THE LIVER
Liver lobules
Blood enters into the liver lobules via
the hepatic arterioles, which then
forms hepatic sinusoids that drain into
the central vein. Kupffer cells, which
are phagocytic cells, are found within
or below the fenestrated sinusoidal
endothelium. These cells serve
important filtering function,
removing and processing antigens
that arrive from the gastrointestinal
tract or systemic circulation. Biliary
canaliculi are located between the
two cords of cells on the contralateral
side of the sinusoids. Biliary
canaliculi get toxins from hepatic
sinusoids. The canaliculi drain into
the bile duct.
Bile is a complex fluid that contains
organic and inorganic secretions from
the liver, which are important in the
digestion and excretion.
Bile is secreted at a fairly constant rate
between meals, with more than 50% of
the daily hepatic bile produced entering
the gallbladder during this
interdigestive period. This volume of
hepatic bile far exceeds the volume of
bile in the gallbladder, and the excess
fluid volume must be absorbed by
epithelial cells lining the gallbladder.
Concentrated bile is more effective
for promoting fat digestion.
The Functions of Bile:
1. It facilitates the assimilation of dietary lipid;
approximately 50% of dietary lipid appears in feces if
bile is excluded from the small intestine. Bile facilitates
fat digestion by promoting its emulsification and
solubilization. This is achieved through the formation of
mixed micelles, which enhance lipase action and then
assist in the delivery of the digestive products to be
absorbed by the enterocytes (see below→…)
- Bile acids are the most abundant
organic molecules in bile and comprise
approximately 70% of the organic
constituents of bile.
Bile salts interact with lipids to form
aggregates known as micelles.
- The micelle provides a polar outer shell,
which interacts with water, and a
hydrophobic inner region. Long-chain
fatty acids, cholesterol, and other
hydrophobic molecules readily dissolve
inside the micelles.
2. It provides a pathway to excrete hydrophobic molecules
(cholesterol, billirubin) that may not be readily excreted by the
kidney.
3. It assists in neutralizing gastric acid because it is an
alkaline solution.
4. Activates lipase, trypsin and amylase, which
contributes to the hydrolysis and absorption of fats,
proteins and carbohydrates.
GASTROINTESTINAL PHYSIOLOGY
Shutova S.V.
48
5. Stimulates the motility of the small and large
intestines, as well as the movement of the villi.
6. Inactivates pepsin in the duodenum.
7. Has a bactericidal effect.
8. Stimulates proliferation and desquamation of
enterocytes.
9. Stimulates bile formation and biliary excretion
The biliary tract consists of a series of
ducts that convey bile from the liver to
the duodenum.
The integrated response to a meal
includes contraction of the
gallbladder during the intestinal
phase.
Bile, along with pancreatic secretions,
reaches the small intestine via the
common bile duct.
The gallbladder is a blind
outpouching of the biliary tree. It is a
distensible muscular organ with a
capacity of only 20–50 mL. The
function of the gallbladder is to store
and concentrate bile between meals
and to eject bile into the duodenum
during the digestion of a meal. During
the interdigestive phase, the
gallbladder is relaxed and the
sphincter of Oddi is contracted,
promoting storage of hepatic bile in
the gallbladder. Contraction of the
gallbladder during the intestinal phase
of a meal delivers the maximum
amount of bile into the small intestine
at the time when nutrients are present.
The liver serves many functions:
- Digestive and excretory (of the
liver are associated with
the secretion of bile via the
biliary tract. )
- Carbohydrate metabolism (The liver is both a source and a
“sink” for glucose.
Glycogenolysis and
gluconeogenesis add glucose
to the blood. Glycogen
synthesis, glycolysis, all
consume blood glucose. )
- Fat metabolism (Dietary
lipids circulating in the
form of chylomicrons (see
fat absorption) are
partially broken down
during passage through
the microcirculation. The
glycerol and fatty acids
that are produced then
enter the adipose and
muscle cells.)
- Cholesterol metabolism (The major sources of
cholesterol are found in
the diet plus de novo
synthesis of cholesterol
by the liver. The major
elimination pathway for
cholesterol is in bile, and
via hepatic synthesis of
bile acids. Bile acids are
subsequently excreted in
feces.)
- Amino acid and protein synthesis (A
large amount of amino
acid is consumed for
synthesis of serum
proteins (e.g., albumin,
clotting factors, and
hormone-binding
proteins).)
- Storage functions (The liver is the main
storage site for the
fat-soluble vitamins
A, D, E, and K, and
vitamin B12, iron,
and copper.)
- Detoxification and biotransformation (Removal of the
bioactivity of many
organic molecules,
including steroids and
hydrophobic drugs,
occurs in the liver.)
- Immune function (Kupffer cells in the liver
are the largest group of
fixed macrophages in
the body. These cells are
responsible for ingesting
foreign bodies entering
the blood via the
gastrointestinal tract.)
