What is the function of the gallbladder in the digestive system?

The Role of the Gallbladder in the Digestive System

The gallbladder primarily functions to store and concentrate bile during fasting, and release it in a controlled manner into the duodenum during digestion, playing a crucial role in fat absorption and maintaining biliary homeostasis. 1

Primary Functions of the Gallbladder

Storage and Concentration of Bile

• The gallbladder stores bile produced by the liver during fasting periods 1
• It concentrates bile by absorbing water and electrolytes, making it 5-10 times more concentrated than when initially secreted by the liver 1
• This concentration process optimizes bile's digestive capabilities when released

Controlled Bile Release

• The gallbladder empties primarily after meals (postprandially) through coordinated contraction of the gallbladder and simultaneous relaxation of the sphincter of Oddi 1
• This coordinated action maintains pressure in the common bile duct higher than in the duodenum or gallbladder during fasting 1
• The process ensures maximal nutrient assimilation within the proximal small intestine 1

Hormonal Control

• Cholecystokinin (CCK) is the principal hormone controlling gallbladder emptying 1, 2
• CCK is a 33-amino acid polypeptide produced in the proximal small bowel in response to food intake 1
• CCK binds to receptors in the gallbladder causing contraction and to receptors on inhibitory neurons innervating the sphincter of Oddi causing relaxation 1

Protective Role of the Gallbladder

• The gallbladder protects the liver, stomach mucosa, and colon from toxic effects of secondary bile acids 3
• By accumulating primary bile acids (cholic acid and chenodeoxycholic acid), it reduces the formation of more hydrophobic secondary bile acids (deoxycholic acid and lithocholic acid) 3
• This protective function helps maintain the balance between hydrophobic and hydrophilic bile acids in the enterohepatic circulation 3

Bile Composition and Formation

• Bile consists of approximately 5% organic and inorganic solutes of considerable complexity 4
• The bile-secretory unit begins with canalicular networks formed by the apical membrane of adjacent hepatocytes 4
• Bile canaliculi (approximately 1 μm in diameter) conduct bile countercurrent to portal blood flow 4
• These connect with the canal of Hering and bile ducts, which increase in diameter before entering the gallbladder and common bile duct 4

Coordination with Digestive Processes

• Gallbladder contraction typically occurs 5-20 minutes after food enters the stomach 3
• This is significantly earlier than when food (gastric chyme) moves from the stomach to the duodenum (1-3 hours later) 3
• Postprandial motor and secretory functions are coordinated to ensure efficient digestion 1
• The gallbladder's contraction and bile release are particularly important for fat digestion, as human pancreatic lipase is less stable than other digestive enzymes 1

Clinical Significance

• Gallbladder dysfunction can lead to various disorders including cholelithiasis, cholecystitis, and functional gallbladder disorder 5
• Impaired gallbladder motility is a key factor in cholesterol gallstone formation 6
• Gallbladder ejection fraction (GBEF) measured by cholecystokinin-cholescintigraphy (CCK-CS) can assess gallbladder function in patients with suspected functional gallbladder disorders 1

Interdigestive Function

• Even during fasting, the gallbladder is not quiescent but functions in concert with interdigestive cyclical motility 1
• This interdigestive activity helps degrade luminal debris, promote antegrade peristalsis, and prevent bacterial overgrowth 1
• The gallbladder's activity during fasting helps maintain intraluminal homeostasis even during prolonged periods without food 1

Understanding the gallbladder's role in bile storage, concentration, and controlled release is essential for comprehending various biliary tract disorders and their management approaches.