What is Dr. Zubair Khalid's research focus?

Dr. Zubair Khalid specializes in molecular virology, mRNA vaccine development, and computational biology, with a focus on avian pathogens like IBDV and Avian Reovirus.

Where is Dr. Zubair Khalid currently working?

Dr. Zubair Khalid is a Postdoctoral Research Associate at the University of Maryland (UMD), specifically within the Department of Animal and Avian Sciences.

Pelvis Renalis: Anatomy of the Kidney Pelvis

Name: Pelvis Renalis: Anatomy of the Kidney Pelvis
Creator: Dr. Zubair Khalid

By Dr. Zubair Khalid, DVM, MS, PhD · 2026-09-20

The pelvis renalis (pelvis of the kidney) is the funnel-shaped expansion of the proximal ureter that sits within the renal sinus and collects urine before it enters the ureter. It is the central urine-collecting chamber of the kidney, continuous with the major and minor calyces where those structures exist, and continuous with the renal crest in species that have a single fused papilla.

This structure matters because it is the anatomical bottleneck of the upper urinary tract. Every drop of urine the kidney produces passes through the pelvis renalis on its way to the bladder. When the pelvis dilates (pyelectasia), when a stone lodges at the ureteropelvic junction, or when a tumor grows from the pelvic urothelium, the clinical signs trace directly back to this small funnel. Understanding its normal shape across species is what lets a clinician recognize the abnormal one.

Definition and Position Within the Kidney

The kidney has an outer cortex, an inner medulla, and a central cavity called the renal sinus. The renal sinus is not empty space. It contains the pelvis renalis, the major and minor calyces (when present), branches of the renal artery and vein, lymphatics, and a variable amount of adipose tissue.

The pelvis renalis is the most proximal part of the ureter that has dilated into a funnel. At its narrow end it continues as the ureter. At its wide end it receives urine from the collecting system. The transition from pelvis to ureter is the ureteropelvic junction, a common site of obstruction in both animals and people [1].

Two anatomical relationships determine how the pelvis behaves clinically. The first is how much of the pelvis sits inside the renal parenchyma versus outside it. A renal pelvic score has been used in human surgery to classify a pelvis as intraparenchymal when more than 50 percent of it is contained within the volume of the renal parenchyma, or extraparenchymal when less than 50 percent is contained [2]. This distinction affects surgical planning and the risk of urine leak after partial nephrectomy [2]. The same principle applies conceptually in veterinary surgery, where an entirely intrarenal pelvis is harder to access and easier to injure.

The second relationship is the angle formed between the pelvis and the infundibula of the lower pole calyces. In humans, a reduced infundibulopelvic angle correlates with stone formation and with lower stone-free rates after surgery [3][4]. The angle between the lower infundibulum and the renal pelvis averaged 112.5 degrees in normal human kidneys and 96.6 degrees in kidneys with lower pole stones, a statistically significant difference [3]. This is human data, but it illustrates a general principle: the geometry of the pelvis and its outflow tracts affects how well urine and small calculi drain.

How the Pelvis Collects Urine

Urine formation ends in the collecting ducts, which open at the tip of the renal papilla or along the renal crest. From that point, urine is no longer modified. It is simply transported.

The pathway runs as follows:

Collecting ducts open at the area cribrosa on the papilla or crest.
Urine drips into the minor calyx (in multipapillary kidneys) or directly onto the renal crest and into the pelvis (in unipapillary kidneys).
Minor calyces merge into major calyces.
Major calyces merge into the pelvis renalis.
The pelvis narrows at the ureteropelvic junction and continues as the ureter.

In species with a renal crest, the crest is a fused ridge of medullary tissue that projects into the pelvic cavity. The camel kidney, which is unipapillary with a complete renal crest, has a main crescentic pelvic cavity that follows the long axis and curvature of the kidney, and a thick renal crest that projects into that cavity [5]. The crest contains large numbers of long loops of Henle and vasa recta, the structures responsible for concentrating urine [5]. The pelvis is not a passive bag. Its close association with the medulla allows it to participate in urea recycling, which supports the countercurrent mechanism of urine concentration [5].

This is a key point for students. The pelvis renalis is both a conduit and, in some species, a contributor to the concentrating mechanism. In the camel, an elaborate pelvis closely associated with the renal medulla is one of the anatomical requisites for efficient water conservation [5]. The equine, caprine, ovine, and canine kidneys share the unipapillary, crest-based architecture that makes this possible [5].

Comparative Anatomy Across Species

The single most important fact in comparative renal anatomy is that not all mammals package their collecting system the same way. The pelvis renalis is present in all domestic species, but what feeds into it differs.

