Equine Renal Physiology: Urine Concentration and Acid-Base Balance

By Dr. Zubair Khalid, DVM, MS, PhD · 2026-08-11

Key Takeaways

• The equine kidney exhibits a wide urine osmolality range (1500-1800 mOsm/kg maximum) due to well-developed loops of Henle, but this capacity is less than many carnivores, limiting water conservation under stress.
• Horses have a high resting renal calcium excretion, a physiological adaptation linked to their high blood calcium, high intestinal absorption, and low vitamin D, predisposing them to calcium-containing sediment and uroliths.
• Renal blood flow autoregulation in horses relies on afferent/efferent arteriolar tone and tubuloglomerular feedback, maintaining glomerular filtration across moderate arterial pressure changes, but becomes pressure-dependent below autoregulatory limits.
• Acid-base balance is regulated by proximal tubule bicarbonate reclamation (>80% of filtered load) and distal nephron bicarbonate regeneration via titratable acid and ammonium excretion, with full metabolic acidosis compensation taking several days.
• Urine specific gravity below 1.020 in a clinically dehydrated horse indicates impaired concentrating ability, while persistently acidic urine (below 6.0) in the presence of systemic acidaemia suggests renal tubular acidosis.
• Fractional excretion of sodium (FE Na) above 1% in a horse with normal renal perfusion suggests tubular dysfunction, and serial FE Na measurements are crucial for distinguishing transient tubular strain from established injury.

This article examines the mechanisms by which the equine kidney concentrates urine and regulates acid-base balance. It is written for veterinary students who already understand nephron anatomy and basic renal terminology, and it provides the physiological foundation needed before approaching clinical assessments of renal function. The focus is on normal physiology, not on renal disease or its management.

The horse presents particular challenges in renal physiology. Its large body mass, high fluid turnover, and herbivorous diet demand a kidney capable of wide swings in urine osmolality and substantial excretion of calcium and other filtered loads. Understanding these mechanisms allows the clinician to interpret laboratory findings, anticipate responses to fluid therapy, and recognize when compensatory physiology is being pushed beyond its limits. Several conditions can directly or indirectly affect renal function on a temporary or permanent basis, and endogenous and exogenous compounds can promote or exacerbate renal injury when renal blood flow is compromised, as reviewed in equine renal anatomy, physiology, and mechanisms of acute kidney injury.

Renal blood flow in the horse is regulated by both extrinsic and intrinsic systems. Intrinsic regulation occurs through the afferent and efferent arterioles and tubuloglomerular feedback mechanisms with activation of the juxtaglomerular apparatus, as described in the same review of equine renal physiology. This autoregulatory capacity maintains glomerular filtration across a range of arterial pressures, but it has limits. When mean arterial pressure falls below the autoregulatory range, or when intrinsic vasoregulation is impaired by drugs, toxins, or sepsis, renal blood flow becomes pressure-dependent and filtration declines. The equine kidney is also unusual among domestic mammals in its high resting renal excretion of calcium, a feature tied to the horse's distinctive calcium economy of high blood calcium, high intestinal calcium absorption, and low vitamin D concentrations, as outlined in studies of calcium-transporting genes in horses.

At a Glance

Glomerular Filtration and Tubular Processing

The glomerulus filters plasma at a rate determined by the balance of hydrostatic and oncotic pressures across the filtration barrier, modified by the ultrafiltration coefficient. In the horse, as in other species, the filtered load of any solute equals the product of GFR and plasma concentration. Tubular reabsorption then determines net excretion. For water and most electrolytes, reabsorption is extensive, and the final urine composition reflects fine adjustments made in the distal nephron.

The proximal tubule reabsorbs the bulk of filtered sodium, chloride, bicarbonate, glucose, amino acids, and water. This segment is highly metabolically active and therefore vulnerable to ischemic injury. The loop of Henle establishes the medullary interstitial osmotic gradient through countercurrent multiplication, driven by active sodium-potassium-chloride cotransport in the thick ascending limb. This segment is impermeable to water, so the fluid delivered to the distal nephron is hypotonic relative to plasma. The collecting duct then responds to antidiuretic hormone by inserting aquaporin channels into its apical membrane, allowing water to move down the osmotic gradient into the hypertonic medullary interstitium.

