# Kidney Medulla: Anatomy, Function, and Structure The kidney medulla is the inner region of the kidney, deep to the cortex, that contains the renal pyramids, the loops of Henle, the vasa recta and the collecting ducts, and it is the site where the countercurrent multiplier builds the osmotic gradient needed to concentrate urine. In domestic mammals the medulla is either a single fused mass (unilobar kidney) or divided into many lobes whose apices form distinct papillae (multilobar kidney). The medulla matters because it is the part of the kidney that decides how much water leaves the body. A dog that has been without water for a day and a horse grazing fresh pasture both filter plasma through the same basic nephron, yet their urine osmolality differs by more than tenfold. Almost all of that difference is created in the medulla. The medulla is also a common site of drug toxicity and of specific renal diseases in animals, from analgesic-associated crest necrosis in horses to collecting duct carcinoma arising in the pyramid [1][2]. Understanding its architecture explains why those lesions look the way they do. ## What the Kidney Medulla Is and Where It Sits The kidney is divided into an outer cortex and an inner medulla. The cortex contains the renal corpuscles (glomeruli with their Bowman's capsules), the proximal and distal convoluted tubules, and the cortical collecting ducts. The medulla contains the straight portions of the nephron, the loops of Henle, the medullary collecting ducts and the vasa recta, which are the long hairpin capillaries that run alongside the loops. The boundary between cortex and medulla is not a straight line. In species with a unilobar kidney, such as the dog and cat, the medulla forms a single continuous cone-shaped mass, and the cortex caps it and sends columns of cortical tissue down between medullary structures. These columns are the renal columns. In species with a multilobar kidney, such as the pig, cow and horse, the medulla is subdivided into many pyramids, each with its own apex, and the cortex extends between them. The medulla itself is usually described in two or three zones. The outer medulla is divided into an outer stripe and an inner stripe. The inner medulla (also called the inner zone) lies deepest and ends at the papilla or crest. The outer stripe contains the thick descending limbs and the straight proximal tubules. The inner stripe contains the thick ascending limbs and the vascular bundles of the vasa recta. The inner medulla contains only thin limbs, medullary collecting ducts and the thin vasa recta. This zonal arrangement is functional, not decorative: each zone houses a different segment of the tubule, and each segment moves a different solute. ## The Functional Unit: Lobes, Pyramids, Papillae and the Crest A renal lobe is the cortex plus the medullary tissue that drains into one papillary duct system. In a unilobar kidney the lobes are fused into one continuous medulla. In a multilobar kidney the lobes remain separate, and the medullary tissue of each lobe forms a pyramid. The apex of the pyramid is the renal papilla. The papilla is perforated by the openings of the papillary ducts (ducts of Bellini), which deliver urine into the renal pelvis or calyces. In some species the medulla does not form a pointed papilla but instead a longitudinal ridge called the renal crest. The horse is the classic example, and the term renal crest is standard in [equine anatomy](/knowledge/veterinary-medicine/veterinary-anatomy-physiology/equine-hindlimb-anatomy-bones-joints-ligaments). In the horse, the crest runs along the inner border of the kidney and receives the papillary ducts along its length. The distinction between crest and papilla is used constantly in veterinary pathology. Phenylbutazone-associated necrosis in horses is described as renal medullary crest necrosis, not papillary necrosis, because the equine kidney has a crest rather than discrete papillae [2]. The lesion is a well-demarcated focal medullary necrosis with sequestration of crest fragments, and the cortical changes (segmental pallor from tubular dilatation, filtrate retention and interstitial edema) are considered secondary to the medullary injury [2]. Using the correct anatomical term makes the diagnosis legible to anyone reading the record. ## Comparative Medullary Anatomy Across Domestic Species ### Unilobar kidneys (dog, cat, small ruminants) In the dog, cat, sheep and goat, the medulla is a single fused mass. There is one renal papilla (or in some species a crest-like ridge) that projects into a single renal pelvis. The renal pelvis is the funnel-shaped expansion of the ureter inside the kidney. Urine formed in the collecting ducts drips from the papilla into the pelvis and then leaves via the ureter. Because the medulla is fused, the loops of Henle from superficial and juxtamedullary nephrons share one continuous interstitial space. Juxtamedullary nephrons, whose glomeruli sit near the corticomedullary