Renal refers to the kidneys and all physiological processes centered on them — filtration of blood, excretion of metabolic waste, regulation of fluid and electrolyte balance, and production of urine. In the horse, both kidneys sit retroperitoneally: the right is heart-shaped and lies in contact with the liver under the last few ribs; the left is bean-shaped and sits slightly more caudal, near the spleen. Together they filter roughly 1,000 liters of blood per day in a 500 kg horse, producing 5-15 liters of urine depending on water intake and work.
Equine renal function draws particular clinical attention because the horse’s urinary concentration capacity is modest compared to many mammals, making adequate daily water intake (25-55 liters at rest, more in work or heat) directly tied to kidney health. Horses on high-protein diets, those heavily medicated with NSAIDs such as phenylbutazone or flunixin, and horses in severe systemic illness are at elevated risk of renal compromise: NSAIDs reduce prostaglandin-mediated blood flow to the kidneys, and sustained reduction causes papillary necrosis or acute tubular injury.
The two most clinically significant renal diagnoses in horses are acute kidney injury (AKI) and chronic kidney disease (CKD). AKI presents with oliguria or anuria, azotemia, and often electrolyte disturbances; common causes include nephrotoxic drug exposure, hemolytic crises, severe dehydration, and oxalate toxicosis from plants such as halogeton or sorrel. CKD is more insidious — gradual loss of functional nephron mass, often detected only at an advanced stage because horses compensate well; weight loss, polyuria-polydipsia, and poor coat condition are the presenting signs. Diagnosis in both cases relies on serum creatinine, blood urea nitrogen, urinalysis including specific gravity and fractional excretion of electrolytes, and renal ultrasound to assess kidney size and architecture. Unlike in small animals, renal biopsy in horses carries higher complication risk and is reserved for cases where histopathology will change management.
Prognosis for equine renal disease depends heavily on cause and chronicity: horses recovering from AKI with prompt supportive care (IV fluids, removal of the offending agent) can return to full work; CKD in advanced stages carries a guarded-to-poor long-term prognosis, and affected horses are typically retired and managed on low-protein hay to reduce nitrogenous load.