The common bile duct of a 4 kg cat is a tube 3 to 5 cm long with a lumen of 2 to 3 mm, and most decisions about feline liver disease pass through that number. At 2.5 mm the duct carries bile at the rate of a slow drip; narrow it to 1.5 mm and the resistance to the same flow rises about 8-fold. What follows covers the 2 junction patterns that build the duct, the 3 variations a sonographer or surgeon has to expect, and the arithmetic that converts a millimetre of lumen into a pressure.
Six segments from lobe to duodenum
Bile leaves a liver of 5 to 6 lobes through 2 hepatic ducts, right and left, which run 1 to 2 cm before merging into a common hepatic duct of 1.5 to 2 mm. The gallbladder sits between the right medial and quadrate lobes, measures 2 to 3 cm long, and holds 2 to 4 mL. Its cystic duct is 1 to 2 cm long and 1 to 1.5 mm wide. Cystic plus common hepatic forms the common bile duct: 2 to 3 mm across, 3 to 5 cm long, wall 0.2 to 0.5 mm thick. It passes dorsal to the duodenum, then runs obliquely through the duodenal wall for 5 to 10 mm before opening at the major duodenal papilla, 2 to 5 cm distal to the pylorus.
Dimensions of each segment
| Segment | Length (cm) | Lumen (mm) | Wall (mm) |
|---|---|---|---|
| Right hepatic duct | 1–2 | 1.0–1.5 | 0.1–0.3 |
| Left hepatic duct | 1–2 | 1.0–1.5 | 0.1–0.3 |
| Common hepatic duct | 1–2 | 1.5–2.0 | 0.2–0.4 |
| Cystic duct | 1–2 | 1.0–1.5 | 0.2–0.4 |
| Common bile duct | 3–5 | 2.0–3.0 | 0.2–0.5 |
| Intramural segment | 0.5–1.0 | 1.0–2.0 | 0.3–0.6 |
Two junction patterns
Pattern one is the high, Y-shaped junction: the cystic duct joins the common hepatic duct within 1 cm of the hilus, so the common bile duct starts early and is 4 to 5 cm long. Pattern two is the low, parallel junction: the cystic duct descends alongside the common bile duct for 1 to 3 cm, sharing a sheath of connective tissue, and enters it within 1 cm of the duodenal wall; occasionally the 2 open side by side on the papilla. The high pattern is the one drawn in most textbooks; the low pattern is common enough in cats that a surgeon should dissect the cystic duct to the gallbladder rather than assume where it ends. On ultrasound the low pattern also produces 2 parallel tubes 1 to 2 mm apart, which read as a double duct if the probe is swept too fast.
The feline difference: 1 pancreatic duct, not 2
Cats have a single pancreatic duct, and it opens on the minor duodenal papilla 1 to 3 cm distal to the bile papilla. Dogs have 2. Cats also usually lack a true hepatopancreatic ampulla, so bile and pancreatic juice reach the gut through separate ostia on the same short segment of duodenum. That arrangement matters clinically: inflammation of the right pancreatic lobe can compress the distal duct from outside without any shared channel, and the intramural course is only 5 to 10 mm long, so 1 mm of mural edema halves the lumen of a 2 mm duct.
Three variations that change the plan
- Accessory hepatic duct. A branch 1 to 2 mm wide drains part of the right medial lobe straight into the gallbladder or cystic duct instead of joining the hilus. If it is missed during cholecystectomy, bile leaks at 0.3 to 0.5 mL/kg/h until it is found.
- Double or septate gallbladder. Two cavities 1 to 3 cm long, either sharing 1 cystic duct or draining through 2. The second cavity is easy to leave behind and easy to mistake for a folded gallbladder on a 3 mm slice.
- Low or separate cystic duct entry. Covered above, but worth listing on its own because it changes both surgical landmarks and the reading of a 2 to 3 mm duct on imaging.