boundary, have the longest loops and reach deepest into the medulla. These long loops are the ones that generate the steepest osmotic gradient. ### Multilobar kidneys (pig, cow, horse) In the pig and cow, the kidney is externally lobated or at least lobulated, and internally it is clearly divided into many pyramids. Each pyramid has its own papilla that projects into a calyx. A calyx is a cup-shaped branch of the renal pelvis that collects urine from one papilla. In the pig, the papillae are distinct and each drains into its own calyx. In the cow, the pyramids are also separate but the kidney is smoother externally than the pig's. The horse kidney is multilobar but externally smooth, and its medullary tissue forms a renal crest rather than separate papillae. The crest is a single longitudinal ridge, so the equine kidney behaves functionally like a unilobar kidney for urine concentration while retaining the lobar architecture internally. The functional consequence of lobation is that each lobe has its own gradient. A multilobar kidney can, in principle, generate a gradient in each pyramid independently, but the maximum concentrating ability still depends on loop length and on the permeability properties of the collecting duct. ### The hamster pelvis as a special case The hamster has an unusual renal pelvis that has been studied in detail. The pelvis is elaborated into peripelvic columns, opercula (sometimes called secondary pyramids), fornices and secondary pouches, and the outer medulla accounts for nearly half of the total pelvic surface area [3]. The inner stripe of the outer medulla has more than twice the pelvic surface area of the outer stripe [3]. The epithelium separating pelvic urine from the outer medullary parenchyma is thin, simple squamous to low cuboidal, while the inner medulla is covered by a cuboidal to columnar epithelium resembling papillary collecting duct epithelium [3]. The authors concluded that contact between pelvic urine and the medulla may be an important aspect of final urine formation [3]. This is a reminder that the pelvis is not just a drain, it is a surface across which solute and water can move in some species. ## How the Medulla Concentrates Urine: The Countercurrent Multiplier The countercurrent multiplier is the mechanism that creates a hyperosmotic medullary interstitium. The term countercurrent refers to fluid flowing in opposite directions in adjacent tubes (descending and ascending limbs of the loop of Henle). The term multiplier refers to the fact that a small single-effect difference in osmotic pressure is multiplied along the length of the loop into a large gradient. The loop of Henle has two limbs with opposite properties. The descending limb is permeable to water and relatively impermeable to solute. The ascending limb is impermeable to water and actively transports solute out of the tubular fluid. The thick ascending limb of the loop of Henle (often abbreviated TAL) is the workhorse of this process. It reabsorbs sodium, potassium and chloride via the Na+-K+-2Cl- cotransporter (NKCC2) and pumps them into the interstitium. Because water cannot follow, the fluid in the ascending limb becomes dilute while the interstitium becomes concentrated. The steps of the multiplier can be listed in order: 1. Filtrate enters the descending limb, which is water-permeable. Water leaves into the hypertonic interstitium, so the tubular fluid becomes more concentrated as it descends. 2. The fluid reaches the hairpin turn and enters the ascending limb, which is water-impermeable. 3. The thick ascending limb actively transports NaCl out of the tubule into the interstitium. The tubular fluid becomes dilute (hypo-osmotic to plasma). 4. The newly deposited NaCl raises interstitial osmolality, which pulls more water out of the descending limb on the next pass. 5. The cycle repeats along the length of the loop, so the osmotic difference between tubular fluid and interstitium grows with each pass. The result is a gradient that increases from the corticomedullary boundary to the papilla tip. 6. Urea recycling adds to the gradient. Urea is reabsorbed from the medullary collecting duct and re-enters the loop, contributing to the inner medullary osmotic gradient. 7. The vasa recta carry blood in hairpin loops that run parallel to the loops of Henle. Because they also run countercurrent, they remove water without washing out the solute gradient, a process called countercurrent exchange. The vasa recta are essential. If medullary blood flow were high and non-countercurrent, the solute deposited by the ascending limb would be swept away and no gradient could form. The hairpin geometry of the vasa recta allows them to act as a countercurrent exchanger, preserving the gradient while still delivering oxygen and nutrients to the medullary tissue. The collecting duct is the final effector. It passes through the medulla on its way to the papilla. In the presence of antidiuretic hormone (ADH, also called