URINARY SYSTEM
The kidneys are bean shaped and reddish brown. The right kidney lies at the level of L1-L3, with the left kidney lying slightly caudal to the right. Both lie equidistant from the midline with their long axis converging cranially.
In healthy rats adipose tissue surrounds the hilus and sides. Like other rodents, the kidney is unipapillate and has one papilla and one calyx entering the ureter directly (Bivin et al. 1979; Hebel & Stromberg 1986g; Sharp & LaRegina 1998). The presence of more long loop nephrons gives the rat an excellent ability to concentrate its urine (28% in comparison to 14% in man) (Bivin et al. 1979).The urinary bladder lies cranial to the rim of the pubis. It is pear shaped when empty and spherical when full, stretching to 25?15 mm or more. In the male, the ventral lobe of the prostate gland attaches to the ventrolateral wall of the bladder and the urethra extends through the penis. In the female the urinary and reproductive openings are totally separate (Hebel & Stromberg 1986g).
Urine output is approximately 15-30 ml daily and proteinuria can be a normal finding (Bivin et al. 1979). Unlike the rabbit, the rat excretes only small quantities of calcium in the urine, even when fed a calcium rich diet (Cheeke & Amber 1973).