After leaving the collecting duct, urine flows through a series of progressively larger structures inside the kidney before traveling down to the bladder. The full path runs from the collecting duct to the renal papilla, into the minor calyces, then the major calyces, the renal pelvis, the ureter, the bladder, and finally out through the urethra. Each structure along the way plays a specific role in moving and storing urine.
From the Collecting Duct to the Renal Papilla
Multiple collecting ducts from different nephrons merge together as they descend deeper into the kidney’s inner tissue. These join to form larger papillary ducts, which open at the tip of cone-shaped structures called renal pyramids. The tip of each pyramid is the renal papilla, and its surface (called the area cribrosa) is dotted with tiny openings where urine drips out. Think of it like multiple streams converging into a single waterfall that empties into a cup below.
Minor Calyces, Major Calyces, and the Renal Pelvis
Each renal papilla is enclosed by a cup-shaped chamber called a minor calyx. Most kidneys contain 7 to 9 minor calyces. These small cups catch the urine as it exits the papilla and channel it inward. Every 2 or 3 minor calyces then converge into a larger chamber called a major calyx.
The major calyces join together to form the renal pelvis, a funnel-shaped space at the center of each kidney. The renal pelvis is essentially the collection point for all urine produced by that kidney, and it marks the transition from kidney tissue to the tube that carries urine downward: the ureter.
How the Ureters Move Urine to the Bladder
Each kidney connects to the bladder through its own ureter, a muscular tube that actively squeezes urine downward. This isn’t a passive drip. Specialized pacemaker cells located in the minor calyces generate rhythmic contractions that ripple down the ureter’s walls, pushing urine in waves toward the bladder. The ureter has three layers of smooth muscle oriented in different directions, giving it the ability to contract powerfully in a coordinated sequence.
This wave-like squeezing (peristalsis) starts at the renal pelvis and travels the full length of the ureter. A healthy adult kidney produces roughly 35 to 70 mL of urine per hour (for someone weighing about 70 kg), so the ureters don’t need to handle large volumes at once. They move small boluses of urine with each contraction.
Where the Ureter Meets the Bladder
The junction where each ureter enters the bladder has a built-in valve mechanism that prevents urine from flowing backward toward the kidneys. The final segment of the ureter passes through the bladder wall at an angle, and as the bladder fills, increasing pressure compresses this tunnel shut. Any activity that raises pressure inside the bladder, like coughing, sneezing, or straining, automatically increases the closing pressure on this segment as well. The ureter in this zone doesn’t contract on its own, which helps it stay sealed. This passive design keeps urine moving in one direction only.
When this mechanism fails, a condition called vesicoureteral reflux allows urine to wash back up toward the kidneys, which can lead to infections and kidney damage over time.
Storage in the Bladder
The bladder is a muscular reservoir that expands as it fills. Women can store up to about 500 mL and men up to about 700 mL, though you’ll feel the urge to urinate well before that point, typically when the bladder holds between 150 and 250 mL.
While urine accumulates, two sphincters keep the exit closed. The internal sphincter is made of smooth muscle and stays contracted automatically through signals from sympathetic nerves. The external sphincter is skeletal muscle under your voluntary control, which is why you can consciously hold urine even when the urge is strong. Together, these “guarding reflexes” maintain continence during storage.
When you decide to urinate, your brain sends signals that relax both sphincters and contract the bladder wall. The bladder muscle squeezes inward, raising internal pressure and pushing urine out through the urethra.
Exit Through the Urethra
The urethra is the final passage. In females, it’s short, only about 3 to 4 centimeters, running from the bladder to an opening in front of the vagina. In males, it’s roughly 20 centimeters long and passes through the prostate gland, the pelvic floor, and the full length of the penis before reaching the external opening. The male urethra has three distinct sections: one through the prostate, a short segment through the pelvic floor muscles, and a longer section running through the penis.
Where Obstructions Commonly Occur
The path from collecting duct to urethra has a few natural narrow points where kidney stones tend to get stuck. The most common trouble spots are the junction between the renal pelvis and the ureter (the ureteropelvic junction) and the lower portion of the ureter near the bladder. Stones lodged at these points block urine flow and cause the intense, wave-like pain characteristic of kidney stone episodes. Stones within the kidney itself often cause no symptoms unless they grow large enough to obstruct the collecting system or become infected.