Left kidney

The left kidney is a paired, retroperitoneal organ responsible for filtering blood, producing urine, and maintaining electrolyte balance. In most domestic mammals, the left kidney lies more caudal than the right one and is often more mobile, especially in ruminants due to the displacement of the gastrointestinal tract.

Position by Species:

• Dog: The left kidney lies approximately from the second to the fourth lumbar vertebrae. It is more mobile than the right and can be displaced during respiration or distention of abdominal organs.

• Cat: Similar to the dog but generally positioned slightly more caudally. The feline kidney is also relatively mobile.

• Horse: The left kidney is located ventral to the last rib and first two or three lumbar vertebrae, and it is more flattened and bean-shaped. The renal hilus is medially oriented.

• Ox (Cattle): The left kidney is displaced to the right side by the rumen. It sits to the right of the midline and is often found near the 2nd to 5th lumbar vertebrae.

• Small Ruminants (Sheep/Goat): Similar to the ox but not as dramatically displaced. The kidney is often situated under the transverse processes of L3-L5.

• Pig: Both kidneys are relatively symmetrical and flattened, with the left kidney lying from the last thoracic to the 4th lumbar vertebrae.

• Rabbit: Kidneys are elongated and smooth, with the left positioned slightly more caudally than the right.

External Morphology

• Shape: Generally bean-shaped in carnivores and horses; lobulated in cattle and smooth in small ruminants and pigs.

• Color: Typically reddish-brown.

• Surfaces: The kidney has a dorsal and ventral surface, cranial and caudal poles, and medial and lateral borders.

• Hilus: Located medially, through which enter and exit the renal artery, vein, lymphatics, nerves, and ureter.

Internal Structure

• Renal Cortex: The outer region, rich in glomeruli and convoluted tubules, giving it a granular appearance.

• Renal Medulla:

  • Unipyramidal (unilobar): Seen in carnivores, small ruminants, and horses. The medulla is fused into a single mass, draining into a single renal crest.

  • Multipyramidal (multilobar): Observed in pigs and cattle. In pigs, the pyramids are fused only at the base; in cattle, each pyramid drains into its own calyx.

• Renal Pelvis: A funnel-shaped structure that collects urine from the papillae. Well-developed in species with unipyramidal kidneys.

• Ureter: Emerges from the hilus and conducts urine to the urinary bladder.

Blood Supply and Innervation

• Arterial Supply: Via the renal artery, a direct branch of the abdominal aorta. It enters at the hilus and divides into segmental, interlobar, arcuate, and interlobular arteries.

• Venous Drainage: Through the renal vein, which drains into the caudal vena cava.

• Lymphatic Drainage: Renal lymph nodes.

• Innervation: Autonomic nerves from the celiac and cranial mesenteric plexuses.

Histology (Common to All Species)

• The nephron, the functional unit, comprises:

  • Renal corpuscle (Bowman's capsule + glomerulus)

  • Proximal and distal convoluted tubules

  • Loop of Henle

  • Collecting ducts