When {HEAD ~ the rivet} is released within an artificial fistula formed through the aorta and vena cava walls, it expands to trap the walls between {TAIL ~ the two expanded heads} 
When { HEAD ~ the rivet } is released within an artificial fistula formed through the aorta and vena cava walls , it expands to trap the walls between { TAIL ~ the two expanded heads }
OTH HEAD HEAD HEAD HEAD HEAD HEAD OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH TAIL TAIL TAIL TAIL TAIL TAIL TAIL TAIL