Urine Concentration in the Horse

The horse's ability to concentrate urine depends on the length of the loops of Henle and the efficiency of urea recycling in the medullary collecting ducts. Horses produce urine that is typically less concentrated than that of dogs or cats, with maximal osmolality in the range of 1500 to 1800 mOsm/kg after prolonged water deprivation. This reflects a medulla that is less developed than in desert-adapted species. The practical consequence is that horses require more water per unit of solute excreted when dehydrated, and they may not concentrate urine as rapidly as a clinician expects after fluid restriction.

Antidiuretic hormone release is triggered by increased plasma osmolality and by hypovolemia via baroreceptor pathways. In the horse, the osmotic threshold for ADH release is similar to that in other mammals, but the response curve is comparatively shallow. The collecting duct response to ADH is modulated by the local urea concentration, which enhances the osmotic gradient in the inner medulla. When medullary urea is depleted, as can occur with chronic low-protein diets or aggressive diuresis, concentrating ability declines even with adequate ADH.

Acid-Base Regulation

The kidney regulates acid-base balance through three principal mechanisms: bicarbonate reclamation in the proximal tubule, bicarbonate regeneration through titratable acid excretion, and ammonium excretion. Bicarbonate reclamation is nearly complete under normal conditions, with the proximal tubule recovering more than 80 percent of the filtered load. This process is coupled to sodium reabsorption and requires carbonic anhydrase on the luminal brush border. When bicarbonate reclamation is overwhelmed, as in metabolic alkalosis or after carbonic anhydrase inhibition, bicarbonate appears in the urine and acts as a buffer.

Bicarbonate regeneration occurs in the distal nephron, where hydrogen ions are secreted by intercalated cells. Each hydrogen ion secreted titrates urinary buffers, primarily phosphate, generating titratable acid. Ammonium excretion provides an additional pathway for hydrogen ion elimination that can be upregulated substantially in response to chronic acidosis. The equine kidney responds to acid loads more slowly than the respiratory system, and full renal compensation for a metabolic acidosis may take several days. Donkeys share these mechanisms but differ in some endocrine and metabolic responses, and their reference ranges for many analytes require species-specific interpretation, as discussed in metabolic and endocrine insights in donkeys.

Calcium Handling and Its Intersection with Acid-Base Status

Renal calcium handling in the horse is distinctive. Epithelial calcium transport occurs by paracellular and transcellular mechanisms, with transcellular transport involving transient receptor potential vanilloid channels, calbindins, the sodium-calcium exchanger, and plasma membrane calcium ATPase, as detailed in comparative analysis of calcium-transporting genes in horses. In the equine kidney, calbindin D28k expression is greatest, and plasma membrane calcium ATPase expression is higher than the sodium-calcium exchanger. This molecular profile supports the horse's high renal calcium excretion.

Acid-base status influences calcium excretion. Chronic metabolic acidosis increases urinary calcium loss by reducing tubular calcium reabsorption, while alkalosis has the opposite effect. In horses, this interaction is clinically relevant because diets high in anions are sometimes used to manage calcium metabolism, and the resulting acidification increases calcium excretion. The clinician should anticipate that any maneuve that alters systemic acid-base balance will also alter urinary calcium concentration and therefore the risk of calcium salt precipitation.

Integration with Systemic Physiology

The kidney does not act in isolation. Renal blood flow, glomerular filtration, tubular transport, and endocrine signaling are integrated with cardiovascular status, respiratory function, and gastrointestinal losses. In the horse, the large hindgut serves as a reservoir of fluid and electrolytes that can buffer acute losses, but this reservoir also means that renal responses to dehydration are slower than in species with smaller gastrointestinal fluid stores. The kidney's capacity to regulate acid-base balance is ultimately limited by the availability of substrates for ammoniagenesis and by the medullary gradient that determines concentrating ability. When these limits are approached, the clinician must recognize that laboratory values reflect the integrated response of the whole animal, not a single tubular process.

