Expansion
Papilla, minor calyx, major calyx, renal pelvis, ureter
Expansion
Urine passes:
- Collecting ducts open at the renal papilla, the apex of a medullary pyramid
- Minor calyx - cups each papilla; 8 to 12 per kidney
- Major calyx - 2 or 3, usually upper, middle and lower
- Renal pelvis
- Ureter, beginning at the pelviureteric junction
Each pyramid plus its overlying cortex forms a renal lobe.
The papilla is the point where the collecting system meets the medulla, and it is vulnerable to ischaemia because it lies at the end of the vasa recta. Papillary necrosis therefore occurs in diabetes, sickle cell disease, analgesic nephropathy, pyelonephritis and obstruction, producing sloughed papillae that can themselves obstruct the ureter.
A duplex collecting system is a common variant, and follows the Weigert-Meyer rule: the upper pole ureter inserts lower and more medially, and is prone to ureterocele and obstruction, while the lower pole ureter refluxes.