Functions of the Liver

Question 1

Functions of Liver

Answer 1

1. Secretory functions: Liver forms and secretes bile into biliary tract.
2. Metabolic functions: Liver is involved in metabolism of major nutrients such as carbohydrate, fat, proteins, and fat soluble and water soluble vitamins. Liver plays a central role in the metabolisms of urea, iron, and alcohol.
3. Synthetic functions: Liver is the major organ for the synthesis of proteins that include clotting factors, acute phase proteins that mediate inflammation, hormone binding proteins, lipids, carbohydrates, vitamins and bile salts
4. Storage functions: Liver stores glucose, protein, fat, and vitamins. These nutrients are released from liver and utilized during their scarcity, and are stored in liver when they are in excess.
5. Detoxifying action: Liver detoxifies many chemicals. Toxins released from infecting organisms are neutralized in liver.
6. Degradation of drugs and chemicals: Liver is the site of inactivation of many drugs. Liver degrades enzymes,
   hormones, cytokines, and various other chemicals.
7. Catabolic and Excretory function: Liver excretes bile pigments, cholesterol, and some metals.
8. Immunity/ Blood cleansing function: Kupffer cells of liver are part of mononuclear phagocyte system (reticuloendothelial system) that forms the nonspecific defenses of the body. These cells phagocytose and kill microorganism.
9. Endocrine functions: Liver converts vitamin D3 to 25-hydroxyvitamin D3. Liver is a major site for conversion of T4 to T3. Somatomedin (insulin-like growth
   factor) that mediates important functions of growth factor is secreted from liver. Many hormones like insulin, glucagon, growth hormone, GI hormones, etc., are
   degraded in liver.
10. functions as a blood reservoir: since it is an expandable organ large quantities of blood can be stored in its blood vessels (about 10% of the body’s total blood volume)

Question 2

Composition of Bile

Answer 2

about 0.6 - 1 litre is excreted daily.
- bile is a greenish yellow (due to the presence of glucuronides of bile pigments - bilirubin and biliverdin) fluid formed in liver and stored in gall bladder.

composition

• water
• bile salts
• cholesterol
• bilirubin
• fatty acids
• lecithin
• na
• k
• ca
• cl
• hco3

Question 3

Bile acids

Question 4

Bile salts

Question 5

Functions of bile salts

Answer 5

The liver cells synthesize about 6 grams of bile salts 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 con-
verted 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 actions in the intes-
tinal tract:
First, they have a detergent action on the fat particles
in the food. This decreases the surface tension of the par-
ticles and allows agitation in the intestinal tract to break
the fat globules into minute sizes. This is called the emul-
sifying or detergent function of bile salts.
Second, and even more important than the emulsify-
ing function, bile salts help in the absorption of (1) fatty
acids, (2) monoglycerides, (3) cholesterol, and (4) 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 semisoluble in the chyme
because of the electrical charges of the bile salts. The
intestinal lipids are “ferried” in this form to the intesti-
nal mucosa, where they are then absorbed into the blood,
as will be described in detail in Chapter 65. Without the
presence of bile salts in the intestinal tract, up to 40 per-
cent of the ingested fats are lost into the feces and the
person often develops a metabolic deficit because of this
nutrient loss

Question 6

Mechanism of Bile secretion

Answer 6

The secretion of bile consists of canalicular flow and
ductular secretion. In the canaliculi, bile acids and Na+
-
K+
ATPase contribute to bile flow. In the ducts, epithelial
cells secrete HCO3
–
and Cl–
actively into the lumen.
Mechanism of Bile Acid Secretion
Bile is formed in hepatocytes. Bile acids are taken up from
the sinusoidal blood by transport proteins that are present
on the basolateral membrane of the hepatocytes.
1. In cytoplasm of hepatocytes, bile acids bind with bile
acid binding proteins. This prevents bile acids to accu-
mulate in higher concentration in cytoplasm. It should
be noted that high concentration of free bile acids
destroys the organelles of hepatocytes.
2. Bile acids are conjugated with glycine or taurine and
then secreted into the lumen of hepatic canaliculi by
facilitated diffusion.
3. Bile acids move into the bile down their electrochemi-
cal gradient. The concentration gradient is maintained
by formation of micelles in the bile present in the can-
aliculi.
Mechanism of Water and Electrolyte Secretin
Water and electrolyte concentration of bile in the bile can-
aliculi is same with that of plasma.
1. The bile duct epithelial cells secrete bicarbonate rich
fluid into the lumen.
2. The ions are transported in a similar way to that of
pancreatic extralobular ducts.
Modification of Bile in Gallbladder
Bile is continuously synthesized and secreted from liver
and is stored in the gallbladder. Gallbladder is a small sac
like structure with the capacity of 30–60 mL.
Gallbladder performs three functions:
1. Storage of bile
2. Acidification of bile: The hydrogen ion is secreted
from gallbladder epithelial cell into the bile to make
the bile acidic.
3. Concentration of bile: H+
is secreted into the lumen
of gallbladder in exchange for Na+
(Na+
-H+
exchanger).
For Na+
transport into the cell, the gradient is created
by Na+
-K+
pump located in the basolateral membrane
of the epithelial cells. Water is reabsorbed from the
gallbladder bile so that bile is concentrated

Question 7

Regulation of Bile secretion

Answer 7

Bile secretion is influenced by choleretics and cholagogues.

Choleretics
The substances that enhance the secretion of bile are
called choleretics. The important choleretics are:
1. Bile salts
2. Secretin
3. Vagal stimulation.
Secretin increases aqueous component of bile.
Cholagogues
The substances that cause contraction of gallbladder to
increase the output of bile from the structure are called
cholagogues. The important cholagogues are:
1. Cholecystokinin
2. Fatty acids.
Cholecystokinin causes contraction of smooth muscles of
gallbladder and empties its contents.

Question 8

Functions of Bile

Answer 8

Functions of bile are mainly due to presence of bile salts
in it.
1. Bile helps in absorption of lipids from intestine by
forming micelles.
2. Bile salts in bile are important choleretics (increase
bile secretion).
3. Bile promotes absorption of fat soluble vitamins (A, D,
E, and K).
4. Bile salts activate pancreatic lipase.
5. Bile pigments (bilirubin and biliverdin) are responsi-
ble for greenish-yellow coloration of gallbladder that
helps to identify the organ especially in small animals.
6. Bile salts are physiological purgative.