Biliary Secretion

Introduction

Bile plays a vital role in digestion by facilitating fat absorption in the intestine. Efficient enterohepatic circulation conserves bile components for reuse, while removal of the gallbladder does not significantly impair digestion and may offer metabolic benefits.

Hepatobiliary System

• The liver secretes approximately 0.5 to 1 liter of bile per day.
• Bile is essential for digestion and absorption of fats in the intestine.

Functional Anatomy

• Bile is produced by hepatocytes and initially enters bile canaliculi located between adjacent liver cells. These canaliculi drain into small bile ductules within the liver.
• Bile ductules, along with branches of the portal vein and hepatic artery, form the portal triad.
• Blood in sinusoids surrounds hepatocytes, allowing close exchange of substances.
• Bile ductules merge to form larger ducts that unite as the right and left hepatic ducts. These ducts combine to form the common hepatic duct.
• The cystic duct from the gallbladder joins the hepatic duct to form the common bile duct.
• The common bile duct joins the pancreatic duct and opens into the duodenum.
• Entry of bile into the intestine is regulated by the sphincter of Oddi.

Composition of Bile

• Bile is a greenish-yellow fluid produced by the liver and stored in the gallbladder. It contains about 98 percent water and 2 percent solids.
• Solids include bile salts, bile pigments, and electrolytes.
• The color is due to bilirubin and biliverdin glucuronides.

Table 42.1: Composition of solids in human bile.

Category	Components
Organic constituents	Bile salts, bile pigments, cholesterol, fatty acids, enzymes
Electrolytes	Sodium, potassium, calcium, magnesium, bicarbonate, chloride, sulfate, phosphate

Table 42.2: Differences in hepatic and gallbladder bile.

Feature	Hepatic Bile	Gallbladder Bile
pH	Alkaline (~8–8.5)	Near neutral (~7)
Water	High (~98%)	Reduced (~85%)
Solids	Low (~2%)	Concentrated (10–15%)
Bile salts/pigments	Lower	Higher

Hepatic and Gallbladder Bile

• Hepatic bile is relatively dilute, whereas gallbladder bile is concentrated and slightly acidic.
• Concentration occurs mainly by absorption of sodium, followed by passive water reabsorption.
• Sodium–potassium adenosine triphosphatase in epithelial cells creates a gradient for sodium uptake.
• Water follows osmotically, reducing bile volume.
• Exchange of sodium with hydrogen ions leads to acidification of bile in the gallbladder.

Bile Acids and Salts

Bile Acids

• Bile acids are derived from cholesterol in hepatocytes and are essential for fat digestion.
• Primary bile acids include cholic acid and chenodeoxycholic acid. They are synthesized in the liver and secreted into bile.
• In the intestine, bacteria convert primary bile acids into secondary bile acids such as deoxycholic acid and lithocholic acid.

Bile Salts

• Bile salts are formed when bile acids are conjugated with glycine or taurine. These conjugated acids combine with sodium or potassium to form bile salts.
• Bile salts are more water-soluble and play a key role in emulsification and absorption of lipids.

Functions of Bile Salts

• Bile salts are essential for digestion and absorption of lipids and fat-soluble nutrients.
• Deficiency of bile salts leads to steatorrhea, characterized by excess fat in stool.
• They promote emulsification of fats by reducing surface tension, increasing surface area for enzyme action. This process facilitates effective digestion by pancreatic enzymes.
• Bile salts form micelles with lipids, phospholipids, and cholesterol.
• Micelles transport fatty acids and monoglycerides to intestinal epithelial cells for absorption.
• They aid in absorption of fat-soluble vitamins such as vitamins A, D, E, and K.
• Deficiency may result in clinical features of vitamin deficiency, especially in obstructive conditions.
• Bile salts contribute to the bile acid pool through enterohepatic circulation.
• They are reabsorbed in the intestine and returned to the liver for reuse.
• They act as choleretic agents, stimulating bile secretion from the liver.
• Bile salts enhance activity of pancreatic lipase, improving fat digestion.
• They help maintain cholesterol in a soluble form with lecithin, thereby preventing gallstone formation.
• Bile salts facilitate normal bowel movement by acting as mild physiological purgatives.
• They indirectly contribute to the normal brown color of stool through bile metabolism.
• They help maintain intestinal environment and limit excessive bacterial growth.

Enterohepatic Circulation

• Enterohepatic circulation refers to repeated recycling of bile acids and salts between the liver and intestine.
• Bile salts are synthesized in the liver (about 200–500 milligrams per day) and secreted into bile.
• They enter the intestine and aid in fat digestion.
• About 95 percent of bile salts are reabsorbed, mainly from the terminal ileum, into portal blood.
• Reabsorbed bile salts return to the liver and are re-secreted into bile.
• In the intestine, some conjugated bile salts are deconjugated and absorbed as free bile acids.
• Intestinal bacteria convert them into secondary bile acids, such as deoxycholic and lithocholic acids. These secondary bile acids are also reabsorbed and recycled.
• Only a small amount, about 200–500 milligrams per day, is lost in feces. This recycling process occurs multiple times daily, typically 4 to 12 cycles per day.

• Enterohepatic circulation conserves important substances and reduces the need for new synthesis. It also applies to compounds such as bile pigments, vitamin B12, vitamin D, hormones, and certain drugs.

