Verse 8.6.2

तद्यथा महापथ आतत उभौ ग्रामौ गच्छतीमं चामुं चैवमेवैता आदित्यस्य रश्मय उभौ लोकौ गच्छन्तीमं चामुं चामुष्मादादित्यात्प्रतायन्ते ता आसु नाडीषु सृप्ता आभ्यो नाडीभ्यः प्रतायन्ते तेऽमुष्मिन्नादित्ये सृप्ताः ॥ ८.६.२ ॥

tadyathā mahāpatha ātata ubhau grāmau gacchatīmaṃ cāmuṃ caivamevaitā ādityasya raśmaya ubhau lokau gacchantīmaṃ cāmuṃ cāmuṣmādādityātpratāyante tā āsu nāḍīṣu sṛptā ābhyo nāḍībhyaḥ pratāyante te'muṣminnāditye sṛptāḥ || 8.6.2 ||

2. Just as a big, broad road connects one village with another one which is far-off, in the same way, the rays emerge from the sun and reach out to a person, connecting one with the other. After entering the veins of that person, they emerge from them and then go back into the sun.

Word-for-word explanation:

Tat yathā, just as; ātataḥ mahāpathaḥ, a long and broad road; ubhau grāmau gacchati, connects two villages; imam ca amum ca, this one with that; evam eva, like that; etāḥ ādityasya raśmayaiḥ these rays [which are blue, yellow, etc.] of the sun; ubhau lokau gacchanti, connect two worlds; imam ca amum ca, this [the human body] with that [the sun]; tāḥ, these [rays]; amuṣmāt ādityāt, from the solar region; pratāyante, emerge; āsu nāḍīṣu, into these veins; sṛptāḥ, enter; te, these [rays]; ābhyaḥ nāḍībhyaḥ pratāyante, emerge from the veins; te amuṣmin āditye sṛptāḥ, [and] they go back into the solar region.

Commentary:

How are the rays of the sun related to the veins of a human body? The Upaniṣad compares the rays to a big, wide road connecting two villages far away from each other. The rays of the sun go out and enter the veins of a human body, and then they return to the sun.