This text is one of the older, ‘primary’ upanishads, and includes an early expression of the subtle system in the yogi:
As a very long highway goes to two places, to one at the beginning, and to another at the end, so do the rays of the sun go to both words, to this one and to the other. They start from the sun, and enter into these arteries; they start from those arteries, and enter into the sun.
And when a man is asleep, reposing, and at perfect rest, so that he sees no dream, then he has entered into those arteries. Then no evil touches him, for he has obtained the light.
When he departs from this body, then he departs upwards by those very rays; or he goes out while meditating on Om. And while his mind is failing, he is going to the sun. For the sun is the door of the world [of Brahman]. Those who know, walk in: those who do not know, are shut out. There is this verse: ‘There are a hundred and one arteries of the heart; one of them penetrates the crown of the head; moving upwards by it a man reaches the immortal; the others serve for departing in different directions.’