Katha-2-2-3-ūrdhvaṃ prāṇam

Sri Shankara’s Commentary (Bhashya) translated by S. Sitarama Sastri

In realising the existence of the âtman an evidence is offered—Upwards] from the heart. Prâba ] the wind, whose function is connected with breath. Unnayati ] leads upwards. Similarly casts the apâna downwards. The word yah (who), should be supplied to complete the sentence. Him seated in the middle, i.e., in the âkâs of the lotus of the heart, with the light of knowledge clearly shining in the intellect and worthy of worship, all the Dêvâs, i.e., the senses, the eyes and the rest propitiate, by bringing in the perceptions of form, etc., as the subjects please the king, by offerings, i.e., they are uninterruptedly active on his account. The drift of the text is that there is another on whose behalf and by whom is directed all the activity of the winds and the senses.

Links
Various resources on the Kathopanishad
Upanishad Series – Katopanishad Commentary by Swami Chinmayananda