The renal papilla is the innermost tip of the kidney’s medulla, the cone-shaped structure that projects into each kidney’s collecting space and serves as the final stop where concentrated urine drips into the renal pelvis before traveling down the ureter to the bladder. Each human kidney typically contains between eight and eighteen of these small, rounded projections, and they play an outsized role in the kidney’s ability to concentrate urine and conserve water. Despite being a small fraction of the kidney’s total tissue, the papilla is a surprisingly common site for kidney stones to begin forming and a structure uniquely vulnerable to damage from reduced blood flow.
Where the Papilla Sits Inside the Kidney
If you imagine the kidney sliced in half from top to bottom, you see two main zones: the outer cortex, where blood is filtered, and the inner medulla, where the filtered fluid is progressively concentrated. The medulla is arranged in cone-shaped sections called renal pyramids, and the papilla is the pointed tip of each pyramid. Each papilla is perforated by the openings of collecting ducts, which is where the final urine exits into a cup-like structure called a calyx. The calyces merge into the renal pelvis, which funnels urine into the ureter.
The papilla sits at the deepest, most interior part of the kidney. It is surrounded by tissue that receives comparatively little blood flow and exists in an extremely concentrated chemical environment. This positioning is essential for the papilla’s job but also makes it one of the most fragile parts of the kidney.
How the Papilla Concentrates Urine
The kidney’s core task is filtering waste while hanging on to water and useful molecules. The papilla is where that balancing act reaches its peak. Fluid entering the medulla passes through progressively saltier tissue on its way to the papilla tip, and this increasing saltiness draws water out of the collecting ducts by osmosis, producing concentrated urine. The hormone vasopressin (also called antidiuretic hormone) plays a direct role in this process. Research has shown that vasopressin increases the absolute amount of sodium stored in the medullary and papillary interstitial fluid, independent of its effects on water, and that reducing vasopressin lowers it.1Kidney International. Vasopressin regulation of renal sodium excretion This means the hormone does not just move water around; it actively raises the salt concentration in the papilla’s surroundings, which in turn pulls more water out of the collecting ducts and back into the body.
The blood vessels serving the papilla, called vasa recta, run in hairpin loops that act as countercurrent exchangers. Blood flowing down into the papilla picks up salt and loses water; blood flowing back out reverses the exchange. This arrangement preserves the high-salt gradient instead of washing it away. Prostaglandins help regulate blood flow through these tiny vessels. When prostaglandin production is blocked, vasa recta blood flow drops significantly, a finding demonstrated in animal studies where prostaglandin-synthesis inhibitors reduced red blood cell velocities in papillary blood vessels by roughly 17 to 24 percent compared to controls.2PubMed. Prostaglandin synthesis inhibitors and vasa recta erythrocyte velocities in the rat This is part of the reason nonsteroidal anti-inflammatory drugs (NSAIDs), which block prostaglandins, can be harmful to the papilla over time.
Why the Papilla Is Vulnerable to Damage
The papilla’s blood supply is precarious by design. The vasa recta that feed it are the tail end of a long vascular chain, and they operate in an environment that is far more concentrated than the rest of the body. This combination of limited blood flow and high osmolarity makes the papilla uniquely susceptible to ischemic injury, meaning damage from insufficient blood supply.3PubMed. Renal papillary necrosis: review and comparison of findings at multi-detector row CT and intravenous urography When any condition further compromises blood delivery to this area, the tissue at the tip can die.
This vulnerability helps explain why such a diverse set of conditions can cause the same type of papillary damage. Anything that narrows blood vessels, thickens blood, raises the surrounding osmotic pressure even further, or directly poisons the cells at the papilla tip can tip the balance from functional to necrotic.
Randall’s Plaques and Kidney Stone Formation
One of the most clinically relevant things that happens at the papilla is the formation of Randall’s plaques, which are tiny deposits of calcium phosphate that accumulate in the tissue just beneath the papillary surface. These plaques are the starting point for most common calcium oxalate kidney stones. The sequence works like this: mineral deposits build up within the basement membranes of the thin loops of Henle deep in the papilla. Over time, these deposits grow and eventually reach the papillary surface. Once the covering layer of epithelial cells is breached and the plaque is exposed to urine in the renal pelvis, calcium oxalate crystals can attach to the plaque surface and grow into a full kidney stone.4PubMed Central. How do stones form? Is unification of theories on stone formation possible?
