Movement of Urine in the Lower Colon and Cloaca of Ostriches

Retrograde movement of urine from the coprodeum to the colon and ceca in birds can be important in the recovery of water and NaCl. Water can be gained both from hypotonic luminal fluid and from hypertonic fluid as long as the rate of Na+-linked water absorption exceeds the rate of osmotic water loss. If the luminal fluid is sufficiently hypertonic, however, there may be a net water loss. As a consequence, post-renal modification of ureteral urine is particularly important in birds with poor renal concentrating ability. In examining the lower colon and cloaca of the Ostrich (Struthio camelus), two opposing possibilities were envisioned: (1) substantial retrograde movement because post-renal modification of urine is vital in the water balance of the Emu and this bird is also a large, flightless ratite; and (2) negligible retrograde movement because Ostriches are able to concentrate their ureteral urine to a much greater extent than emus and, in a previous report, no traces of uric acid were found in the terminal portion of the colon. The cloaca was divided into a coprodeum and proctodeum, but no urodeum was evident. Urine was stored mainly in the proctodeum which acted as a "bladder" and which changed dramatically in size. In eight-week-old chicks, it was 6-8 cm in diameter prior to urination when normally hydrated and 1-2 cm in diameter following urination. Mean cloacal urine osmolality was 395 ± 167 mOsm/kg. Considerable uric acid was observed in the coprodeum of all specimens but only traces were found on or within feces in the terminal 5 cm of the colon. Most of a barium sulfate suspension administered into the coprodeum of an adult Ostrich moved aborad into the proctodeum within 20 min; no orad movement was observed radiographically. A contrast medium injected intravenously into Ostrich chicks was cleared by the kidneys into the coprodeum, then moved immediately into the proctodeum, and was held there until excreted. Urine and feces were excreted separately (urine first) and the birds were coprophagic. We conclude that Ostriches rely mainly on kidney function for final composition of voided fluid.