Verse 3.12.4

यद्वै तत्पुरुषे शरीरमिदं वाव तद्यदिदमस्मिन्नन्तः पुरुषे हृदयमस्मिन्हीमे प्राणाः प्रतिष्ठिता एतदेव नातिशीयन्ते ॥ ३.१२.४ ॥

yadvai tatpuruṣe śarīramidaṃ vāva tadyadidamasminnantaḥ puruṣe hṛdayamasminhīme prāṇāḥ pratiṣṭhitā etadeva nātiśīyante || 3.12.4 ||

4. That which is in this human body is in this human heart, for all these prāṇas are based in this heart and cannot exist independent of it.

Word-for-word explanation:

Yat vai tat puruṣe śarīram, that which is in this human body; idam vāva tat, it is that; yat idam asmin antaḥ puruṣe hṛdayam, which is in this human heart; hi, because; asmin, in this [heart]; ime prāṇāḥ, these prāṇas [the vital forces]; pratiṣṭhitāḥ, are based; etat eva na atiśīyante, they cannot go beyond this [heart—i.e., they cannot exist independent of this heart].

Commentary:

The human body has been described as the gāyatrī. But where is that gāyatrī? It is in the heart. In fact, the heart is the gāyatrī. In what sense can the heart be called the gāyatrī? In the sense that all the prāṇas are in the heart and cannot exist separate from it. And since the body is the gāyatrī, the heart is also the gāyatrī.