The urinary system has two major functions

  • it removes waste products from the blood stream
  • it regulates the amount of water present in the body

    In mammals, the kidneys are ventral to the vertebral column in the anterior lumbar region. The kidneys of pigs and sheep are oval in shape while the kidneys of cattle are each divided into approximately 20 lobules, as shown below.

    Pork kidneys are not lobulated, are flat, and are usually pale (not as dark as the image shown below with its scale in good old-fashioned inches).

    In healthy well-fed animals, the kidneys are usually surrounded by perirenal fat, which is called leaf fat in the pork carcass.

    As shown in the image above, where a pork kidney has been sliced longitudinally, each kidney has a depression or hilum where the renal artery enters the kidney, and where the renal vein and ureter leave the kidney (Figure 1-17). The ureter from each kidney carries urine to the bladder. In the image below, the large pale bladder on the left of the screen was taken at slaughter from a boar.

    Although sometimes difficult to see, when a kidney is cut open, a pale inner medulla may be seen surrounded by a dark red cortex (Figure 1-18). The wide entrance to the ureter is called the pelvis of the kidney. Running through the medulla, towards the pelvis of the kidney, are many small collecting tubules. Each of these terminates at a small conical mound called the pyramid, so that the pyramids project into the pelvis of the kidney. Urine is produced from the blood by a functional unit of the kidney called a nephron. There are large numbers of nephrons in each kidney. Urine leaves the bladder in a single tube, the urethra, that runs to the penis or to the vagina.

    In poultry, the kidneys are pressed closely against the ventral surface of the vertebral column, posterior to the lungs. Urine from each kidney leaves in a ureter but passes directly to the cloaca (Figure 1-20). Here, the urine rapidly loses water and the main component of nitrogenous excretion for poultry, uric acid, is precipitated as a sludge.