vasopressin), the medullary collecting duct becomes permeable to water through aquaporin-2 channels. Water then leaves the duct down the osmotic gradient into the hypertonic interstitium, and the urine left behind becomes concentrated. Without ADH, the duct stays water-impermeable and dilute urine is produced. This is why the medulla is often described as the site where the kidney decides the final urine concentration. ## The Collecting Duct in the Medulla The collecting duct is not a passive tube. It is a site of regulated sodium, potassium, acid and water transport, and its properties change along its length. The outer medullary collecting duct and the inner medullary collecting duct have different cell compositions and different transport capacities. Intercalated cells are a major cell type in the collecting duct. In the rabbit, the number of alpha-intercalated cells increases from the cortical collecting duct into the outer medullary collecting duct, rising from about 11 cells per 200 micrometers in the cortex to about 15 in the outer stripe and about 32 in the inner stripe [4]. The morphology of these cells also changes: in the cortex they are pyramidal or conical, while in the medulla they become elongated and shallow, giving a more rectangular profile [4]. Metabolic acidosis reversibly changes cortical alpha-intercalated cells toward this rectangular shape and shifts the basolateral anion exchanger AE1 and the apical V-ATPase [4]. The takeaway for students is that the medullary collecting duct is not simply a continuation of the cortical collecting duct, it is a distinct transport environment. The collecting ducts also converge as they approach the papilla. This convergence has a measurable hemodynamic consequence. When flow resistance was calculated along the rat renal tubule using the Hagen-Poiseuille equation, the maximum pressure drop occurred in two segments: the thin descending limbs of Henle and the inner medullary collecting ducts [5]. The high resistance in the thin descending limbs is due to their small diameter, while the steep pressure drop in the inner medullary collecting ducts is due to the convergent structure that channels flow into fewer and fewer tubules toward the papillary tip [5]. This is a useful reminder that the medulla is a low-flow, high-resistance environment, which is part of why it is vulnerable to ischemia. ## How the Medulla Is Studied and Observed ### Imaging The collecting ducts can be seen on urography. In a study of 636 patients with normal renal function and no upper urinary tract obstruction, contrast medium filled thin cylindrical structures in the renal pyramids that followed the course of the collecting ducts [6]. The cylinders appeared within 4 minutes of contrast injection and remained distinct for 35 to 50 minutes, with diameters between 0.05 and 0.30 millimeters [6]. The prevalence of visible cylinders was much higher with the low-osmolality contrast medium iohexol (21 of 158 studies, 13.3%) than with the high-osmolality medium amidotrizoate (2 of 529 studies, 0.4%) [6]. The authors noted that the cylinders were more distinct and numerous with iohexol, and that the configuration suggested each cylinder might be an individual collecting duct [6]. For veterinary students, the practical point is that the medullary collecting ducts are large enough to be resolved radiographically under the right contrast conditions, which is why they can be mistaken for pathology when they are simply normal anatomy. ### Histology and immunohistochemistry Medullary segments can be identified by segment-specific markers. Aquaporin-1 labels proximal tubules and thin descending limbs of Henle, uromodulin labels distal tubules, and aquaporin-2 labels collecting ducts [7]. These markers are used in stereology studies to estimate segment lengths. In the CD-1 mouse kidney, all tubular segments increased significantly in length between postnatal day 21 and 2 months of age: proximal tubules by 602%, distal tubules by 200%, thin descending limbs of Henle by 35%, and collecting ducts by 53% [7]. Between 2 and 12 months, only proximal tubules increased further (76%) [7]. Mice on a high-salt diet for 12 months had a 27% greater length of the thin descending limb of Henle but no significant change in the other segments [7]. These numbers show that the medullary thin limb is a plastic structure that can remodel with dietary salt load. ### Biomarkers Loop of Henle injury can be detected with urinary biomarkers. Olfactomedin 4 (OLFM4) is a secreted glycoprotein that localized to the loop of Henle in septic mice and was detectable in urine [8]. In critically ill pediatric patients, urine OLFM4 was increased in those with acute kidney injury compared with no or stage 1 injury (p = 0.044) and in those with sepsis compared with no sepsis (p = 0.026) [8]. Immunofluorescence showed OLFM4 colocalized with NKCC2 and uromodulin, confirming its loop of Henle origin [8]. This is an example of how segment-specific medullary