It is a previous death with unresolved karma which creates the seed body for a future birth--see Law of the Last Thought. There are three ways for karma to be exhausted: through bhoga (enjoyment or suffering), jnana (wisdom), or by the grace of God. If the last two methods are not experienced, the only recourse is to helplessly suffer or enjoy and this is why rebirth takes place.
The vital energy which percolates through every being is broadly divided into five vital breaths, the pancha vayu: prana, apana, udana, samana and vyana. The opposite pull of the prana and apana, the upward and downward winds, creates the life principle in us, and death takes place when one finally gives way to the other.
The function of the prana vayu with its location in the heart is to move the lungs for inhalation and exhalation of breath. The apana air throws waste outside and has its location below the navel; udana vayu, centered in the throat, connects and disconnects the mind and its functions during the time of deep sleep and the wakeful states of consciousness; samana vayu digests and distributes food all over this body and is located in the navel; vyana vayu, pervading the whole body, gives the experience of touch.
Upa vayus or the subsidiary airs, are naga, kurma, krakara, devadatta and dhananjaya. Their functions are to help vomiting, movement of the eyelids, sneezing, and yawning, respectively. Dhananjaya vayu lives in the body even after death takes place and departs only after burning or decomposition of the whole body.
(Essence of Bhagavad Gita and Bible, p. 115-117.)