Q. Write an Essay on the Composition, Secretion, and Function of Bile
Composition of Bile
The key components of bile are; bile salts that facilitate enzymatic fat digestion; bile pigments, such as bilirubin, that are the waste product of hemoglobin degradation; and cholesterol, which is excreted in the feces
Moreover, drugs and other xenobiotics are cleared from the blood by hepatic processing and are also excreted by bile. Bile salts, which act as detergents to solubilize fats during digestion, are made from steroid bile acids combined with amino acids.
Bile is composed of about 98% water. The most abundant substances secreted in the bile are bile salts, which account for about one half of the total solutes also in the bile. Also secreted in large concentrations are bilirubin, cholesterol, lecithin, and usual electrolytes of plasma.
In the concentrating process in the gallbladder, water and large portions of the electrolytes (except calcium ions) are reabsorbed by the gallbladder mucosa; essentially all other constituents, especially the bile salts and the lipid substances cholesterol and lecithin, are not reabsorbed and, therefore, becomes highly concentrated in the gallbladder.
Secretion of Bile
Bile is a non-enzymatic solution secreted from hepatocytes, or liver cells. Bile is continuously secreted by the liver and is diverted to the gallbladder between meals. The opening of the bile duct into the duodenum is guarded by the sphincter of Oddi, which prevents bile from entering the duodenum except during digestion of meals.
When this sphincter is closed, most of the bile secreted by the liver is diverted back up into the gallbladder, a small saclike structure tucked beneath but not directly connected to the liver.
Thus, bile is not transported directly from the liver to the gallbladder. The bile is subsequently stored and concentrated in the gallbladder between meals. After a meal, contraction of the gallbladder sends bile into the duodenum through the common bile duct, along with a watery solution of bicarbonate and digestive enzymes from the pancreas.
The amount of bile secreted per day ranges from 250 ml to 1 liter, depending on the degree of stimulation.
Functions of Bile
Bile serves two important functions:
First, bile plays an important role in fat digestion and absorption, not because of any enzymes in the bile that cause fat digestion, but because bile acids in the bile do two things:
- they help to emulsify the large fat particles of the food into many minute particles, the surface of which can then be attacked by lipase enzymes secreted in pancreatic juice, and
- they aid in absorption of the digested fat end products through the intestinal mucosal membrane
Second, bile serves as a means for excretion of several important waste products from the blood. These include especially bilirubin, an end product of hemoglobin destruction, and excesses of cholesterol.
Function of Bile Salts in Fat Digestion And Absorption
The liver cells synthesize about 6 grams of bile slats daily. The precursor of the bile salts is cholesterol, which is either present in the diet or synthesized in the liver cells during the course of fat metabolism.
The cholesterol is first converted to cholic acid or chenodeoxycholic acid in about equal quantities.
These acids in turn combine principally with glycine and to a lesser extent with taurine to form glyco– and tauro-conjugated bile acids. The salts of these acids, mainly sodium salts, are then secreted in the bile.
The bile salts have two important functions in the intestinal tract:
First, they have a detergent action on the fat particles in the food. This decreases the surface tension of the particles and allows agitation in the intestinal tract to break the fat globules into minute sizes. This is called the emulsifying or detergent function of bile slats.
Second, and even more important than the emulsifying function, bile salts help in the absorption of
- fatty acids
- cholesterol, and
- other lipids from the intestinal tract
They do this by forming small physical complexes with these lipids; the complexes are called micelles, and they are semi-soluble in the chyme because of the electrical charges of the bile salts. The intestinal lipids are ferried in this form to the intestinal mucosa, where they are then absorbed into the blood. In the absence of bile salts in the intestinal tract, up to 40% of the ingested fats are lost into the feces and the person often develops a metabolic deficit because of this nutrient loss.