In the physical body the Third Eye chakra relates to the eyes, face, brain, lymphatic and endocrine system.
The Crown Chakra is located just behind the top of the skull. It is the entry point for higher consciousness and for the Higher Self to incarnate into the body. This chakra is almost always blocked in humans for if it was not we would have conscious connection with our higher selves and cosmic consciousness. We would be able to make contact with many multi-dimensional beings and receive clear communication from higher dimensions. We could easily take our consciousness upwards through the Column into higher dimensions and become aware of our multi-dimensional nature.
The Crown Chakra is intimately tied with the pineal gland which releases large amounts of growth hormone among other chemicals when we are babies. As we grow up the pineal gland shrinks to a tiny shriveled up pea. This is due to the closure of the Crown Chakra after the chitta and ego structures do their work. Reactivating the pineal gland can only come along with the activation of the Crown Chakra which will indeed rejuvenate the entire body. All bodily systems would run smoother and we would live long and free of sickness. Of course the main benefit would be the ecstasy and grace acquired through the incarnation of higher consciousness. The Crown Chakra is intimately tied with the Base Chakra and will not open and become activated without the kundalini and sexual energy rising from the Base and Sex Chakras. These forces will activate the crown along with the pineal and lead to the actual incarnation of your Higher Self. All ego structures will be dissolved during this process and the spirit may finally dwell within the body.
The Crown Chakra is also the proper exit point for the soul at death. If the Crown is open enough the subtle bodies will leave through the crown and begin to ascend into higher dimensions. If on the other hand the Crown Chakra is very blocked (which does happen here on Earth) the soul may leave out another hole such as the eyes, ears, mouth or even the anus. If the soul leaves through one of these holes it may end up in some negative astral realms during afterlife wandering until the astral body