The calyces are cup-shaped chambers inside each kidney that collect urine as it drains from the kidney tissue and funnel it toward the bladder. They sit between the microscopic filtering units of the kidney (nephrons) and the larger renal pelvis, acting as the first collection point in the urinary drainage system. Each kidney typically contains 7 to 9 of these small cups, organized into a branching system that merges into progressively larger channels.
Minor and Major Calyces
The calyceal system has two tiers. The smaller units, called minor calyces, wrap around the tip of each cone-shaped section of kidney tissue (called a renal pyramid). Each minor calyx catches the urine that exits through tiny openings at the pyramid’s tip. Think of it like a series of small funnels, each positioned beneath a spout.
Two or three minor calyces then merge to form a major calyx, a wider channel that pools urine from a larger section of the kidney. Most kidneys have two or three major calyces, which in turn empty into the renal pelvis, the broad, flat reservoir at the center of the kidney that connects to the ureter. From the ureter, urine travels down to the bladder. The full path looks like this: nephron filtering units → collecting ducts → minor calyces → major calyces → renal pelvis → ureter → bladder.
How the Calyces Move Urine
The calyces don’t just passively hold urine. Their walls contain smooth muscle and, importantly, specialized pacemaker cells that generate spontaneous electrical signals. These signals trigger coordinated, wave-like contractions (peristalsis) that push urine from the calyces down through the renal pelvis and into the ureter. In species with multiple kidney papillae, including humans, these contractions originate in the calyces themselves. Pacemaker cells are most densely concentrated in the minor and major calyces, with their numbers tapering off closer to the renal pelvis.
This active pumping matters because urine production is continuous, and the system needs to keep fluid moving even when you’re lying down and gravity isn’t helping. The rhythmic contractions ensure urine doesn’t pool and stagnate inside the kidney.
What Lines the Calyces
The inner surface of the calyces is lined with a specialized tissue called urothelium, the same stretchy, waterproof lining found throughout the urinary tract. This tissue has multiple cell layers, including large “umbrella cells” on the surface that can flatten and expand as the calyx fills and empties. The urothelium’s primary job here is forming a barrier that prevents urine (which contains waste products and can be quite acidic) from leaking back into the kidney tissue. At the very tips of the renal pyramids, the lining gradually transitions from a simpler, thinner epithelium to the full multilayered urothelium found deeper in the collecting system.
Kidney Stones and the Calyces
The calyces are one of the most common locations where kidney stones form or become lodged. Most stones begin as tiny mineral deposits at the junction where the collecting ducts meet the renal pelvis, right at the papillary tips that project into the minor calyces. These deposits start beneath the lining, then gradually grow until they break through into the calyx. Once exposed to urine pooling in the calyx, the deposit serves as a seed crystal that accumulates more mineral layers over time.
A stone sitting in a calyx may cause no symptoms at all if it’s small and not blocking drainage. Problems start when the stone grows large enough to obstruct the narrow neck of a calyx or when it dislodges and moves into the ureter. That’s when the sharp, intense flank pain most people associate with kidney stones typically begins.
Calyceal Diverticula
Some people have small pouch-like outgrowths that branch off from a calyx into the surrounding kidney tissue. These are calyceal diverticula, found in roughly 0.2% to 0.6% of adults. They connect to the main collecting system through a narrow channel, so urine passively fills them but drains poorly. About one-third to one-half of people with a diverticulum eventually develop flank pain, urinary tract infections, or blood in the urine. Stones form inside the diverticulum in up to half of cases, largely because urine sits stagnant in the pouch long enough for minerals to crystallize.
Calyceal Blunting on Imaging
When doctors image the kidneys with a CT scan or similar study, the calyces normally appear as sharp, pointed cups. If those cups look rounded or “clubbed” instead, that’s called calyceal blunting. It signals that something has damaged or destroyed the papillary tips that normally project into the calyces. Several conditions can cause this, including chronic kidney infections, sickle cell disease, diabetes, and long-term overuse of certain pain medications. A condition called renal papillary necrosis, where the papillary tissue dies from reduced blood flow or toxic injury, is one of the more common causes. In advanced cases, the dead papillary tissue sloughs off entirely, leaving the calyx with a blunt, rounded shape visible on imaging.
Calyceal blunting can also appear with hydronephrosis, a backup of urine that causes the calyces and renal pelvis to swell. The increased pressure stretches and flattens the normally sharp calyceal cups. This can happen from a kidney stone blocking the ureter, an enlarged prostate, or anything else that obstructs urine flow downstream.