markers are moving into clinical use. ## Comparative Table: Medullary Structure and Urine Concentrating Ability The table below summarizes the medullary anatomy and concentrating capacity of five domestic species. Concentrating ability is expressed qualitatively because it depends on hydration status, diet and individual variation, and because precise maximal values are not fixed constants. | Species | Kidney type | Medullary apex | Papillae | Relative urine concentrating ability | Notes | |--|--|--|--|--|--| | Dog | Unilobar | Single papilla or crest | One | Moderate to high | Fused medulla, long juxtamedullary loops | | Cat | Unilobar | Single papilla | One | High | Obligate carnivore, well-developed medullary gradient | | Horse | Multilobar, externally smooth | Renal crest | None (crest) | Low to moderate | Crest necrosis is the correct term for phenylbutazone lesions [2] | | Cow | Multilobar | Multiple papillae | Many | Moderate | Separate pyramids, each with its own calyx | | Pig | Multilobar | Multiple papillae | Many | Moderate | Distinct externally visible lobation | The pattern is that species with longer loops of Henle and a deeper inner medulla, such as the cat and the desert-adapted mammals, achieve higher urine osmolality. The horse, with a crest rather than long papillae and a relatively modest inner medulla, concentrates urine less effectively. This is consistent with the horse's high water turnover on pasture. ## Clinical Relevance, Limitations and Common Mistakes The medulla is vulnerable to ischemia because medullary blood flow is low and the tissue operates at the edge of hypoxia. This is why nonsteroidal anti-inflammatory drugs that reduce renal prostaglandin synthesis can cause medullary injury. The equine case is the best documented in [veterinary medicine](/blog/careers/veterinary-medicine-careers-from-clinical-practice-to-public-health): renal medullary crest necrosis associated with phenylbutazone therapy in horses produces a well-demarcated focal medullary necrosis with sequestration of crest fragments, and the cortical lesions are secondary [2]. Ischemia in concert with phenylbutazone is the suggested etiology [2]. Medullary scarring is another pattern. In reflux nephropathy, small medullary scars involving only portions of the pyramids extend from the inner medulla to the cortex and obliterate collecting ducts, vasa recta and recurrent loops [9]. These sublobar scars are thought to result from single duct medullary disruptions, possibly through obstruction of the thousands of nephrons subtended by each papillary duct and through localized disruption of the renal microvasculature [9]. The clinical lesson is that a small medullary lesion can have a large functional footprint because each papillary duct drains a large number of nephrons. The medulla is also the site of specific tumors. Collecting duct carcinoma, also called Bellini duct carcinoma, arises in the renal medulla and pyramid and is composed of large cells resembling collecting duct cells, arranged in tubular, microcystic and papillary structures [1]. It is rare and usually has a poor prognosis [1]. In veterinary medicine, renal tumors are more often diagnosed as renal cell carcinoma or lymphoma, but the anatomical principle holds: tumors that arise from medullary structures will be centered in the pyramid. Stone formation has a medullary component. Calcium microliths larger than the collecting duct diameter can damage the duct wall, pass into the interstitium of the pyramid and accumulate beneath the epithelium of the pitted zone, forming the subepithelial plaque of Randall [10]. Prolonged stagnation of urine in the collecting duct is also necessary for microliths to form [10]. The papillary surface itself can favor calcium deposition through factors independent of the Randall plaque [10]. This explains why some stones recur even after the stone is removed. Common mistakes students make: 1. Confusing the medulla with the pelvis. The pelvis is the urine-collecting space, the medulla is the tissue that makes the gradient. 2. Calling every medullary apex a papilla. In the horse it is a crest, and the correct term matters in pathology [2]. 3. Thinking the loop of Henle is entirely in the medulla. The thick ascending limb begins in the outer medulla and the loop turns at varying depths, and the cortical thick ascending limb is also part of the segment. 4. Assuming the collecting duct is passive. It is a regulated transport segment with changing cell populations along its length [4]. 