Gallbladder

The bile reservoir. The gallbladder stores and concentrates the liver's bile between meals, then squeezes it into the duodenum the moment fat arrives.

Primary Role: Bile storage, concentration, and timed release
Metabolic Pathways: Bile-salt enterohepatic cycling; water and ion absorption
Energy Budget: Concentrates bile up to tenfold by pumping out water
Key Cell Types: Gallbladder epithelium, Smooth muscle coat

Structure & Metabolism

A pear-shaped sac under the liver's right lobe, holding 30 to 50 milliliters. Its mucosa absorbs water and electrolytes aggressively, concentrating bile's salts and pigments several-fold; a muscular coat encircles the reservoir for the mealtime squeeze. The cystic duct joins the hepatic duct to form the common bile duct — the system's single-lane road, gallstone territory.

Metabolic Pathways

Bile salts cycle liver to duodenum to terminal ileum and back — the enterohepatic loop the biliary liver depends on.
Concentration by isotonic water reabsorption keeps bile's detergent power high in a small volume.
Bilirubin passes through on its way to becoming fecal pigment — the color chain from spleen culling to stool.

Regulation

Cholecystokinin from duodenal I cells contracts the sac and relaxes the sphincter of Oddi — fat in the duodenum is the trigger.
Fasting keeps the sphincter closed and the gallbladder filling.
Vagal input augments the mealtime response.

Interactions

Downstream partner of the liver's biliary secretion — maker and reservoir.
Delivers the bile that lets pancreatic lipase work on emulsified fat.
Ileal resection or Crohn disease breaks the recycling loop and drains the bile-acid pool.

Clinical Relevance

Gallstones: cholesterol supersaturation plus stasis — biliary colic when they obstruct.
Cholecystitis: obstructed cystic duct with inflammation; cholecystectomy is among the commonest surgeries.
Post-cholecystectomy life is nearly normal — the bile duct adapts; fat tolerance usually survives.
Sludge forms in fasting and pregnancy; most gallstones are cholesterol, the rest pigment from hemolysis.
Porcelain gallbladder and gallbladder cancer are the rare complications that justify follow-up of persistent stones.

Sources

Wikipedia: Gallbladder
Wikipedia: Bile