The urinary tract consists of the kidneys, ureters, urinary bladder, and urethra. Most kidneys are located against the sublumbar muscles, under the peritoneum, but the left kidney of ruminants is pendulous and surrounded by a layer of peritoneal membrane. The left kidney is more caudal than the right in domestic animals, with the exception of pigs who typically have a more caudally positioned right kidney. The blood supply to the kidneys is the renal artery, usually the fifth branch off of the abdominal aorta (after the unpaired celiac artery and cranial mesenteric artery and the paired adrenal arteries). The renal veins drain directly into the caudal vena cava. The adrenal glands are located medial to the kidneys and cranial to the renal vessels.

The morphology of mammalian kidneys varies considerably by species (see Table 10.1 and Figure 10.1). All mammalian kidneys are composed of an outer cortex (site of filtrate production in the glomeruli) and an inner medulla (site of filtrate processing and concentration in the tubules). The tubules are arranged into groups (pyramids) which terminate in the renal pelvis at the renal papilla, which channels the processed filtrate into a cup-shaped region of the renal pelvis (calyx). In general, the larger the animal, the more complex the kidney. In small animals, like rodents and rabbits, each kidney has just one pyramid with one papilla, covered by a single layer of cortex (unilobular, unipyramidal). Many larger animals, like horses, camelids, dogs, cats, and small ruminants, have kidneys with multiple pyramids covered by a single layer of cortex, but the papillae are fused together to form a renal crest (unilobular with renal crest). Pigs and cattle have kidneys composed of multiple, discrete pyramids of medullary tissue, each with a separate papilla and each covered by a distinct layer of cortex (multilobular, multipyramidal). Cat kidneys are further distinguished by color (yellow to tan rather than the reddish-brown kidneys of other species) and the prominent subcapsular veins on the surface of the kidneys. In most species, the left and right kidneys have a similar shape, but in horses, the right kidney is heart shaped while the left is bean shaped.

The ureters run along the psoas muscle before entering the bladder. In males, the ureters cross the ductus deferens prior to connecting with the bladder. Before opening into the lumen of the bladder, the ureters continue a short distance within the smooth muscle of the wall in the trigone area. The bladder varies greatly in size, and the wall varies in thickness, depending on the degree of distension by urine.

The components of the reproductive tract vary depending on the species, sex, and whether the animal has been neutered. For intact females, the tract consists of ovaries, oviducts, uterine horns +/– body, cervix/cervices, vagina, and vulva. In spayed females, the ovaries and uterus have been removed, leaving only a uterine stump. Intact male reproductive tracts include testes, epididymides, ductus (or vas) deferens, and accessory sex glands (vary by species; see Table 10.2). In neutered males, the testes and epididymides are absent.

The ovaries are located at the caudal poles of the kidneys. They are often encased in adipose tissue. In spayed animals, the remnant of the ovarian pedicle and possibly suture material can usually be found at this location. The size of the ovaries varies based on the stage of the reproductive cycle. The normal follicular size for different species is listed in Table 10.3. The morphology of the uterus varies somewhat by species, with most domestic mammalian species having two uterine horns connecting to a uterine body which terminates in the cervix (bicornuate uterus; see Figure 10.2). Rabbits have two cervices and no uterine body (duplex uterus; see Figure 10.2), while rodents have separate uterine horns, with no uterine body, but a single cervix (bipartite uterus). The cervix is often not apparent from an external view of the reproductive tract but can be palpated as a thick band of smooth muscle at the junction between the uterus and the vagina.

In males, the testes descend into the scrotum before (ruminants, pigs) or after (carnivores, equids, camelids, rodents, and rabbits) birth. Rodents and rabbits can retract their testes into the abdominal cavity when they are stressed or cold. In young or cryptorchid animals, the testes may be located anywhere from the caudal aspect of the kidney to the inguinal canal. The size of the testes is not proportional to body size, and the orientation varies so that the long axis may be vertical (ruminants), horizontal (horses and dogs), or tilted (cats and pigs). Each testis has a thick capsule (tunica albuginea) to which the head of the epididymis is attached. The epididymis contains a single, convoluted duct which is continuous with the ductus deferens. The ductus deferens travels, along with the testicular vessels, in the spermatic cord, through the internal inguinal ring before joining the urethra via the ejaculatory duct.

Accessory sex glands vary by species (see Table 10.3). The prostate gland, located at the base of the bladder, is present in all domestic mammals, although the shape varies from spherical (dog and tom cat) to ring shaped (bull) to butterfly shaped (stallion). The prostate is the only accessory sex gland in the ferret. In general, the ampullary glands (not present in tom cats or boars) are dilations of the ductus deferens, located medially where the ductus deferens enter the urethra. The vesicular glands (also known as seminal vesicles or paraprostate; absent in carnivores and camelids) and coagulating glands (also known as proprostate; only found in rodents and rabbits) are located more laterally and are sac-like structures. The bulbourethral glands are found in all domestic mammals except the dog and are more caudal than the other glands, usually found near the arch of the ischium. The bulbourethral glands are most prominent in the stallion and boar. Figure 10.3 shows the location of accessory sex glands in a ram.

Mammalian penises are classified as fibroelastic or musculovascular. Fibrovascular penises contain more connective tissue and less erectile tissue, while musculovascular penises contain abundant erectile tissue and little connective tissue. Musculovascular penises may (dogs, most cats, and rodents) or may not (pigs, rabbits) contain a bony os penis.