Liver has many functions. One of them is to secrete bile. The liver cells per day secrete about 600 to 1000 ml of bile. Bile secretion is a continuous process. Certain substances are actively secreted and certain other substances are passively transported. Water molecules follow the transported substances mechanism.
Histology of Liver:
Figure 5.22 shows basic histology of liver.
Stages of Bile Secretion:
Bile secretion occurs in two stages:
Primary secretion:
Occurs at the level of the hepatocytes. This initial secretion is rich in bile acids, cholesterol, bile pigments and other organic materials. From the cells, these substances are secreted into the bile canaliculi. This bile then enters the bile ducts. Here, sodium and bicarbonates are added by active secretion under the influence of the hormone secretin.
The bile that has entered the hepatic duct normally flows along the cystic duct and gets stored in the gallbladder for some-time. During this time, the composition of the bile gets changed again. Sodium, bicarbonates, chlorides and water get absorbed. This brings about concentration of the remaining substances to the extent of about 10 to 14 times (secondary secretion—composition alteration in gallbladder).
Composition of the Liver Bile and Gallbladder Bile:
Table 5.1 shows the composition of liver bile and gallbladder bile.
Functions of Liver:
1. Secretion of bite:
The hepatocytes are responsible for secretion of bile.
2. Synthetic function:
All the plasma proteins except gamma globulin fraction are synthesized in liver.
3. Storage function:
Many of the vitamins, glycogen, iron, etc. are stored in liver.
4. Excretory/function:
Cholesterol, bile pigments, and alkaline phosphatase are excreted along with bile.
5. Hemopoietic function:
In fetal life liver, liver is one of the sites of erythropoiesis.
6. Detoxification function:
Several drugs, toxins, are detoxified in liver. Detoxification may involve oxidation, reduction, hydrolysis, etc.
7. Inactivation of hormones:
Most of the hormones especially steroidal group of hormones is inactivated in liver.
Composition of Bile:
i. Bile is golden yellow aqueous solution.
ii. Volume secreted per day is about 600-1000 ml.
iii. pH is around 8.
Composition:
i. Water 97%
ii. Solids 3%
Organic constituents:
Bile salts, bile pigments, cholesterol, alkaline phosphatase, etc.
Inorganic constituents:
Sodium, hydrogen ion, calcium, bicarbonate, etc.
Bile Salts:
They are derived from bile acids. The two important bile acids are cholic acid and chenodeoxy cholic acid which are produced in the liver from cholesterol. These acids are conjugated with glycine or taurine and then form salt with sodium or potassium. Hence bile salts are nothing but sodium or potassium glycocholate or taurocholate.
Bile salts enter jejunum along with bile. In the intestine, bile salts are subjected to bacterial actions. This brings about the formation of lithocholic acid and deoxycholic acid. About 95% of bile salts that have reached the intestine are returned to liver (Figs 5.24 and 5.25) by enterohepatic circulation (portal circulation).
The absorption of bile salts into enterohepatic circulation occurs in the terminal part of ileum and at the beginning of large intestine. Enterohepatic circulation is repeated about 4-8 times per day. The total pool of bile salts in the body is about 1.6 g.
Regulation of secretion of bile (Fig. 5.23):
Neural influence is through the vagus nerve. This influence is not much on secretion of bile.
Hormonal influence:
Hormones secreted from duodenum stimulate secretion of bile. Any agent which stimulates the secretion of bile is known as choleretic.
Apart from bile salts some of the other choleretic agents are:
a. Secretin
b. Acetylcholine
Functions of the Gallbladder:
i. It stores bile. It can store about 30-60 ml of bile.
ii. Absorption: While it is being stored sodium, chloride, bicarbonates and water are reabsorbed.
iii. By absorption of water, it tries to maintain a low pressure in the biliary tree so that the bile secretion remains a continuous process.
iv. By absorption of water, it maintains a high concentration ratio between bile salts and cholesterol. This prevents the precipitation of cholesterol and, therefore, the formation of gallstones (Fig. 5.26).
v. Contraction of the gallbladder will bring about expulsion of the bile into the intestine.
Any substance that brings about the contraction of the gallbladder and releases or increases the bile flow into the intestine is known as a cholagogue substance. Bile salts, CCK-PZ are very powerful cholagogues.
Fate of bile salts:
Bile acids are synthesized in the liver from cholesterol by hepatocytes. Bile acids are cholic acid and chenodeoxycholic acid. They combine with glycine or taurine to form glychocholic or taurocholic acid. This in turn combines with sodium or potassium to form sodium and potassium salts of glychocholic or taurocholic acid. Contraction of the gallbladder leads to entry of bile salts into the duodenum.
Functions of the Bile/Bile Salts:
1. Bile salts are very powerful surface tension lowering agents. This helps in the emulsification of fats. The larger fat molecules are broken down to smaller ones so that larger surface area is made available for the pancreatic lipase to act.
2. Bile salts with phospholipids, cholesterol, free fatty acids and monoglycerides form micelles and help in the absorption of fats.
3. They also help in the absorption of fat-soluble vitamins A, D, E, and K.
4. They are very powerful choleretic agents. Choleretic agents are those substances, which increase the secretion of bile from the liver cells.
5. They activate pancreatic lipase.
6. They also act as mild laxative agents.
In the small intestine, about 90 to 95% of the bile salts are absorbed into the portal circulation. The site of absorption is the ileum, and the process involved is an active process. Rest of it enters the large intestine, gets converted into secondary bile salts—deoxycholic acid and lithocholic acid.
A part of it is lost in the stools. About 200 to 600 mg of the bile salt is lost in the stools per day. This quantity synthesized by the liver daily and added to the bile salt pool so that the total quantity of bile salt constant is kept constant.
Cholecystography:
It is a radiological procedure to visualize gallbladder. In oral cholecystography, radiopaque dye (tetroiodophenolphathalene) is given orally. This dye gets secreted by liver cells. So the dye reaches gallbladder along with bile. In the gallbladder, dye is concentrated due to absorption of some amount of water from bile.
About 2-3 hours after giving the dye, if X-rays are taken, the details of its passage from liver through gallbladder and biliary duct can be made out. To facilitate the contraction of gallbladder, usually a fatty food (cheese) is given. When the radiopaque substance is injected intravenously and the radiographic study of gallbladder is done, it is known as cholangiography. Gallbladder can be visualized by ultrasonography.
Removal of the gallbladder is known as cholecystectomy, and this has no harmful effect on the person.
The effects of cholecystectomy are:
1. Bile ducts undergo enlargement to accommodate bile in the absence of concentration function of gallbladder.
2. If sphincter of Oddi does not have adequate tone, bile may dribble into intestine.
3. If the tone is high in sphincter of Oddi, accumulation of bile in the biliary tract leads to back pressure development and may affect the functions of hepatocytes.