{HEAD ~ Fluid} returning to {TAIL ~ the surface} passes through mud return line 14 via various surface equipment components as known to those skilled in the art, and then into the mud pits, thereby completing the closed loop 
returning to
{ HEAD ~ Fluid } returning to { TAIL ~ the surface } passes through mud return line 14 via various surface equipment components as known to those skilled in the art , and then into the mud pits , thereby completing the closed loop
HEAD HEAD HEAD HEAD HEAD REL REL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH