Probably because of the importance for the JIva of all the physical functions of the body and the necessity of the JIva to monitor them, this heart is also kept ‘semi-physical’.
Even though it is semi-physical, once the continued practice goes on with the thought that ego is to be dissolved, in due time it becomes subtle and becomes almost just space.
The Atma-sthAnaM (the locale for the Atman) however is more subtle; it is kAraNa-AkAshaM (causal space) – it is the centre point of the heart.
This subtler thing cannot be approached by anything physical or semi-physical.
Only by lightening the ego, making it subtle, it can enter the subtler space.
This lightening of the ego (*ahamkAra-kArshyaM*) is what is done by Bhakti.