Unipapillary Kidneys: Dog and Cat

The dog and cat have smooth, bean-shaped, unipapillary kidneys. All the medullary pyramids are fused into a single renal crest, also called the renal papilla or common papilla. There are no true calyces. The crest projects into the pelvis renalis, and urine drains from the crest directly into the pelvic cavity.

In the dog, fetal kidney development has been tracked ultrasonographically. The fetal renal pelvis is initially visible and measurably distended, with a diameter ranging from 0.06 cm to 0.17 cm during gestation. By the end of gestation, the renal pelvis is no longer dilated and its diameter can no longer be measured [6]. This is a normal developmental sequence, not a disease. A distended fetal pelvis that persists would be abnormal.

In the cat, the renal pelvis has lower stiffness than the renal cortex on acoustic radiation force impulse elastography, appearing as a white (more deformable) region on qualitative elastography [7]. This is a normal tissue property, not a sign of disease, but it helps explain why the pelvis deforms differently from surrounding tissue under pressure.

Multipapillary Kidneys: Pig and Cattle

The pig and cow have multipapillary kidneys. Individual medullary pyramids remain separate, each with its own papilla. Each papilla is cupped by a minor calyx. Minor calyces drain into major calyces, and major calyces drain into the pelvis renalis.

This architecture means the bovine and porcine pelvis renalis is a true collecting chamber with calyceal tributaries, structurally closer to the human kidney than to the dog or cat kidney. The practical consequence is that in cattle and pigs, the pelvis is a larger and more complex space, and the calyces are distinct anatomical entities that can be individually affected by disease.

The bovine kidney is also lobated externally in the fetus and neonate, though the lobes fuse to varying degrees in the adult. The internal multipapillary architecture persists regardless of external appearance.

Renal Crest with Terminal Recesses: Horse

The horse has a unipapillary kidney with a renal crest, similar in principle to the dog and cat. The distinguishing feature is the presence of terminal recesses (recessus terminales) at the cranial and caudal poles of the kidney. These are extensions of the pelvis renalis that project into the polar parenchyma.

The equine pelvis is therefore more elaborate than the simple funnel of the dog. The terminal recesses increase the surface area of pelvic urothelium in contact with medullary tissue, which may support the urine-concentrating mechanism in a species adapted to variable water availability.

Summary Table

Species	Kidney type	Papilla or crest	Calyces	Pelvis characteristics
Dog	Unipapillary, smooth	Single renal crest	Absent	Simple funnel, crest projects into pelvis
Cat	Unipapillary, smooth	Single renal crest	Absent	Simple funnel, crest projects into pelvis
Pig	Multipapillary	Multiple separate papillae	Present (minor and major)	Complex chamber receiving calyceal drainage
Cattle	Multipapillary	Multiple separate papillae	Present (minor and major)	Complex chamber receiving calyceal drainage
Horse	Unipapillary with terminal recesses	Single renal crest	Absent	Funnel with cranial and caudal terminal recesses

The Renal Crest and Urine Collection

The renal crest is the fused termination of all medullary pyramids in unipapillary kidneys. It is not a papilla in the strict sense because it is a ridge rather than a cone, but it serves the same function: it is the surface where collecting ducts open and urine enters the pelvis.

The crest is rich in long loops of Henle and vasa recta [5]. These structures create the osmotic gradient that allows the kidney to produce concentrated urine. The proximity of the crest to the pelvic cavity means that urine entering the pelvis is in close contact with the medullary tissue that will ultimately determine its concentration.

In the camel, the renal crest is thick and extensive, projecting into the pelvic cavity [5]. The camel kidney is described as bean-shaped, smooth, multilobar, and unipapillary, with complete fusion of the renal papillae into a common crest that channels urine into a central renal pelvis [5]. This description applies broadly to the equine, caprine, ovine, and canine kidney as well [5].

For students, the key relationship is this: collecting ducts empty at the crest, the crest projects into the pelvis, and the pelvis continues as the ureter. There is no calyx in the dog or cat. Any structure between the crest and the ureter in these species is pelvis, not calyx.

Clinical Relevance, Limitations and Common Mistakes

The pelvis renalis is involved in a wide range of clinical conditions. Dilation of the pelvis, called pyelectasia or pyelectasis, is one of the most common ultrasonographic findings in veterinary medicine.

In cats with confirmed ureteral obstruction, sonographic measurement of the renal pelvis is used for diagnosis and management. A study comparing subjective and objective measurement methods found that subjective assessment had the lowest interobserver agreement in every plane (transverse kappa = 0.326, sagittal kappa = 0.311, dorsal kappa = 0.473). The standardized measurements with the highest interobserver agreement measured the distance between the renal crest and the ureter in dorsal (ICC = 0.91) and transverse planes (ICC = 0.907) [8]. This means that when you measure the pelvis, you should measure from the crest to the ureter using a standardized technique, not estimate by eye.

