Bile is a digestive juice that is secreted by the liver and stored in the gallbladder. It has two important functions :
- Assists with fat digestion and absorption in the gut.
- Is a means for the body to excrete waste products from the blood.
Bile does not contain enzymes like other secretions from the gastrointestinal tract. Instead it has bile salts (acids) which can :
- Emulsify fats and break it down into small particles. This is a detergent-like action of bile.
- Helps the body absorb the breakdown products of fat in the gut. Bile salts bind with lipids to form micelles. This is then absorbed through the intestinal mucosa.
The other important function of bile is that it contains waste products from hemoglobin break down. This is known as bilirubin and is normally formed by the body as it gets rid of old red blood cells which are rich in hemoglobin. Bile also carries excess cholesterol out of the body and ‘dumps’ it into the gastrointestinal tract where it can be passed out with other waste matter.
The liver cells (hepatocytes) produce bile which collects and drains into the hepatic duct. From here it can enter the small intestine to act on fats by traveling down the common bile duct, or it can enter the gallbladder through the cystic duct, where it is stored.
The liver manufactures between 600ml to 1 liter of bile in a day. As bile travels down the ducts, the lining of these passages, secrete water, sodium and bicarbonate ions into the bile, thereby diluting it. These additional substances help to neutralize the stomach acid which enters the duodenum with partially digested food (chyme) from the stomach. Refer to Gastric Acid Secretion.
Bile salts are constantly recycled in the body. It is secreted into the duodenum along with other compounds as bile. It binds with lipids to form micelles and eventually reenters the blood stream. By passing through the portal cirulation, bile salts enter the liver. Here it goes through the venous sinuses of the liver and is absorbed by the liver cells (hepatocytes). It is then ready to be secreted again by the liver cells to make up bile. This allows for over 90% of bile salts to be recycled in this manner (enterohepatic circulation of bile) and after about 15 cycles, it is discarded in the feces. The liver, nevertheless, constantly produces small amounts of bile salts to replenish the amounts that cannot be recirculated.
The liver is constantly secreting bile, up to 1 liter in a 24 hour period, but most of it is stored in the gallbladder. This hollow organ can only hold 30 to 60 ml of bile and is able to store the large quantities of bile from the liver by concentrating it. The gallbladder is able to achieve this by resorption of water, sodium, chloride and other electrolytes through its lining. The other constituents of bile, like the bile salts, cholesterol, lecithin and bilirubin, stays in the gallbladder.
The gallbladder has to concentrate the bile so that it can store the bile salts and waste products of the liver bile. It actively transports sodium through the gallbladder mucosa. The other constituents like water, chloride and electrolytes then diffuse through the gallbladder lining.
By doing this, gallbladder bile is 5 to 20 times more concentrated than liver bile. At this point, gallbladder bile is mainly composed of bile salts, and smaller volumes of bilirubin, cholesterol, lecithin and other electrolytes stay in the gallbladder.
Bile, whether from the liver or gallbladder, contains the following substances :
- bile salts
- fatty acids
- bicarbonate ions
As mentioned, gallbladder bile is concentrated compared to liver bile. Bile salts make up the largest volume of gallbladder bile and can be 6 times more concentrated than bile salts in liver bile.
Liver bile, however, has higher concentrations of :
Between 20 to 30 minutes after eating a meal, the partially digested food enters the duodenum of the small intestine from the stomach as chyme (gastric emptying). The presence of food, especially fatty foods, in the stomach and duodenum stimulates the gallbladder to contract due to the action of cholecystokinin (CCK). The gallbladder then forces out bile and relaxes the sphincter of Oddi thereby allowing bile to enter the duodenum.
The other stimulus for gallbladder contraction is nerve impulses from the vagus nerve and enteric nervous system. Refer to Stomach Nerves for more information on the nerve supply of the upper gastrointestinal tract.
If sufficiently stimulated for a prolonged period (due to the presence of fatty foods), the gallbladder can empty its entire contents within an hour.
Secretin, the digestive hormone that stimulate pancreatic secretion, also increases bile secretion. Its main effect, however, is to increase the secretion of water and sodium bicarbonate from the lining of the bile duct. This bicarbonate solution, along with pancreatic bicarbonate, is essential for neutralizing the stomach acid that is present in the duodenum.