[box type=”download”] Appreciation of both its storage and metabolic roles (bile concentration region) Stimulants to bile formation (bile salts, secretin, glucagons, gastrin) Role of CCK and vagal stimulation in bile release[/box]
Bile and the gallbladder
The hepatocytes secrete bile which is isotonic and resembles plasma ionically.
It also contains bile salts, bile pigments, cholesterol, lecithin and mucus.
This fraction of bile is called the bile acid-dependent fraction.
As it passes along the bile duct, the bile is modified by the epithelial cells lining the duct by the addition of water and bicarbonate ions; this fraction is called the bile acid-independent fraction.
Overall, the liver can produce 500–1000 mL of bile per day.
The bile is either discharged directly into the duodenum or stored in the gallbladder.
The bile acid-independent fraction is made as required, i.e. during digestion of the chyme.
The bile acid-dependent fraction is made when the bile salts are returned from the GI tract, and is then stored in the gallbladder when the sphincter of Oddi is closed.
About 95% of the bile salts that enter the small intestine in bile are recycled and reabsorbed into the portal circulation by active transport mechanisms in the distal ileum (the enterohepatic circulation).
Many of the bile salts are returned unaltered, some are broken down by intestinal bacteria into secondary bile acids and then reabsorbed, and a small proportion is excreted in the faeces.
The gallbladder not only stores the bile, but also concentrates leaving the bile acids and pigments.
The process of concentration is mainly by active transport of Na+ ions into the intercellular spaces of the lining cells and this, in turn, draws water, HCO3− and Cl− ions from the bile and into the extracellular fluid, thereby concentrating the bile held in the gallbladder.
The formation of bile is stimulated by bile salts, secretin, glucagons and gastrin.
The release of bile stored in the gallbladder is stimulated by the secretion of CCK into the bloodstream when chyme enters the duodenum and, to a lesser extent, by the actions of the vagus nerve.
Within a few minutes of a meal, particularly when fats are consumed, the muscles of the gallbladder contract; this forces the contents into the duodenum through the now relaxed sphincter of Oddi.
CCK relaxes the sphincter and stimulates the pancreatic secretions at the same time.
The gallbladder empties completely 1 h after a fat-rich meal and maintains the level of bile acids in the duodenum above that necessary for the function of the micelles.