Renal

“Renal” refers to anything relating to the kidneys (from Latin ren, “kidney”). The kidneys are paired organs located in the retroperitoneal space of the abdomen, responsible for maintaining the body’s internal environment through filtration, excretion, and regulation.

Key Renal Functions

Filtration and Excretion

The kidneys filter approximately 25% of cardiac output, producing an ultrafiltrate of blood in the glomeruli. From this filtrate, the tubular system reabsorbs what the body needs (water, glucose, amino acids, electrolytes) and excretes what it does not (urea, creatinine, toxins, drug metabolites). The final product is urine.

Fluid and Electrolyte Balance

The kidneys regulate body water volume, sodium, potassium, calcium, phosphorus, and acid-base balance through precise adjustment of what is reabsorbed versus excreted. This regulatory function is why kidney disease disrupts so many body systems simultaneously.

Hormone Production

• Erythropoietin (EPO): stimulates red blood cell production in the bone marrow. Renal failure leads to decreased EPO and non-regenerative anemia.
• Active vitamin D (calcitriol): the kidneys perform the final activation step of vitamin D, which is essential for calcium absorption and bone health. Loss of this function contributes to renal secondary hyperparathyroidism.
• Renin: the kidneys produce renin, which initiates the renin-angiotensin-aldosterone system (RAAS) for blood pressure regulation.

Renal Function Markers

• Creatinine: waste product of muscle metabolism filtered by the kidneys. Elevation indicates reduced glomerular filtration rate (GFR). Does not rise above normal range until approximately 75% of renal function is lost.
• SDMA: symmetric dimethylarginine, a more sensitive renal marker that detects kidney function loss earlier than creatinine — typically when 25-40% of function is lost.
• Blood urea nitrogen (BUN): influenced by diet, hydration, and renal function. Less specific than creatinine.
• Urinalysis: urine specific gravity (USG) assesses concentrating ability. Dilute urine (USG < 1.030 in dogs) suggests impaired renal concentrating function. Proteinuria indicates glomerular damage.
• Urine protein-to-creatinine ratio (UPC): quantifies protein loss in urine

Chronic Kidney Disease

Chronic kidney disease (CKD) is one of the most common chronic diseases in aging dogs. It is staged using the International Renal Interest Society (IRIS) system based on creatinine or SDMA, substaged by proteinuria and blood pressure. Because renal function declines silently before clinical signs appear, routine screening blood work and urinalysis in senior dogs is one of the most effective early-detection strategies.