Verse 3.13.5

अथ योऽस्योर्ध्वः सुषिः स उदानः स वायुः स आकाशस्तदेतदोजश्च महश्चेत्युपासीतौजस्वी महस्वान्भवति य एवं वेद ॥ ३.१३.५ ॥

atha yo'syordhvaḥ suṣiḥ sa udānaḥ sa vāyuḥ sa ākāśastadetadojaśca mahaścetyupāsītaujasvī mahasvānbhavati ya evaṃ veda || 3.13.5 ||

5. Next, the door at the top of the heart is udāna. It is also vāyu [air], and it is ākāśa [space]. Worship this [Brahman in the form of udāna] as strength and greatness. He who knows this becomes strong and great.

Word-for-word explanation:

Atha, next; yaḥ asya ūrdhvaḥ suṣiḥ, that which is its passage at the top [of the heart]; saḥ udānaḥ, that is udāna; saḥ vāyuḥ, that is [also] vāyu [air]; saḥ ākāśaḥ, [and] that is ākāśa [space]; tat etat, it is that [Brahman as udāna]; ojaḥ ca mahaḥ ca

Commentary:

There is an aperture at the upper part of the heart, and this is known as udāna. Air passes through this aperture from the feet to the head to help maintain the necessary operations of the body. The air that passes thus is also called udāna. Ākāśa (space) is the support of this udāna. Because udāna is one of the gateways to Brahman, it is meditated on as Brahman. If you do this, you become strong and famous. But, more important, you also attain heaven.