Notes
Liver and Gallbladder Physiology
Sections
Overview
The liver and gallbladder secrete and store bile to aid in digestion.
Additional roles of the liver:
- Nutrient metabolism
- Synthesis of plasma proteins
- Hormone secretion and modification
- Storage of essential molecules
- Removal of old red blood cells
- Detoxification
Bile
Bile is cholesterol-derived and alkaline.
It contains cholesterol, lecithin (phospholipids), bile pigments, and trace minerals.
Bile is secreted by the liver and stored and concentrated in the gallbladder.
It is released into the digestive tract postprandially (after a meal) when the Sphincter of Oddi opens.
Enterohepatic Circulation & Bile Recycling
The liver produces and releases bile into the hepatic ducts, which merge to form the common hepatic duct.
Review Biliary Tree
The gallbladder stores bile; it sits below the liver, and is drained by the cystic duct; the cystic and common hepatic ducts merge to form the common bile duct.
Bile exits the biliary system of ducts and enters the duodenum of the small intestine; the Sphincter of Oddi (aka hepatopancreatic sphincter), regulates passage of bile from the common bile duct to the duodenum.
Bile secretion is triggered by hormones that act on the liver and gallbladder; both hormones are secreted by the duodenum in response to chyme.
Secretin acts on the liver to promote bile's release to the common hepatic duct; secretin is secreted in response to the presence of acidic chyme in the duodenum – the bicarbonate in bile neutralizes the acidity of chyme.
CCK (cholecystokinin) stimulates gallbladder contraction and release of bile; it also works on the Sphincter of Oddi to allow bile to move from the common bile duct to the duodenum. CCK is secreted in response to fatty acids in the duodenum– as we'll see, bile salts emulsify lipids.
Bile salts are recycled in the enterohepatic circulation as follows:
Bile salts leave the ileum and are reabsorbed by nearby capillary beds that drain into the hepatic portal system; recall that the hepatic portal system routes venous blood from the digestive system through the liver for processing.
When the bile salts reach the liver, they are recycled and re-secreted into newly formed bile.
A small portion, about 5%, of bile salts escape this pathway and are excreted in the feces. The liver synthesizes new bile salts to account for this loss.
Bile Salts
Finally, let's learn about bile salts and the role they play in lipid digestion.
Large lipid droplets present an obstacle to digestion in the water environment of the small intestine, so bile salts emulsify them to form smaller droplets.
Bile salts have hydrophobic and hydrophilic sides (they are amphipathic molecules):
- The hydrophobic portion is a cholesterol precursor, composed of mainly non-polar hydrocarbons that can interact with lipid droplets.
- The hydrophilic side is studded with polar hydroxyl and carboxyl groups, which can be exposed to the aqueous environment.
Fat emulsification: large lipid droplets, which are hydrophobic, are exposeed to bile salts and phospholipids. These emulsifying agents break the large droplet into smaller droplets of triglycerides.
Now, digestion can occur: lipase, with the help of colipase, can break
triglycerides into their smaller components: a monoglyceride (glycerol) and two fatty acids.
These small molecules can be absorbed by the small intestine in the form of micelles.
Micelles are water-soluble units that act as holding stations for the lipids for transport out of the small intestine.
They have a spherical arrangement: the fatty acids and triglycerides are on the inside, and the bile salts are on the outside. We use double-sided arrows to show that these micelles are continuously formed and broken.