Verse 3.13.3

अथ योऽस्य प्रत्यङ्सुषिः सोऽपानः सा वाक्सोऽग्निस्तदेतद्ब्रह्मवर्चसमन्नाद्यमित्युपासीत ब्रह्मवर्चस्यन्नादो भवति य एवं वेद ॥ ३.१३.३ ॥

atha yo'sya pratyaṅsuṣiḥ so'pānaḥ sā vākso'gnistadetadbrahmavarcasamannādyamityupāsīta brahmavarcasyannādo bhavati ya evaṃ veda || 3.13.3 ||

3. Next, the western door of the heart is apāna. It is also vāk, and it is fire. Worship this [Brahman in the form of apāna] as the radiance of Brahman and as food. He who knows this becomes radiant with the light of Brahman and a great eater of food.

Word-for-word explanation:

Atha, next; yaḥ asya prātyāṅsuṣiḥ, that which is its western passage; saḥ apānaḥ, that is apāna; sā vāk, that is [also] vāk [word]; saḥ agniḥ, [and] that is fire; tat etat, it is that [Brahman as apāna]; brahmavarcasam ca annādyam iti upāsīta, meditate on as the radiance of Brahman [that comes from living a disciplined life and from scholarship] and as food; yah evam veda, he who knows this;

Commentary:

Apāna’s function is to remove all waste from the body. It is connected with speech and fire. Brahmavarcasa is the radiance a person acquires when he has much scholarship and has led a disciplined life. One who meditates on apāna as Brahman acquires this radiance and also food.