Introduction
Mammals and birds are the only vertebrates known to produce a concentrated urine by means of a renal medullary countercurrent system. These two countercurrent systems, however, exhibit functional and anatomical differences which appear to be related to the fact that mammals excrete most of their waste nitrogen in the form of urea, while birds excrete uric acid.

Fig 1. Phylogeny of urine excretion.
In mammalian kidneys, urea accumulation in the renal medulla plays an important role in the mechanism which concentrates the urine in the collecting ducts. The mammalian renal medulla is surrounded by a muscular funnel shaped pelvic wall, which leaves an elaborate urinary space between the renal medulla and the inside of the pelvic wall.

Fig 2. Funnel-Shaped Pelvis.
In bird kidneys, there is no urea accumulation in the renal medulla. The medulla is surrounded by tight sheets of connective tissue leaving no space for the urine to contact the renal medulla.  

Fig 3. Bird Renal Pelvis.
The mammalian renal pelvis makes it possible for urine to contact the epithelium covering the inner and outer renal medulla.

Fig 4. Kidneys with pelvis contracted and dilated.

Peristaltic contractions of the muscular pelvic wall exert a rhythmic pumping action on the renal papilla. The functional significance of these two aspects of the renal pelvis will be explored in this film.

[Respiratory Rate = 140/min]
[Pelvic Contractions = 10/min]

Video Clip 1. Rhythmic pelvic contractions.