Kidneys are paired organs situated high up against the roof of the abdomen, and in most animals lying one on either side of the spinal column.


The kidneys of the horse differ from each other in both shape and position. The right has the outline of a playing-card heart, and lies under the last 2 or 3 ribs and the transverse process of the first lumbar vertebra, while the left is roughly bean-shaped and lies under the last rib and the first 2 or 3 lumbar transverse processes. They are held in place by the surrounding organs and by fibrous tissue, called the renal fascia. Each of them moves slightly backwards and forwards during the respiratory movements of the animal.


The kidneys are lobulated, each possessing from 20 to 25 lobes separated by fissures filled with fat in the living animal. The right kidney lies below the last rib and the first 2 or 3 lumbar transverse processes, and is somewhat elliptical in outline. The left occupies a variable position. When the rumen is full, it pushes the left kidney over to the right side of the body into a position slightly below and behind the right organ, but when it is empty the left kidney lies underneath the vertebral column about the level of the third to the fifth lumbar vertebra. It may lie partly on the left side of the body in this position in some cases.


In the sheep the kidneys are bean-shaped and smooth. In position they resemble those of the ox, except that the right is usually a little farther back.


In this animal the kidneys are shaped like elongated beans, and they are placed almost symmetrically on either side of the bodies of the first 4 lumbar vertebrae. They sometimes vary in position.

Dogs and Cats

In these animals the kidneys are again bean-shaped, but they are thicker than in other animals, and relatively larger. As in most animals, the right kidney is placed farther forward than the left, the latter varying in position according to the degree of fullness of the digestive organs. In the cat the left kidney is very loosely attached and can usually be felt as a rounded mass which is quite movable in the anterior part of the abdominal cavity.


Birds have paired kidneys, seen as elongated brown organs closely attached on each side of the vertebrae.


Fish have a single kidney which is seen in salmonids as a long black strucure in the dorsal part of the abdomen extending from the back of the head to the vent. The vena cava runs through the centre of the organ. The kidney also has a role in the development of blood cells and in combating infection.


The organ is enveloped in a fibrous coat continuous with the rest of the peritoneal membrane, and attached to the kidney capsule. This capsule does not permit of much swelling or enlargement of the organ, and consequently any inflammation of the kidney is attended with much pain. On the inner border there is an indentation called the hilus, which acts as a place of entrance and exit for vessels, nerves, etc. Entering each kidney at its hilus are a renal artery and renal nerves; leaving the kidney are renal vein or veins, lymphatics, and the ureter. If the kidney is cut across, there are 2 distinct areas seen in its substance. Lying outermost is the reddish-brown granular cortex, which contains small dark spots known as Malpighian corpuscles.

Within the cortex is the medulla, an area presenting a radiated appearance, whose periphery is of a deep red colour.

The kidney tissue contains many thousands of filtration units called nephrons. Each of these comprises the glomerulus (almost a spherical arrangement of capillaries on an arteriole); Bowman’s capsule, the blind end of a proximal tubule which expands so as almost to surround the glomerulus; the convoluted tubule itself (with its loop of Henle); and the distal convoluted tubule which leads on to an arched collecting tubule. The latter continues with a straight tubule in the cortex of the kidney, and on into the medulla, where papillary ducts are formed to take the urine to the pelvis of the kidney.

The Malpighian corpuscle, comprising the glomerulus and inner and outer layers of Bowman’s capsule, is where most of the filtration of fluid from the blood occurs; but only a small percentage of this fluid is finally excreted as urine.

In birds, the glomeruli are of 2 different kinds; 1 type is similar to mammalian glomeruli; the other is more akin to the type found in reptiles.


The kidney’s 2 main functions are:first, the excretion of waste (and excess) materials from the bloodstream; and, second, the maintenance of the correct proportions of water in the blood, the correct levels of its chemical constituents, and the correct pH. (See HOMEOSTASIS.)

Blood pressure in the arteries determines pressure in each glomerulus and has an important bearing on the quantity of fluid filtered from the blood.

For its controlling effect on the kidney, see ANTIDIURETIC HORMONE.

The proximal tubules reabsorb a high percentage of the water, sodium chloride and bicarbonate. The distal tubules reabsorb sodium, or exchange sodium ions for hydrogen, potassium or ammonium ions; determining thereby the pH of the urine.

The kidney also secretes the hormone erythropoietin (see under ERYTHROPOIESIS) and produces RENIN. Additionally, the kidney converts vitamin D1 into its active form.