The hepatorenal recess, the lowest part of the peritoneal cavity in the supine patient
Mnemonic
“The lowest point of the peritoneal cavity in a supine patient”, the hepatorenal recess between the liver and the right kidney.
It is the first place free fluid collects when lying flat, which makes it the primary window in a FAST scan in trauma, along with the perisplenic space, the pelvis and the pericardium.
In the erect position the lowest point is instead the rectouterine pouch of Douglas in women or the rectovesical pouch in men, which is why a pelvic collection is approached transrectally or transvaginally.
Fluid tracks from the right paracolic gutter into Morison’s pouch and then over the liver, which is why a perforated appendix can produce right upper quadrant signs and a subphrenic collection.
Expansion
Morison’s pouch is the hepatorenal recess: the space between the visceral surface of the right lobe of the liver and the right kidney, part of the right subhepatic space.
It is the most dependent part of the peritoneal cavity when supine in the upper abdomen, so free blood or pus collects there first.
That makes it the primary window of the FAST scan (Focused Assessment with Sonography in Trauma), together with the perisplenic space, the pelvis and the pericardium. A thin anechoic stripe between liver and kidney indicates free fluid, taken as blood in the context of trauma.
It also connects upwards with the right subphrenic space and downwards with the right paracolic gutter, so it lies on the main route by which lower abdominal sepsis reaches the diaphragm.