From the glossary of Philokalia:
HEART - (καρδιά - kardia): not simply the physical organ but the spiritual centre of man’s being, man as made in the image of God, his deepest and truest self, or the inner shrine, to be entered only through sacrifice and death, in which the mystery of the union between the divine and the human is consummated. ' “I called with my whole heart”, says the psalmist - that is, with body, soul and spirit' (John Klimakos, The Ladder of Divine Ascent). ‘Heart’ has thus an all-embracing significance: ‘prayer of the heart’ means prayer not just of the emotions and affections, but of the whole person, including the body.