The horse has two kidneys, paired retroperitoneal organs responsible for filtering metabolic waste from the blood, regulating fluid and electrolyte balance, and producing urine. In anatomy they differ from most mammals: the right kidney is heart-shaped and the left kidney is bean-shaped, a distinction unique to Equidae and a landmark in equine anatomical study.
Anatomy and Location
Each kidney lies against the dorsal abdominal wall, embedded in perirenal fat. The right kidney sits in contact with the base of the cecum and the right lobe of the liver; the left sits more caudally and freely, in proximity to the spleen. Because the left kidney is more mobile and palpable per rectum, rectal palpation of the left kidney is a standard component of the clinical examination for urinary tract disease and abdominal pain.
Equine kidneys are large relative to body mass (roughly 680 grams for the right, 560 grams for the left in a mature light-breed horse) and handle unusually high urine volumes. Horses produce between 5 and 15 liters of urine per day, and normal equine urine is characteristically cloudy and mucoid, high in calcium carbonate crystals, which owners unfamiliar with the species often mistake for pathology. The mucoid character results from mucus secreted by glands in the renal pelvis and is normal.
Disease
Chronic renal failure (CRF) is the dominant kidney disease in the horse. It develops insidiously; horses compensate well until roughly 75 percent of nephron mass is lost, so clinical signs (weight loss, ventral edema, polyuria, polydipsia, and oral ulceration from uremia) typically appear late. Causes include nephrotoxic drugs (notably aminoglycosides, NSAIDs in excessive doses, and certain plant toxins), glomerulonephritis secondary to systemic infection, and pyelonephritis. Acute kidney injury (AKI) is less common but recognized after severe dehydration, hemodynamic shock, or iatrogenic nephrotoxin exposure. Serum creatinine and blood urea nitrogen are the standard biochemical markers; creatinine above 2 mg/dL is considered elevated in the horse.