Importance of Enterohepatic Circulation

• The primary role of enterohepatic circulation is to maintain the bile acid pool within the body.
• The total bile acid pool is approximately 2 to 4 grams.
• Bile acids are recycled multiple times daily, especially during meals. This allows a small pool to efficiently support fat digestion and absorption.
• A small fraction of bile acids is lost in feces during each cycle.
• Approximately 500 milligrams per day are lost and replaced by new synthesis from cholesterol. This process provides a pathway for elimination of excess cholesterol from the body.
• Efficient recycling reduces the metabolic burden of continuous bile acid synthesis.

Determinants of Enterohepatic Circulation

Integrity of Intestinal Epithelium

• Bile salts are mainly reabsorbed in the terminal ileum by active transport mechanisms.
• Only a small proportion reaches the colon under normal conditions.
• Intestinal bacteria convert bile salts into bile acids, which are more lipophilic and easily absorbed.
• Secondary bile acids formed in the colon are also reabsorbed into portal circulation.
• Damage to intestinal mucosa reduces absorption and leads to fat malabsorption.
• Chronic intestinal diseases impair recycling efficiency.

Efficiency of Hepatic Uptake

• In portal blood, bile salts are transported bound to albumin or lipoproteins.
• Hepatocytes extract more than 80 percent of bile salts from portal circulation.
• Efficient hepatic uptake ensures continuous secretion of bile salts into bile. This step is essential for maintaining effective circulation and bile production.
• Impairment of liver function reduces bile salt uptake and disrupts the cycle. Thus, both intestinal absorption and hepatic uptake determine the efficiency of enterohepatic circulation.

Bile Secretion

Mechanism of Secretion

• Bile secretion has two components: canalicular flow and ductular secretion.
• Canalicular flow depends mainly on bile acids and ionic transport.
• Ductal epithelial cells add bicarbonate-rich fluid to bile.

Mechanism of Bile Acid Secretion

• Bile acids are taken up from sinusoidal blood into hepatocytes by transport proteins.
• Inside hepatocytes, they bind to bile acid binding proteins, preventing cellular injury.
• Bile acids are conjugated with glycine or taurine to increase solubility.
• Conjugated bile acids are secreted into bile canaliculi. Their movement follows an electrochemical gradient maintained by micelle formation.
• Secretion of bile acids is the primary driving force for bile flow.

Mechanism of Water and Electrolyte Secretin

• Canalicular bile initially has a composition similar to plasma.
• Ductal epithelial cells actively secrete bicarbonate and chloride ions.
• Water follows osmotically, increasing bile volume. This process resembles secretion in pancreatic ducts.

Modification of Bile in Gallbladder

• Bile is continuously produced and stored in the gallbladder, which has a capacity of about 30 to 60 milliliters.
• The gallbladder performs three major functions:
  • Storage of bile between meals.
  • Concentration of bile by reabsorption of sodium and water.
  • Acidification of bile by secretion of hydrogen ions.

• Sodium is actively transported out of epithelial cells by sodium–potassium adenosine triphosphatase, creating a gradient.
• Sodium enters cells via exchange with hydrogen ions.
• Water follows sodium absorption, resulting in concentrated bile.
• These modifications prepare bile for efficient release during digestion.

Regulation of Bile Secretion

Bile secretion is regulated by choleretics and cholagogues.

Choleretics

• Choleretics are substances that increase bile formation by the liver.
• Important examples include bile salts, secretin, and vagal stimulation.
• Secretin specifically enhances the aqueous component of bile.

Cholagogues

• Cholagogues promote contraction of the gallbladder and bile release.
• Cholecystokinin and fatty acids are key cholagogues.
• Cholecystokinin causes gallbladder contraction and bile expulsion into the intestine.

Functions of Bile

• Bile facilitates absorption of lipids by forming micelles in the intestine.
• It promotes absorption of fat-soluble vitamins such as vitamins A, D, E, and K.
• Bile salts stimulate bile secretion and act as choleretic agents.
• Bile salts enhance activity of pancreatic lipase, aiding fat digestion.
• Bile pigments contribute to the characteristic color of bile.
• Bile salts act as mild physiological purgatives, supporting normal bowel movement.

Applied Physiology

Gallstones

• Gallstones are solid deposits formed in the gallbladder or bile ducts.
• The condition is termed cholelithiasis.
• Common risk factors include female sex, middle age, obesity, and multiparity.

Types

• Two major types are cholesterol stones and pigment stones.
• Cholesterol stones are more common and usually larger in size.
• Pigment stones, composed of calcium bilirubinate, are often multiple and smaller.
• Bile salts and phospholipids normally keep cholesterol dissolved.
• Stone formation occurs when bile becomes supersaturated with cholesterol.
• Contributing factors include:
  • Bile stasis due to reduced gallbladder emptying.
  • Increased cholesterol concentration in bile.
  • Presence of nucleating factors such as mucous glycoproteins.

Features

• Many gallstones remain asymptomatic and are termed silent stones.
• Migration of stones into bile ducts causes severe colicky abdominal pain.
• Complications include:
  • Cholecystitis, due to inflammation of the gallbladder.
  • Obstruction of bile flow leading to obstructive jaundice.
  • Chronic inflammation causing structural changes in the gallbladder.

Diagnosis

• Ultrasonography is the preferred and most sensitive diagnostic method.
• Imaging techniques such as cholecystography and radiography may also detect stones.

Treatment

• Medical dissolution therapy has limited effectiveness.
• Definitive treatment is cholecystectomy, which involves removal of the gallbladder.

Effects of Cholecystectomy

• The gallbladder is not essential for survival or normal digestion.
• Most individuals maintain adequate nutrition after surgery.
• Some patients may develop mild fat malabsorption when consuming large fatty meals.
• Reduced fat tolerance may assist in weight control in certain individuals.