Electron microscopy work has pinpointed the earliest deposits as tiny spherical crystals, as small as 50 nanometers, sitting within the basement membranes of the thin loops of Henle and the walls of nearby vasa recta. At these initial sites, the surrounding cells and tissue appear completely normal, meaning the process begins silently with no detectable injury to the surrounding structures.5JCI Insight. Randall’s plaque of patients with nephrolithiasis begins in basement membranes of thin loops of Henle More recent research has refined this picture further, showing that the plaques form preferentially in the ascending thin limbs of the loop of Henle rather than the descending limbs, a finding that supports the theory that local fluid dynamics and solute transport patterns drive where crystals first appear.6PubMed Central. Randall’s plaque in stone formers originates in ascending thin limbs
When urologists look at the papillae of stone-forming patients during endoscopy, Randall’s plaques are extremely common. One study found that 83 percent of stone-forming patients had visible plaques, and only about 4.5 percent of patients showed no papillary abnormality at all.7PubMed Central. Classification of the renal papillary abnormalities by flexible ureteroscopy: evaluation of the 2016 version and update Erosions of the papillary surface were present in over half the patients, and signs of crystal plugging within the ducts were seen in about 16 percent. These observations have led researchers to develop standardized grading systems for papillary appearance, scoring features like ductal plugging, surface pitting, loss of normal contour, and the extent of plaque coverage.8PubMed Central. A Proposed Grading System to Standardize the Description of Renal Papillary Appearance at the Time of Endoscopy in Patients with Nephrolithiasis The goal is to make papillary findings comparable between doctors and studies, and to use them to guide metabolic workups for stone prevention.
Renal Papillary Necrosis
When the papilla’s already-thin blood supply is compromised enough, the tissue at the tip dies. This condition is called renal papillary necrosis, and it most commonly occurs in people with diabetes, sickle cell disease, urinary tract obstruction, urinary infection, or chronic NSAID use.9PubMed Central. Renal Papillary Necrosis Complicating Immune Checkpoint Inhibitor Therapy Diabetes damages the small blood vessels feeding the papilla. Sickle cell disease causes red blood cells to become rigid and block the already-narrow vasa recta. NSAIDs suppress prostaglandins, which as noted earlier are important for maintaining papillary blood flow. Urinary obstruction raises pressure within the collecting system, further squeezing the papilla’s tenuous blood supply.
The dead tissue can remain in place or slough off and travel downstream. When a necrosed papilla detaches, it can lodge in the ureter and obstruct urine flow, mimicking a kidney stone. A case report describes a 75-year-old woman who presented with flank pain, visible blood in her urine, and a high fever; endoscopic examination revealed that sloughed papillary tissue had migrated into her upper ureter and calyces, causing obstruction and kidney swelling.10PubMed Central. Renal papillary necrosis with urinary tract obstruction: A case report In a separate study of 15 patients with hydronephrosis from papillary necrosis, necrosed tissue was found filling the ureter at the point of blockage, and ureteroscopic removal was performed in 13 of those cases.11PubMed. Sonographic features of necrosed renal papillae causing hydronephrosis
There are also rarer triggers. Immune checkpoint inhibitors, a class of cancer therapy that works by unleashing the immune system, have been linked to papillary necrosis as a complication.9PubMed Central. Renal Papillary Necrosis Complicating Immune Checkpoint Inhibitor Therapy Infections can cause it as well. In one unusual case, a kidney transplant recipient developed papillary necrosis when disseminated histoplasmosis, a fungal infection, invaded the transplanted kidney and caused a papilla to slough off and obstruct the urinary tract.12PubMed. Disseminated histoplasmosis presenting as urinary tract obstruction in a renal transplant recipient
Detecting Papillary Problems on Imaging
The challenge with early papillary necrosis is that it can be invisible on routine imaging. Uncomplicated cases may not show up on standard ultrasound or non-contrast CT scans.13PubMed Central. MR urogram findings and diffusion restriction in the renal papilla and calyx in papillary necrosis-a new finding: preliminary report By the time imaging picks up a problem, it may be because a sloughed papilla has already caused obstruction. CT scanning with contrast (CT urogram) is considered an important tool because it can detect necrosis before surgery and can prevent complications like transplanting a kidney that unknowingly has papillary damage.14PubMed Central. Renal Papillary Necrosis: Role of Radiology MR urography has also shown promise, with studies describing the ability to detect filling defects and restricted diffusion in the calyx and the tip of the renal pyramid, potentially catching necrosis at an earlier stage.13PubMed Central. MR urogram findings and diffusion restriction in the renal papilla and calyx in papillary necrosis-a new finding: preliminary report
Treatment and Management of Papillary Necrosis
There is no way to regenerate a papilla that has already died. Treatment centers on addressing whatever caused the necrosis and managing complications as they arise. In practice, that means optimizing blood sugar control in people with diabetes, stopping NSAIDs or other drugs that may be reducing papillary blood flow, treating any urinary infections, and relieving obstruction.15Acta Medica Colombiana. Renal papillary necrosis. A forgotten entity If a sloughed papilla is blocking the ureter, endoscopic removal is usually the approach. In extreme cases, like emphysematous pyelonephritis (a severe gas-forming kidney infection), nephrectomy may be necessary.
