Xin 
regulates Blood pumping through the entire body, and nourishes Pi. Pi 
regulates Blood circulation in the vessels and prevents its extravasation. Blood circulation needs the control of Pi –Qi to contain it within the vessels, but depends on the pulse of Xin –Qi to move it. The Xin –Pi relationship is manifested by the generation and circulation of Blood.
Insufficient Xin -Blood or disordered Xin -Spirit negatively affects Pi ; dysfunction of Pi leads to disorders of Xin .