Applied Assessment of Urinary Concentration and Acid-Base Status

Indications for Renal Function Testing

Renal concentrating capacity and acid-base handling are assessed when historical or physical findings suggest impaired tubular function. Common triggers include polyuria with polydipsia, unexplained metabolic acidosis, poor performance with electrolyte abnormalities, or administration of potentially nephrotoxic drugs. The clinician should also evaluate renal function before general anesthesia in horses with suspected systemic disease, since anesthetic agents can reduce renal blood flow and unmask subclinical tubular dysfunction. Relevant equine renal anatomy, physiology, and mechanisms of acute kidney injury emphasizes that acute kidney injury frequently follows changes in renal blood flow, so a baseline assessment of concentrating ability is prudent before procedures that may compromise perfusion.

Urine Collection and Sample Handling

A free-catch midstream sample is adequate for most biochemical and pH determinations. Catheterization is reserved for cases requiring quantitative urine output measurement or when contamination from the vagina or prepuce is suspected. Urine should be collected into a clean container without preservatives. Analysis should occur within 30 minutes of collection for pH and specific gravity, because CO2 loss and bacterial urease activity can alkalinize the sample and alter measured parameters. Refrigeration delays but does not eliminate these changes.

Urine Parameters and Their Interpretation

The following table summarizes the key urine parameters used to assess concentrating ability and acid-base status in horses. Reference ranges reflect healthy adult horses at maintenance.

Urine specific gravity is the most practical field test for concentrating ability. A value below 1.020 in a dehydrated horse indicates impaired urine concentration. However, the clinician must confirm dehydration clinically, because a well-hydrated horse may appropriately produce dilute urine. The water deprivation test remains the definitive method for assessing maximal concentrating capacity, but it carries risk in horses with suspected renal disease and should only be performed when the baseline specific gravity is below 1.020 and the horse is otherwise stable.

Water Deprivation Test Protocol

The test requires accurate body weight measurement and access to dry feed. Water is withheld for 24 to 48 hours, with body weight, urine specific gravity, and packed cell volume monitored every 6 hours. The test is stopped when specific gravity exceeds 1.025, when body weight loss reaches 5%, or when the horse becomes clinically dehydrated. A horse that cannot concentrate urine above 1.020 despite 5% weight loss has a concentrating defect. This protocol is contraindicated in horses with azotemia, hypercalcemia, or evidence of dehydration at baseline, because further water restriction may precipitate prerenal or intrinsic renal failure.

Fractional Excretion of Electrolytes

Fractional excretion (FE) of sodium, potassium, and calcium provides a quantitative assessment of tubular handling. The formula is:

FE% = (urine electrolyte / serum electrolyte) x (serum creatinine / urine creatinine) x 100

A paired blood and urine sample is required. The samples should be collected within the same hour. FE of sodium above 1% in a horse with normal renal perfusion suggests tubular dysfunction, although prerenal azotemia can lower FE values and mask tubular injury. FE of calcium is normally higher in horses than in most domestic species, reflecting the horse's unique calcium physiology with high renal calcium excretion and low vitamin D concentrations. Calcium-transporting genes in horses documents that the kidney expresses calbindin D28k most strongly among calcium transport proteins, and plasma membrane calcium ATPase 1 expression is higher in the kidney than in the intestine. These molecular features support the horse's substantial renal calcium flux.

Acid-Base Assessment in the Horse

Venous blood gas analysis with simultaneous urine pH measurement provides the core assessment of acid-base status. The normal equine urine pH is alkaline, typically 7.0 to 8.5, because the herbivorous diet generates a large bicarbonate load. A persistently acidic urine in a horse without systemic acidosis suggests renal tubular acidosis, a condition where the distal nephron fails to excrete hydrogen ions appropriately.

The anion gap helps differentiate causes of metabolic acidosis. A high anion gap acidosis in the horse most commonly results from lactic acidosis, renal failure, or ketoacidosis associated with hyperlipemia. A normal anion gap acidosis suggests bicarbonate loss from the gastrointestinal tract or renal tubular acidosis. Essentials of equine renal and urinary tract physiology notes that the kidney regulates acid-base balance alongside fluid and electrolyte homeostasis, so disruption of tubular function often produces combined abnormalities instead of isolated acid-base defects.

