There are two kidneys lying in the cranial abdominal cavity, one on each side of the midline ventral to the lumbar hypaxial muscles (Fig. 10.1). Each kidney is closely attached to the lumbar muscles by a covering of parietal peritoneum. There is no mesenteric attachment, as seen in other abdominal organs, and the kidney is described as being retroperitoneal. The right kidney lies slightly cranial to the left because the stomach has evolved to lie on the left side of the abdomen, pushing the left kidney out of position. Lying close to the cranial pole of each kidney are the ovaries of the female and the adrenal glands (Fig. 10.2).

Fig. 10.1 The position of the urinary system in the bitch.

Fig. 10.2 Ventrodorsal view of the urogenital system of the bitch.

Macroscopic structure

The kidneys of the cat and dog have a characteristic bean shape and the indented area is known as the hilus. This is the point at which blood vessels, nerves and the ureters enter and leave the kidney. The kidneys are normally a deep reddish-brown but the colour may be affected by any substance filtering through them. On a lateral radiograph of the abdomen, a normal kidney can be seen to be equivalent in size to approximately 2.5 vertebrae (Fig. 10.3). The outer surface may be surrounded by a layer of fat, which acts as an energy reserve and protects the kidney from external damage.

Fig. 10.3 Radiograph of the kidney area. The bladder has been inflated with air for contrast and the kidney (arrows) can be seen to measure about 2.5 times the length of the lumbar vertebrae.

When examining the cut surface of a normal kidney cut longitudinally, it is possible to see four layers (Fig. 10.4). From the outside inwards these are:

1 Capsule – protective layer of irregular dense fibrous connective tissue closely attached to the cortex. This can be easily peeled away from a healthy kidney but any adhesions may indicate previous infection or damage

2 Cortex – dark red outermost layer of the kidney containing the renal corpuscles and convoluted tubules of the nephrons

3 Medulla – slightly paler than the cortex and it may be possible to see the triangular-shaped pyramids, which contain the collecting ducts and, between them, tissue containing the loops of Henle of the nephrons

4 Pelvis – this is basin-shaped and made of fibrous connective tissue which gives it a whitish appearance. Urine formed by the nephrons drains into the pelvis and out of the kidney via a single ureter.

Fig. 10.4 Parasagittal section through the dog kidney.

Blood supply

Arterial blood is carried from the aorta in a single renal artery to each kidney (Fig. 10.4). This carries 20% of cardiac output. Within the tissue of the kidney, the renal artery divides into several interlobar arteries, which pass between the renal pyramids and into the cortex. Here capillaries supply the renal tubules and also give off numerous capillary networks known as glomeruli (sing. glomerulus) (Fig. 10.5). Each glomerulus supplies an individual nephron. The capillaries then recombine to form interlobar veins, which enter the single renal vein. This carries venous blood to the caudal vena cava.

Fig. 10.5 A renal nephron.

Blood entering the kidney carries oxygen, nutrients and waste products from the tissues of the body; blood leaving the kidney carries carbon dioxide produced by the kidney tissues, but the nitrogenous waste products have been removed via the glomeruli.

Microscopic structure

The functional unit of the kidney is the nephron (Fig. 10.5). Each kidney contains about a million nephrons, which are closely packed together. They are responsible for the filtration of blood and the production of urine. Each nephron is a long tubule divided into several parts:

• Glomerular capsule – a cup-shaped structure enclosing a network of blood capillaries called the glomerulus (Fig. 10.6). The capsule may also be known as Bowman’s capsule. The capsule and the glomerulus form the renal corpuscle. The basement membrane of the inner surface of the capsule, which is in close contact with the endothelium of the glomerular capillaries, is lined by podocyte cells between which are tiny pores (Fig 10.6). These pores are of such a size that they will allow the passage of fluid and small molecules, but restrict the passage of larger molecules. The outer surface of Bowman’s capsule is continuous with the epithelium of the proximal convoluted tubule. Fluid filtered by the capsule drains into the space between the two layers and continues into the next part of the nephron.

• Proximal convoluted tubule – a long, twisted tube leading from the neck of the capsule and lying in the renal cortex (Fig. 10.5). The tubules are lined in simple cuboidal or columnar epithelium. The side of the epithelium directed towards the lumen of the tubule is lined by fine microvilli forming a ‘brush border’. This increases the surface area for the reabsorption of water and electrolytes.

• Loop of Henle – a U-shaped part of the tube leading from the proximal convoluted tubule and dipping down into the renal medulla. The tubule is lined in simple squamous epithelium which is thicker in the ascending loop than in the descending loop (Fig. 10.5).

• Distal convoluted tubule – a short but less twisted part of the tube than the proximal convoluted tubule. It lies in the renal cortex and is lined in cuboidal epithelium without a brush border.

• Collecting duct – each duct receives urine from several nephrons and conducts it through the pyramids into the renal pelvis. The ducts are lined in columnar epithelium.