Gallstones pass through a series of narrow ducts that connect the gallbladder to the small intestine. The journey starts when a stone slips out of the gallbladder into the cystic duct, then enters the common bile duct, and finally reaches the duodenum (the first section of the small intestine) through a tight muscular valve. From there, the stone travels through the rest of the intestines and exits the body in stool. That’s the best-case scenario. In many cases, stones get stuck along the way.
The Route From Gallbladder to Intestine
The gallbladder is a small, pear-shaped organ that sits just beneath the liver. It stores bile, a digestive fluid produced by the liver, and releases it into the intestine after you eat. Gallstones form inside this organ and can sit there for years without causing problems. Trouble begins when a stone moves.
The first structure a gallstone enters is the cystic duct, a short tube that connects the gallbladder to the rest of the biliary system. This duct is only a few millimeters wide, making it the first common place for a stone to get stuck. If the stone clears the cystic duct, it drops into the common bile duct, a wider channel that carries bile from both the liver and gallbladder down toward the intestine.
At the bottom of the common bile duct sits a ring of muscle called the sphincter of Oddi. This valve controls the flow of bile into the duodenum and prevents intestinal contents from backing up into the bile duct. It opens and closes rhythmically, with a resting pressure of about 10 mmHg and periodic contractions that can reach 50 to 140 mmHg. When you eat, a hormone triggers this sphincter to relax, allowing bile (and potentially a small stone) to pass into the intestine. Once a stone reaches the duodenum, it moves through the rest of the small intestine and colon and eventually passes in stool.
Where Stones Get Stuck
Each narrowing along this path is a potential trap. The three most common places for a gallstone to lodge are:
- The neck of the gallbladder or cystic duct. A stone caught here blocks bile from leaving the gallbladder. This causes the gallbladder itself to become inflamed and swollen, a condition called cholecystitis.
- The common bile duct. A stone lodged here blocks bile drainage from both the liver and gallbladder. This can cause jaundice (yellowing of the skin and eyes), dark urine, and pale stools because bile pigments can’t reach the intestine.
- The sphincter of Oddi. Because the pancreatic duct also empties through this valve, a stone stuck at the very end of the common bile duct can block pancreatic drainage too, potentially triggering pancreatitis. The trauma of a stone forcing its way through this sphincter can also cause scarring that leads to long-term dysfunction of the valve.
What Size Can Pass Naturally
The common bile duct in a healthy adult measures roughly 6 to 8 mm in diameter. Stones smaller than this generally have the best chance of passing on their own. Most gallstones that pass without intervention are small, but there are documented cases of stones larger than 1 cm making it through the common bile duct and into the duodenum spontaneously. Gallstones themselves range enormously in size, from a grain of sand to a golf ball, so the vast majority of larger stones will not pass on their own and require treatment.
Cholesterol stones, the most common type, are usually yellow-green and pebble-like. Pigment stones, made from bilirubin, are smaller and dark brown or black. If a stone does pass completely, you’re unlikely to notice it in stool unless you’re looking carefully, and even then small stones are easy to miss.
What It Feels Like When a Stone Moves
The pain caused by a gallstone trying to pass through a duct is called biliary colic. It typically hits suddenly in the upper right abdomen or just below the breastbone and can radiate to the back between the shoulder blades or into the right shoulder. The pain builds rapidly and stays intense. A single episode lasts anywhere from 20 minutes to several hours, often starting after a meal when the gallbladder contracts to release bile.
Biliary colic is different from the dull, achy discomfort of indigestion. It tends to be steady and severe rather than crampy, and it often comes with nausea or vomiting. If the pain lasts longer than a few hours, is accompanied by fever, or your skin turns yellow, the stone may be fully obstructing a duct and causing infection or inflammation that needs urgent treatment.
The Rare Alternate Route
In unusual cases, a large gallstone bypasses the bile duct system entirely. When the gallbladder becomes severely inflamed and presses against the intestine wall over time, the tissues can erode into each other, forming an abnormal opening called a fistula. About 60% of these fistulas connect the gallbladder directly to the duodenum. Through this opening, a large stone can drop straight into the intestine.
The problem is what happens next. Stones that enter the gut this way are often too large to pass through the entire intestinal tract. They most commonly get stuck at the terminal ileum, the narrowest section of the small intestine near its junction with the large intestine. This is called gallstone ileus, and it causes a bowel obstruction. Stones under 2 cm may still pass on their own through the intestine, but larger ones typically require surgical removal.
How Stuck Stones Are Found and Removed
When doctors suspect a stone is lodged in the common bile duct, they use imaging to confirm its location. A specialized MRI scan of the bile ducts (called MRCP) detects stones with about 88% sensitivity and 94% specificity compared to direct visualization. It’s noninvasive and serves as a reliable first step.
If a stone is confirmed in the common bile duct, the standard approach is a procedure called ERCP, where a flexible scope is passed through the mouth, down the stomach, and into the duodenum. The doctor can then access the opening of the bile duct, widen the sphincter of Oddi, and extract the stone directly. This is both a diagnostic and treatment tool. For stones stuck in the gallbladder neck or cystic duct, the typical treatment is surgical removal of the gallbladder itself, since stones that have caused one episode of symptoms tend to cause more.