Monitoring Parameters During Treatment

When a horse is receiving fluid therapy to correct dehydration or electrolyte abnormalities, serial monitoring guides adjustment. The following parameters should be reassessed at intervals determined by the severity of the initial derangement:

Urine specific gravity will fall as hydration improves, so the clinician must interpret this parameter in the context of fluid balance. A horse that remains azotemic but produces dilute urine after rehydration has intrinsic renal dysfunction instead of prerenal azotemia.

Species Differences in Donkeys

Donkeys differ from horses in several metabolic and endocrine parameters, and these differences extend to renal function. Metabolic and endocrine insights in donkeys highlights that donkeys have a high prevalence of obesity, hyperlipemia, and insulin dysregulation, conditions that can affect renal perfusion and tubular function. Donkey-specific reference ranges for serum and urine parameters should be used when available, because extrapolation from equine values may misclassify normal findings. The same reference notes that dynamic testing is recommended when resting hormone concentrations fall in non-diagnostic ranges, and this principle applies to renal function testing as well: a single borderline urine specific gravity should prompt serial assessment instead of immediate diagnosis.

Documentation and Reporting

Findings should be recorded with the collection method, time of day, and the horse's hydration status at sampling. Urine specific gravity, pH, and fractional excretion values should be reported with the concurrent serum chemistry values. Serial measurements are more informative than single values, particularly when monitoring response to fluid therapy. The clinician should document the clinical decision point that triggered testing and the specific question the test was designed to answer, because this context determines whether a borderline result warrants further investigation or treatment adjustment.

Recognized Complications and Failure Modes

The concentrating and acid-base functions of the equine kidney fail through several recognizable pathways, each with distinct early indicators. Medullary washout occurs when chronic polyuria or prolonged diuresis depletes the interstitial solute gradient. The horse produces large volumes of dilute urine despite adequate antidiuretic hormone, and the water deprivation test fails to concentrate urine above plasma osmolality. Early detection relies on serial urine specific gravity measurements during treatment with fluids or diuretics. A trend toward isosthenuria, specific gravity between 1.008 and 1.012, before therapy ends signals that the medullary gradient has been compromised.

Tubular dysfunction from ischemic or toxic injury presents with enzymuria and cylindruria before azotaemia develops. The review of equine renal anatomy and physiology by Divers notes that acute tubular necrosis and apoptosis are common after ischemic or toxic insults and in sepsis-associated acute kidney injury, and that sepsis-related injury often involves both functional and obstructive changes in intrarenal circulation. Fractional excretion of sodium rises early because proximal tubular reabsorptive capacity fails before glomerular filtration rate declines measurably. Serial fractional excretion measurements, instead of a single value, distinguish transient tubular strain from established injury.

Acid-base failure manifests as an inappropriate inability to excrete an acid load. The horse with impaired distal acidification maintains a urine pH above 6.0 despite systemic acidaemia. Early detection requires simultaneous measurement of blood gas parameters and urine pH, because a normal blood pH with inappropriately alkaline urine may be the first abnormality. Hyperchloraemic metabolic acidosis with a normal anion gap in a horse with dilute urine should prompt evaluation of tubular acidification capacity.

Common Errors in Assessment

Less experienced clinicians frequently interpret a single urine specific gravity measurement without reference to hydration status, recent fluid therapy, or time of day. A horse that has just received intravenous fluids will predictably produce dilute urine, and this does not indicate renal disease. The corrective action is to interpret urine concentration only in the context of the horse's water intake, fluid administration, and plasma osmolality.

A second error is relying on urine dipstick pH without confirming the sample is fresh. Urine left at room temperature loses carbon dioxide, and pH rises artefactually. The corrective action is to measure pH within 30 minutes of collection or to refrigerate the sample and measure promptly. A third error is calculating fractional excretion of electrolytes from a single urine sample without simultaneous serum values. The formula requires paired samples, and using a reference serum value from a textbook introduces error.

Students often misinterpret the water deprivation test result when the horse was not truly water-deprived. The test is valid only if water intake is verifiably zero for the full period, and the horse must be monitored to prevent access to bedding, puddles, or other water sources. The MSD Veterinary Manual provides the standard framework for interpreting water deprivation testing in horses, including the expected maximum urine specific gravity and the contraindications to performing the test in dehydrated or azotaemic animals.

