How Do a Cat’s Kidneys Work? Anatomy and Physiology

While often thought of as simple filters, the feline kidneys are sophisticated, multitasking organs essential for maintaining homeostasis, the state of steady internal, physical, and chemical conditions maintained by living systems. Located in the retroperitoneal space, behind the abdominal cavity lining, under the lumbar spine, these bean shaped organs perform complex chemical balancing acts every second of your cat's life.

To understand Chronic Kidney Disease (CKD), it is vital to understand the physiology of a healthy kidney. The kidney's functions can be categorized into three main areas: excretion of waste, regulation of fluids and minerals, and endocrine or hormone production.

The Functional Unit: The Nephron

The workhorse of the kidney is the nephron. A healthy cat has approximately 200,000 to 500,000 nephrons per kidney. Each nephron consists of two main parts:

  • The Glomerulus: A microscopic ball of capillaries, or tiny blood vessels, that acts as a sieve. Blood flows through the glomerulus under pressure, and water and small molecules are pushed out into a collection tube. This initial fluid is called the ultrafiltrate.

  • The Tubules: A winding tube attached to the glomerulus. As the ultrafiltrate flows through the tubules, the kidney cells reabsorb nearly 99% of the water and essential nutrients like glucose and amino acids back into the bloodstream. They also secrete specific waste products into the urine.

Excretion of Metabolic Waste

The most well known function of the kidney is the removal of metabolic waste products from the blood. These wastes are the byproducts of normal cellular functions and protein metabolism.

  • Urea (measured as BUN): Produced by the liver during the breakdown of proteins.

  • Creatinine: A waste product generated by muscle metabolism. Because it is produced at a constant rate, it is the primary marker used to estimate Glomerular Filtration Rate (GFR).

  • Uremic Toxins: Various other compounds that, if allowed to accumulate, lead to a toxic condition called uremia or azotemia.

In a healthy kidney, these toxins are filtered at the glomerulus and excreted in the urine. In CKD, the GFR drops, meaning the cleaning speed of the blood slows down, leading to the accumulation of these toxins in the bloodstream.

Water Conservation and Osmoregulation

Cats evolved as desert animals, and their kidneys are uniquely adapted to conserve water. This process is controlled by the Loop of Henle, a specific section of the nephron tubule, and the hormone Vasopressin (also known as Antidiuretic Hormone or ADH).

When a cat is dehydrated, the brain releases Vasopressin, which tells the kidney tubules to become permeable to water. The water is drawn out of the urine and back into the blood, resulting in a small volume of highly concentrated urine.

Veterinarians measure this concentrating ability using Urine Specific Gravity (USG).

  • Healthy USG: typically greater than 1.035 (highly concentrated).

  • Isosthenuria: A range of 1.008 to 1.012. This indicates the kidneys are doing nothing to the urine; it has the same concentration as plasma. This loss of concentrating ability is often the very first clinical sign of kidney insufficiency, occurring before blood values rise.

Electrolyte and Acid Base Balance

The kidneys are the ultimate chemists of the body, strictly regulating the concentration of electrolytes, or charged minerals, in the blood.

  • Potassium (K+): Essential for nerve function and cardiac rhythm. The kidneys excrete excess potassium. In CKD, this mechanism can fail, leading to hypokalemia (low potassium), which causes muscle weakness and cervical ventroflexion (inability to lift the head).

  • Phosphorus: Critical for energy and bone structure. The kidneys are the primary route for phosphorus excretion. As GFR declines, phosphorus builds up in the blood (hyperphosphatemia), which makes the cat feel ill and drives further kidney damage.

  • Acid Base Balance: The body's pH must be kept within a narrow range of 7.35 to 7.45. The kidneys help maintain this by excreting hydrogen ions (acid) and reabsorbing bicarbonate (a buffer). Failing kidneys can lead to metabolic acidosis, where the blood becomes too acidic.

Endocrine Functions: The Kidney as a Gland

Beyond filtering, the kidneys produce hormones that influence other body systems.

  • Erythropoietin (EPO): This hormone stimulates the bone marrow to produce red blood cells (RBCs). Specialized cells in the kidney detect low oxygen levels and release EPO in response. In advanced CKD, these cells are damaged, EPO production drops, and the cat develops non regenerative anemia.

  • Renin: An enzyme involved in the Renin Angiotensin Aldosterone System (RAAS). This system regulates blood pressure. When kidneys perceive low blood flow (common in scarred CKD kidneys), they release Renin to constrict blood vessels and raise blood pressure. Chronic over activation of this system leads to systemic hypertension, which can cause retinal detachment (blindness) and strokes.

  • Calcitriol (Active Vitamin D): The kidneys convert Vitamin D into its active form, Calcitriol. This hormone helps the gut absorb calcium. When this fails, calcium balance is disrupted, triggering the parathyroid glands to strip calcium from the bones, a condition known as Renal Secondary Hyperparathyroidism.

Summary

The feline kidney is a complex organ responsible for filtration, water retention, electrolyte balance, pH regulation, and hormone production. When a cat is diagnosed with CKD, it is rarely just a filtration problem; it is a multisystem breakdown affecting blood pressure, bone health, hydration, and red blood cell production. Understanding these mechanisms helps owners appreciate why comprehensive management, fluids, diet, binders, and blood pressure meds, is necessary.

Cited Research

  • Sparkes, A. H., et al. (2016). ISFM Consensus Guidelines on the Diagnosis and Management of Feline Chronic Kidney Disease. Journal of Feline Medicine and Surgery, 18(3), 219 to 239.

  • DiBartola, S. P. (2012). Fluid, Electrolyte, and Acid Base Disorders in Small Animal Practice (4th ed.). Elsevier.

  • Guyton, A. C., and Hall, J. E. (2020). Textbook of Medical Physiology (14th ed.). Elsevier.

  • Brown, S. A. (2016). Evaluation of the Kidney in Dogs and Cats. Veterinary Clinics: Small Animal Practice, 46(6).

  • Jepson, R. E. (2011). Feline Systemic Hypertension: Classification and Pathogenesis. Journal of Feline Medicine and Surgery, 13(5), 311 to 318.