Bile
Functions of bile are the functions of its different constituents. It is particularly important for digestion and absorption of lipids and also for absorption of fat soluble vitamins.
image credit by-Blamb
Functions of bile salts
Bile salts are good choleretic and increase bile secretion from
the liver. Next important function is formation of micelles, which are loose
aggregates of bile salt, phospholipid and cholesterol formed in such a manner
that the hydrophilic ends of the molecules are kept outside and the hydrophobic
ends inside the aggregates. This way the bile salts keep the hydrophobic
substances like cholesterol in aqueous solution. In the intestine, the bile
salts are essential to emulsify the lipids of the food which are present in
droplet form. The lipase gets more surface area for action on the emulsified
fat. (Emulsification means breaking the fat droplets into innumerable fine
particles which increases the surface area. This is achieved by the surfactant
action of bile salts and lecithin).
The bile salts help to keep the products of lipid digestion in solution by forming mixed micelles where the products of lipid digestion are kept inside the micelles. This also helps in transport of lipids to the enterocytes and thus in absorption of lipid.
Bile salts are also responsible for absorption of fat soluble Vitamins A, D, E and K by forming mixed micelles.
Another important function of bile salts is activation of a Bile salts also act as mild cathartic (cathartics are pancreatic lipase. substances which promote defaecation).
Functions of bilirubin:
It acts as a colouring agent and is responsible
for the yellow colour
of the stool. When bilirubin cannot enter the intestine, stool becomes white or
clay coloured (china clay). As bilirubin
is a waste product so bile helps in its
excretion.
Functions of bile lipids:
Lecithin helps in the formation of micelles and cholesterol is an
excretory product. Excretion of cholesterol is very much important as its
accumulation is associated to diseases. But this excretion of cholesterol
through bile does not help very much because this cholesterol is reabsorbed
from the intestine again. To reduce the cholesterol content of the body, this
reabsorption is sometimes prevented by using different drugs.
Functions of bile proteins:
The immunoglobulins provide secretory immunity
against microbes
invading the gut. Along
with the above, bile also helps in absorption of some metals, excretion of
some drugs, hormone metabolites, dyes,
etc. Bicarbonate in bile helps to neutralise the
acidic chime from
the stomach to some extent.