Hydration is a consistent theme across management strategies. Generous fluid intake helps maintain urine flow and reduces the concentration of harmful substances in the already-stressed medulla. Monitoring urine output closely, while guarding against fluid overload in patients whose kidney function is already reduced, is part of acute management.15Acta Medica Colombiana. Renal papillary necrosis. A forgotten entity Urine alkalinization, which raises the pH of urine, remains an important strategy in chronic settings, especially for people with sickle cell disease, where the acidic and concentrated environment of the papilla is particularly hostile to sickled red blood cells. Any nephrotoxic medication should be avoided when possible, and if contrast dye must be used for imaging, kidney-protective measures are essential.
Papillary Renal Cell Carcinoma
The term “papillary” appears in kidney cancer as well, though the connection to the anatomical papilla is indirect. Papillary renal cell carcinoma gets its name from the microscopic growth pattern of the tumor cells, which form finger-like projections, rather than from the location in the kidney. It accounts for roughly 15 to 20 percent of all renal cell carcinoma cases and is divided into two main subtypes.16PubMed Central. A Review of Papillary Renal Cell Carcinoma and MET Inhibitors
Molecular analysis has confirmed that type 1 and type 2 papillary renal cell carcinomas are fundamentally different cancers. Type 1 tumors tend to have alterations in a receptor called MET, which has made MET-targeting drugs an area of active research. Type 2 tumors are more heterogeneous; they are characterized by a different set of genetic changes and can be further split into subgroups with different survival outcomes.17PubMed Central. Comprehensive Molecular Characterization of Papillary Renal-Cell Carcinoma Type 2 generally carries a worse prognosis than type 1. Because the subtypes behave differently, treatment strategies increasingly depend on molecular testing rather than a one-size-fits-all approach.
Reflux Nephropathy and Compound Papillae
Not all papillae are equally shaped, and the differences matter in pediatric kidney disease. Most papillae have a convex, simple shape that resists backward flow of urine. But some, especially those in the upper and lower poles of the kidney, are “compound” papillae with a flatter shape and a round orifice that allows urine to flow backward into the kidney tissue when pressure rises. This is why vesicoureteral reflux, where urine backs up from the bladder toward the kidney, tends to damage the upper and lower poles first. The compound papillae in those regions facilitate what is called intrarenal reflux, where urine is pushed into the kidney parenchyma itself, causing inflammation and scarring.18Clinical Kidney Journal. Developmental pathology of congenital kidney and urinary tract anomalies If the reflux goes untreated, adjacent simple papillae can gradually transform into compound papillae, spreading the damage across the entire kidney.
This is one reason persistent reflux in young children is taken seriously even when infections have not yet occurred. The structural vulnerability of compound papillae means that even relatively low-grade reflux can cause parenchymal damage if it reaches the right (or wrong) part of the kidney.
How the Papilla Develops Before Birth
The papilla forms during fetal development through the coordinated branching of the ureteric bud, the embryonic precursor of the collecting system. A gene called Esrrg plays a critical role in this process. In mouse studies, expression of Esrrg concentrates in the collecting ducts of the developing papilla by late gestation and persists in the newborn kidney. When both copies of Esrrg are knocked out, the papilla fails to form entirely while the cortex and the rest of the medulla develop normally. Instead of the usual cone-shaped papilla projecting into the pelvis, the medulla forms a concave disc sitting above a widely dilated pelvic space.19Oxford Academic (Human Molecular Genetics). Esrrg functions in early branch generation of the ureteric bud and is essential for normal development of the renal papilla This finding highlights how the papilla is not simply an extension of the medulla but a distinct developmental structure with its own genetic program. Defects in papillary formation can lead to impaired urine concentration from birth.
Papilla Length and Urine-Concentrating Ability Across Species
The papilla is one of the best examples of form following function in comparative anatomy. Desert-dwelling mammals that must conserve every drop of water tend to have extremely long papillae, while aquatic or semi-aquatic mammals that have water readily available often have short or barely developed ones. The morphological features that vary across species include the length of the papilla, the percentage of long-looped nephrons that extend deep into it, the architecture of medullary vascular bundles, and the degree of collecting duct convergence at the papilla tip.20American Journal of Physiology. Urinary concentrating ability: insights from comparative anatomy A longer papilla creates more distance for the osmotic gradient to build, which means the kidney can produce more concentrated urine. Some macaque species, in contrast to humans, have a single-papilla kidney with a relatively underdeveloped papilla and fewer long loops of Henle, resulting in more limited concentrating ability.21PubMed Central. Normal Anatomy, Histology, and Spontaneous Pathology of the Kidney, and Selected Renal Biomarker Reference Ranges in the Cynomolgus Monkey
This evolutionary lens puts human kidney stones in an interesting context. Our papillae are long enough to produce reasonably concentrated urine but also create the conditions where mineral supersaturation can occur. The very architecture that lets us survive without constant water intake is also the architecture that seeds Randall’s plaques. It is a tradeoff baked into the design of the organ itself.