What 2 to 3 mm looks like
A 2 mm lumen is the width of a grain of rice at 1.5 to 2 mm and the diameter of a 2 mm pencil lead; 3 mm is the thickness of a matchstick. On ultrasound at 7.5 to 10 MHz the axial resolution is 0.3 to 0.6 mm, so the duct spans 4 to 10 pixels and must be measured inner wall to inner wall, perpendicular to its axis. Most references take 3 mm as the upper limit for a normal cat, 3 to 4 mm as equivocal, and above 4 mm as abnormal. Different individuals and laboratories differ slightly — operator, machine, and depth add about 0.5 mm of spread — so a single measurement on one day carries less weight than 2 measurements 4 weeks apart.
Worked conversion: lumen to resistance
Flow through a narrow tube follows the fourth power of the radius. Take a 2.5 mm duct: radius 1.25 mm, cross-section 3.14 × 1.25² = 4.9 mm². Take a 1.5 mm duct: radius 0.75 mm, cross-section 1.8 mm². The area falls by 2.8-fold, but resistance rises by (2.5 ÷ 1.5)⁴ = 7.7-fold, which is why 1 mm of swelling does more than 1 mm of stone. Flow gives the other half of the picture: hepatic bile production is 0.3 to 0.5 mL/kg/h, so a 4 kg cat makes 1.2 to 2 mL/h, or 29 to 48 mL/day. The gallbladder concentrates that 5 to 10 times, so 3 mL of stored bile represents 15 to 30 mL of hepatic bile — and a duct narrowed to 1.5 mm still passes 30 mL/day, slowly, which is exactly why obstruction can run for weeks before jaundice appears.
Reference values and the signs that override them
Bilirubin above 2 to 3 mg/dL turns the gums and sclera yellow; alkaline phosphatase in cats often rises 5 to 20 times the top of the reference interval with extrahepatic obstruction, while alanine aminotransferase may stay near normal. Cats also dilate less than dogs: a duct obstructed for a week may sit at 3.5 to 4 mm rather than the 8 to 10 mm a dog would show, so a duct that looks only borderline on a single scan is not reassurance. Vomiting for more than 24 hours, jaundice, pale stools, or a cat that stops eating for 2 days is a same-week appointment, and jaundice with fever is an emergency.
Four practical steps
- Measure inner-to-inner, at 90 degrees to the duct, and write the number with the machine and probe frequency beside it.
- Compare against 3 mm, then repeat in 4 weeks before calling a borderline duct normal or abnormal.
- During surgery, dissect the cystic duct to the gallbladder wall — the low junction pattern and an accessory duct 1 to 2 mm wide both live in that 1 to 3 cm.
- Pair the anatomy with function: read the bile acid cycle for what the duct is carrying, and ocular anatomy for why jaundice shows in the sclera first. The breed quiz is a lighter way in.
Terms mentioned in this article
Frequently asked
What is a normal common bile duct diameter in a cat, and how should it be measured?
Most references put the normal lumen at 2 to 3 mm, with 3 mm as the practical upper limit, 3 to 4 mm as equivocal, and above 4 mm as abnormal. Measure inner wall to inner wall, perpendicular to the duct axis, at 7.5 to 10 MHz. Cats dilate less than dogs, so an obstructed duct may sit at only 3.5 to 4 mm after a week rather than the 8 to 10 mm seen in dogs.
Why does the cat have only one pancreatic duct, and why does that matter?
Cats carry a single pancreatic duct that opens on the minor duodenal papilla 1 to 3 cm distal to the bile papilla, while dogs have two, and cats usually lack a true hepatopancreatic ampulla. Bile and pancreatic juice therefore reach the gut through separate ostia, so right-lobe pancreatitis can compress the 2 to 3 mm bile duct from outside without any shared channel being involved.
How much does a small narrowing actually reduce bile flow?
Resistance in a narrow tube scales with the inverse fourth power of the radius, so a duct that falls from 2.5 mm to 1.5 mm keeps only 36 percent of its cross-section but faces about 7.7 times the resistance. Flow itself is 0.3 to 0.5 mL/kg/h, roughly 29 to 48 mL a day in a 4 kg cat, which is slow enough that obstruction can run for weeks before jaundice shows.