Limitations of Current Evidence

The equine renal physiology literature is comparatively sparse, and several areas rest on extrapolation from other species. The molecular identity of equine renal calcium transporters has been characterized, with calbindin D28k expression greatest in the kidney and plasma membrane calcium ATPase 1 expression higher in the kidney than in the intestine, but the functional regulation of these proteins in the horse remains incompletely defined. The unique calcium physiology of the horse, including high blood calcium, high intestinal calcium absorption, and high renal calcium excretion with low vitamin D concentrations, complicates direct extrapolation from other mammals.

Expert opinion still differs on the threshold urine specific gravity that defines adequate concentrating ability in the horse. Some authorities accept 1.020 as adequate, while others require 1.025 or higher. The discrepancy reflects variation in diet, environmental temperature, and prior fluid therapy. Similarly, the normal range for fractional excretion of sodium in horses is debated, and reference intervals vary between institutions. The review of equine renal and urinary tract physiology by Toribio emphasizes that several conditions can temporarily or permanently affect renal function, and that endogenous and exogenous compounds can promote or exacerbate renal disease when combined with inappropriate renal blood flow.

Donkey-specific reference intervals for urine concentration and electrolyte handling are not well established. The metabolic and endocrine insights in donkeys review notes that donkey-specific cut-off values and reference ranges need to be established for several diagnostic tests, and this applies to renal function parameters as well. Clinicians should interpret donkey urine values with caution and use serial trends instead of absolute thresholds.

Referral and Escalation Criteria

Referral to a specialist or teaching hospital is warranted when the horse fails to concentrate urine after 24 hours of water deprivation, when fractional excretion of sodium remains elevated despite correction of hypovolemia, or when acid-base disturbances persist beyond 48 hours of supportive treatment. Horses with suspected leptospiral interstitial nephritis, which may occur after Leptospira infection or secondary to tubular necrosis, warrant specialist involvement for biopsy and targeted therapy.

Laboratory involvement is indicated when point-of-care testing produces results that conflict with clinical findings. Confirmatory testing for urine osmolality, quantitative proteinuria, and blood gas analysis should be performed at a reference laboratory when the clinical picture is ambiguous. Regulatory reporting may be required when renal failure is suspected to result from a notifiable disease or from exposure to a reportable toxin, and the WOAH terrestrial animal health standards should be consulted for disease-specific reporting obligations.

Frequently Asked Questions

How Should I Interpret a Urine Specific Gravity Value When the Sample Was Collected After Fluid Therapy?

Fluid therapy expands extracellular volume and suppresses antidiuretic hormone release, which directly reduces renal concentrating ability. A post- fluid urine specific gravity below 1.020 does not confirm renal disease if the sample was collected during or shortly after intravenous fluid administration. Wait at least 12 to 24 hours after discontinuing fluids before interpreting concentrating capacity. If the horse remains polyuric with dilute urine beyond this period, pursue water deprivation testing only after confirming normal hydration status and electrolyte balance. Remember that the normal horse produces urine with specific gravity ranging from approximately 1.020 to 1.050, with values above 1.050 expected after water deprivation. Review the equine renal physiology review by Divers for the relationship between renal blood flow and concentrating function.

What Can I Do When a Hydrometer or Refractometer Is Unavailable in the Field?

A refractometer remains the preferred method for urine specific gravity measurement in equine practice. When unavailable, use urine dipstick protein and creatinine concentration to calculate a urine protein to creatinine ratio, which provides a crude estimate of concentration but does not replace specific gravity. Alternatively, measure urine osmolality if a laboratory osmometer is accessible, though this is rarely practical in ambulatory settings. The most pragmatic field approach is to assess urine color and clarity alongside clinical hydration parameters, then submit a refrigerated urine sample to a diagnostic laboratory within 24 hours. Urine specific gravity remains stable for several days when refrigerated, so sample submission is a reliable option. The MSD Veterinary Manual provides guidance on urine sample handling and storage for laboratory analysis.

How Does the Water Deprivation Test Differ in Donkeys Compared with Horses?

