-------4:25:21------purport).

Śrīmad-Bhāgavatam also describes how yogīs can travel to all the planets in the universe. When the vital force is lifted to the cerebellum, there is every chance that this force will burst out from the eyes, nose, ears, etc., as these are places that are known as the seventh orbit of the vital force. But the yogīs can block these holes by complete suspension of air. The yogī then concentrates the vital force in the middle position, that is, between the eyebrows. At this position, the yogī can think of the planet to which he wants to go after leaving the body. He can then decide whether he wants to go to the abode of Kṛṣṇa in the transcendental Vaikuṇṭhas, from which he will not be required to descend into the material world, or to travel to higher planets in the material universe.
(Sri Caitanya Caritamrta------1:5:22------purport).

Purañjana inquired from the woman about those eleven men and their wives and the snake. The woman gave a brief description of them. She was obviously without full knowledge of her surrounding men and women and the snake. As stated before, the snake is the vital force of the living being. This vital force always remains awake even when the body and the senses become fatigued and do no work. Even in the state of unconsciousness, when we sleep, the snake, or the life-force, remains intact and awake. Consequently we dream when we sleep. When the living entity gives up this material body, the vital force still remains intact and is carried to another material body. That is called transmigration, or change of the body, and we have come to know this process as death. Actually, there is no death. The vital force always exists with the soul, and when the soul is awakened from so-called sleep, he can see his eleven friends, or the active senses and the mind with their various desires (wives). The vital life-force remains. Even during our sleeping hours we can understand by virtue of our breathing process that the snake lives by eating the air that passes within this body. Air is exhibited in the form of breathing, and as long as breath is there, one can understand that a sleeping man is alive. Even when the gross body is asleep the vital force remains active and alive to protect the body. Thus the snake is described as living and eating air to keep the body fit for life.
(Srimad Bhagavatam-------4:25:35------purport).

As one's body engages in sense gratification, it becomes weaker and weaker daily. Finally the vital force becomes so weak that it is herein compared to a weak serpent. The life air has already been compared to the serpent. When the vital force within the body becomes weak, the body itself also becomes weak. At such a time the death symptoms—that is, the dangerous soldiers of death's superintendent, Yamarāja—begin to attack very severely. According to the Vedic system, before coming to such a stage one should leave home and take sannyāsa to preach the message of God for the duration of life. However, if one sits at home and is served by his beloved wife and children, he certainly becomes weaker and weaker due to sense gratification. When death finally comes, one leaves the body devoid of spiritual assets. At the present time, even the oldest man in the family does not leave home, being attracted by wife, children, money, opulence, dwelling, etc. Thus at the end of life one worries about how his wife will be protected and how she will manage the great family responsibilities. In this way a man usually thinks of his wife before death.
(Srimad Bhagavatam-------4:28:2------purport).