5. Forgetting the vasa recta. Without countercurrent exchange, the gradient would be washed out. 6. Treating concentrating ability as a fixed species constant. It varies with hydration, diet and ADH status. This article is educational and is not a substitute for veterinary diagnosis or treatment. ## Quick Review - The kidney medulla is the inner renal region containing pyramids, loops of Henle, vasa recta and collecting ducts. - Unilobar kidneys (dog, cat, small ruminants) have a fused medulla with one papilla or crest. Multilobar kidneys (pig, cow, horse) have multiple pyramids, with distinct papillae in the pig and cow and a crest in the horse. - The countercurrent multiplier depends on the water-permeable descending limb and the water-impermeable, salt-transporting ascending limb, and it is powered by NKCC2 in the thick ascending limb. - The vasa recta preserve the gradient through countercurrent exchange. - The collecting duct is the final effector, and ADH controls its water permeability. - The medulla is vulnerable to ischemia, which is why NSAID-associated crest necrosis occurs in horses [2]. - Medullary collecting ducts converge toward the papilla, creating high flow resistance and a steep pressure drop [5]. ## Frequently Asked Questions ### What is the kidney medulla? The kidney medulla is the inner region of the kidney, deep to the cortex, that contains the renal pyramids, loops of Henle, vasa recta and collecting ducts. It is the site where the osmotic gradient for urine concentration is generated. ### What is the difference between the renal cortex and the renal medulla? The cortex contains glomeruli and convoluted tubules, while the medulla contains the straight tubules, loops of Henle, vasa recta and medullary collecting ducts. The cortex filters and reabsorbs the bulk of the filtrate, and the medulla fine-tunes water and solute balance. ### What is the countercurrent multiplier? The countercurrent multiplier is the mechanism in which fluid flowing in opposite directions in the descending and ascending limbs of the loop of Henle multiplies a small osmotic difference into a large medullary gradient. It depends on the descending limb being water-permeable and the ascending limb being water-impermeable and salt-transporting. ### Which animals have a renal crest instead of a papilla? The horse has a renal crest, a longitudinal ridge rather than discrete papillae. This is why equine medullary lesions are called crest necrosis rather than papillary necrosis [2]. ### Do all domestic species concentrate urine equally well? No. Species with longer loops of Henle and a deeper inner medulla, such as the cat, concentrate urine more effectively than species with a shallower medulla, such as the horse. Concentrating ability also depends on hydration, diet and ADH status. ### Why is the renal medulla prone to drug injury? Medullary blood flow is low and the tissue operates near the limit of oxygen delivery, so drugs that reduce renal blood flow or prostaglandin synthesis can cause ischemic medullary injury. Phenylbutazone-associated crest necrosis in horses is the classic veterinary example [2]. ## Related Articles - [Equine Hoof Anatomy: Structures and Function](/knowledge/veterinary-medicine/veterinary-anatomy-physiology/equine-hoof-anatomy-structures-function) - [Comparative Anatomy of the Mammalian Kidney](/knowledge/veterinary-medicine/veterinary-anatomy-physiology/comparative-anatomy-mammalian-kidney) - [Equine Larynx Anatomy and Function in Respiration](/knowledge/veterinary-medicine/veterinary-anatomy-physiology/equine-larynx-anatomy-function-respiration) - [Canine Lymphatic System: Anatomy and Function](/knowledge/veterinary-medicine/veterinary-anatomy-physiology/canine-lymphatic-system-anatomy-function) - [Feline Renal Anatomy and Physiology: A Clinical Correlation](/knowledge/veterinary-medicine/veterinary-anatomy-physiology/feline-renal-anatomy-physiology-clinical-correlation) - [Canine Liver Anatomy and Lobular Structure](/knowledge/veterinary-medicine/veterinary-anatomy-physiology/canine-liver-anatomy-lobular-structure) - [Renal Corpuscle: Structure and Filtration Explained](/knowledge/veterinary-medicine/veterinary-anatomy-physiology/renal-corpuscle-structure-and-filtration-explained) ## Sources 1. [[Bellini duct carcinoma or kidney collecting duct carcinoma].](https://pubmed.ncbi.nlm.nih.gov/1304115/) 2. [Renal medullary crest necrosis associated with phenylbutazone therapy in horses.](https://pubmed.ncbi.nlm.nih.gov/6649337/) 3. [Anatomy of the renal pelvis in the hamster.](https://pubmed.ncbi.nlm.nih.gov/433785/) 4. [Distinct α-intercalated cell morphology and its modification by acidosis define regions of the collecting duct.](https://pubmed.ncbi.nlm.nih.gov/26084929/) 5. [Flow resistance along the rat renal tubule.](https://pubmed.ncbi.nlm.nih.gov/30089029/) 6. [Normal collecting ducts: visualization at urography.](https://pubmed.ncbi.nlm.nih.gov/2909118/) 7. [Lengths of nephron tubule segments and collecting ducts in the CD-1 mouse kidney: an ontogeny study.](https://pubmed.ncbi.nlm.nih.gov/27654893/) 8. [Olfactomedin 4 as a novel loop of Henle-specific acute kidney injury biomarker.](https://pubmed.ncbi.nlm.nih.gov/36117416/) 9. [Morphological characteristics of segmental renal scarring in vesicoureteral reflux.](https://pubmed.ncbi.nlm.nih.gov/1433595/) 10. [[Renal lithogenesis (author's transl)].](https://pubmed.ncbi.nlm.nih.gov/7276600/)