Donkeys evolved in arid environments and maintain water balance more efficiently than horses, with lower basal water turnover and greater tolerance of dehydration. Standard equine water deprivation protocols may therefore overestimate renal concentrating capacity in donkeys, and reference intervals for urine specific gravity and osmolality derived from horses should not be applied directly. Donkeys also differ in calcium metabolism, with higher renal calcium excretion and lower vitamin D concentrations than horses, which can influence urine composition and sediment interpretation. The metabolic and endocrine insights in donkeys by Mendoza et al. highlight that donkey-specific reference ranges remain incompletely established, so interpret water deprivation results cautiously and compare against age matched healthy donkeys when possible.

What Documentation Should Accompany a Urine Concentration or Acid-Base Assessment in the Medical Record?

Record the collection method, time of day, whether the sample was free catch, catheterized, or obtained via cystocentesis, and the interval since last urination. Document concurrent fluid therapy rates, recent drug administration, and any sedatives used during collection, since alpha-2 agonists can transiently alter urine output. For acid-base assessment, record the analyzer used, sample handling time, and whether the sample was collected anaerobically. Include the clinical question that prompted testing, the specific results, and the interpretation in the assessment and plan. Serial measurements are more informative than single values, so record trends over time. The AVMA practice resources offer general guidance on medical record standards that apply to diagnostic testing documentation.

How Should I Explain Abnormal Concentrating Ability to an Owner Without Causing Unnecessary Alarm?

Frame the finding as a functional observation instead of a diagnosis. Explain that the kidney's ability to concentrate urine is one of several filters that can be checked, and that a single abnormal value requires confirmation. Describe the water deprivation test as a controlled, monitored procedure that answers a specific question about kidney performance. Avoid speculating about prognosis before repeat testing and additional diagnostics are complete. Emphasize that many horses with reduced concentrating ability maintain good quality of life and that treatment decisions depend on the underlying cause, which may be reversible. Provide the owner with a written summary of the test results and the planned next steps, and invite questions after they have had time to process the information.

When Should I Refer a Horse with Suspected Concentrating or Acid-Base Abnormalities?

Refer when the horse is clinically unstable, when azotaemia progresses despite fluid therapy, when the water deprivation test cannot be performed safely in the field, or when the diagnostic question requires specialised equipment such as osmolality measurement or blood gas analysis with ionised calcium. Refer also when the clinician suspects a systemic disease process that requires advanced imaging or biopsy, or when the horse fails to respond to initial management within 48 to 72 hours. The equine renal physiology review by Divers notes that acute kidney injury mechanisms are frequently associated with changes in renal blood flow, so horses with hemodynamic instability or sepsis merit early referral. Establish communication with the receiving facility before transfer and provide the complete diagnostic record, including serial laboratory values and fluid therapy details.

Related Clinical & Scientific Guides

• Canine Respiratory System: Anatomy and Physiology
• Comparative Anatomy of the Mammalian Kidney
• Feline Cardiopulmonary Physiology: Heart-Lung Interactions

References and Further Reading

• Relevant Equine Renal Anatomy, Physiology, and Mechanisms of Acute Kidney Injury: A Review.. 2022.
• Essentials of equine renal and urinary tract physiology.. 2007.
• Cloning, comparative sequence analysis and mRNA expression of calcium-transporting genes in horses.. 2010.
• Metabolic and Endocrine Insights in Donkeys.. 2024.
• Equine glucagon-like peptide-1 receptor physiology.. 2018.
• Constitutively Activating Mutants of Equine LH/CGR Constitutively Induce Signal Transduction and Inactivating Mutations Impair Biological Activity and Cell-Surface Receptor Loss In Vitro.. 2021.
• NCBI Bookshelf: Veterinary and Comparative Biomedical Sciences. NCBI Bookshelf.
• MSD Veterinary Manual, Professional Edition. MSD Veterinary Manual.
• American Veterinary Medical Association Practice Resources. American Veterinary Medical Association.

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This article is educational professional reference material for veterinary audiences. It is not a substitute for veterinary diagnosis, individual clinical judgment, current product labeling, or applicable regulatory requirements.