Bile – Function, Composition and Diseases
Bile is a yellowish-green digestive fluid produced by the liver and stored in the gallbladder. It plays a key role in the digestion and absorption of dietary fats in the small intestine.
Things worth knowing about "Bile"
Bile is a yellowish-green digestive fluid produced by the liver and stored in the gallbladder. It plays a key role in the digestion and absorption of dietary fats in the small intestine.
What is Bile?
Bile (also called bile fluid or bile juice) is a vital digestive fluid continuously produced by liver cells (hepatocytes). It has a characteristic yellowish-green colour and a slightly bitter taste. The liver produces approximately 600 to 1000 millilitres of bile per day. The fluid is transported through the bile ducts to the gallbladder, where it is concentrated and stored until it is needed for digestion.
Composition of Bile
Bile is composed of several substances that work together to fulfil its digestive functions:
- Bile salts: The most important components for fat digestion. They are synthesised from cholesterol in the liver.
- Cholesterol: A lipid component synthesised in the liver and excreted via bile.
- Bilirubin: A breakdown product of the red blood pigment haemoglobin, which gives bile its characteristic colour.
- Lecithin (phospholipids): Support the action of bile salts in fat emulsification.
- Water and electrolytes: Make up the majority of bile fluid.
Functions of Bile
Bile performs several essential functions in the human body:
Fat Digestion and Absorption
The most important function of bile is the emulsification of dietary fats. Bile salts act like a detergent: they surround large fat droplets and break them down into tiny droplets called micelles. This greatly increases the surface area of fats, enabling the fat-digesting enzymes (lipases) from the pancreas to work efficiently. The resulting fatty acids and fat-soluble vitamins (A, D, E, K) are then absorbed through the intestinal wall.
Excretory Function
Bile also serves as an excretory route for various substances that the body needs to eliminate, including bilirubin (from the breakdown of red blood cells), excess cholesterol, certain medications, and metabolic waste products.
Regulation of the Gut Microbiome
Bile salts have a mild antibacterial effect and help regulate the balance of the gut microbiome by inhibiting the excessive growth of certain bacteria in the small intestine.
How is Bile Release Controlled?
When fatty or protein-rich food enters the duodenum (the first part of the small intestine), the intestinal wall releases the hormone cholecystokinin (CCK). This hormone causes the gallbladder to contract and release bile into the duodenum, where it mixes with the food. After digestion, bile salts are reabsorbed in the ileum (the final section of the small intestine), transported back to the liver, and recycled. This recycling process is known as the enterohepatic circulation.
Diseases Related to Bile
Disruptions in the production, transport, or composition of bile can lead to various medical conditions:
- Gallstones (cholelithiasis): Form when components of bile (mainly cholesterol or bilirubin) crystallise and create solid deposits. They can cause severe pain known as biliary colic.
- Cholecystitis (gallbladder inflammation): Often triggered by gallstones, this condition causes fever, upper abdominal pain, and nausea.
- Cholestasis (bile stasis): A backup of bile caused by gallstones, tumours, or inflammatory conditions. A key sign is jaundice (icterus).
- Cholangitis (bile duct inflammation): Inflammation of the bile ducts, typically caused by a bacterial infection associated with bile stasis.
- Bile acid deficiency: Can lead to reduced fat absorption and deficiency of fat-soluble vitamins.
Clinical Relevance and Diagnosis
Bile-related conditions are commonly diagnosed using abdominal ultrasound. Blood tests measuring liver parameters such as bilirubin, ALT, AST, gamma-GT, and alkaline phosphatase provide information about disruptions in bile flow or liver function. When more detailed imaging is required, procedures such as MRCP (magnetic resonance cholangiopancreatography) or ERCP (endoscopic retrograde cholangiopancreatography) are used to visualise the bile ducts.
References
- Guyton A.C., Hall J.E. Textbook of Medical Physiology. 14th edition. Elsevier, Philadelphia 2020.
- Boron W.F., Boulpaep E.L. Medical Physiology. 3rd edition. Elsevier, Philadelphia 2017.
- World Gastroenterology Organisation (WGO): Global Guidelines on Gallstone Disease. WGO, 2020. Available at: https://www.worldgastroenterology.org
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