In dogs with ureteral calculi, pyelectasis was present in 12 of 14 cases, indicating partial to complete obstruction of urinary flow [9]. The presence of pyelectasis did not differ significantly between dogs with and without abdominal pain or azotaemia [9]. This is a critical point: a dilated pelvis does not always mean the animal is in pain or azotemic, and a normal-appearing pelvis does not rule out obstruction.

The pelvis can also be the site of primary disease. A malignant peripheral nerve sheath tumor was reported in the renal pelvis of a Border Collie, presenting with hematuria and a vascularized renal mass [10]. The tumor was located beneath the lamina propria of the transitional epithelium of the renal pelvis and infiltrated the renal medulla [10]. This is rare, but it demonstrates that the pelvis is a site of primary neoplasia, not just a conduit.

Inflammation of the renal pelvis, or pyelitis, is a component of pyelonephritis. In a dog with emphysematous pyonephrosis, ultrasonography revealed a dilated right renal pelvis containing echogenic fluid with free gas, and Escherichia coli was isolated from pus from the right kidney [11]. The pelvis was both the site of infection and the source of the gas-producing process.

Developmental abnormalities of the pelvis and ureter also occur. Prenatal ultrasound in two puppies detected ectopic ureter and bilateral renal hypoplasia, with reduced, hyperechogenic fetal kidneys without a distinguishable renal pelvis [12]. The absence of a visible pelvis was a diagnostic feature of hypoplasia.

Common Mistakes

Mistake 1: Calling the pelvis a calyx in the dog or cat. Dogs and cats do not have calyces. The structure between the renal crest and the ureter is the pelvis renalis. Calyces are found in multipapillary kidneys (pig, cattle) and in humans.

Mistake 2: Assuming all pelvic dilation is obstruction. Pyelectasia can be physiologic, as seen in fetal dogs where the pelvis is transiently distended and then returns to normal by the end of gestation [6]. It can also be seen with reflux, infection, or after diuresis.

Mistake 3: Measuring the pelvis subjectively. Subjective assessment has poor interobserver agreement [8]. Use a standardized measurement from the renal crest to the ureter.

Mistake 4: Forgetting the terminal recesses in the horse. The equine pelvis extends into the cranial and caudal poles as terminal recesses. A lesion in a terminal recess may not be visible on a standard longitudinal image of the kidney.

Mistake 5: Confusing the renal pelvis with the renal sinus. The sinus is the entire central cavity. The pelvis is one structure within it, along with vessels, nerves, lymphatics, and fat.

This article is educational and is not a substitute for veterinary diagnosis or treatment.

Quick Review

The pelvis renalis is the funnel-shaped expansion of the proximal ureter within the renal sinus.
It receives urine from the renal crest (dog, cat, horse) or from major and minor calyces (pig, cattle).
Dogs and cats have unipapillary kidneys with a single renal crest and no calyces.
Pigs and cattle have multipapillary kidneys with distinct minor and major calyces.
Horses have a renal crest with terminal recesses at the cranial and caudal poles.
Pyelectasia is pelvic dilation and is a common sign of obstruction, but it can be physiologic.
Standardized measurement of the pelvis (crest to ureter) has higher interobserver agreement than subjective assessment.

Frequently Asked Questions

What is the pelvis renalis?

The pelvis renalis is the funnel-shaped expansion of the ureter inside the kidney. It collects urine from the renal crest or from the calyces and funnels it into the ureter.

Do dogs and cats have renal calyces?

No. Dogs and cats have unipapillary kidneys with a single renal crest and no true calyces. Urine drains directly from the crest into the pelvis renalis.

Which domestic animals have renal calyces?

Pigs and cattle have multipapillary kidneys with distinct minor and major calyces. Each medullary papilla is cupped by a minor calyx, and the calyces drain into the pelvis.

What is pyelectasia?

Pyelectasia is dilation of the renal pelvis. It is a common ultrasonographic finding and can indicate obstruction, infection, or reflux, though it can also be physiologic in some situations.

What is the renal crest?

The renal crest is the fused ridge of medullary tissue in unipapillary kidneys. Collecting ducts open at the crest, and urine passes from the crest into the pelvis renalis.

How is the renal pelvis measured on ultrasound?

The most reproducible measurement is the distance between the renal crest and the ureter, taken in the dorsal or transverse plane. Subjective assessment is less reliable.