Liver And Gallbladder

 Introduction : 

What is liver?

• The liver is the heaviest gland of the body, weighing about 1.4 kg

(about 3 lb) in an average adult. Of all of the organs of the body,

• it is second only to the skin in size. The liver is inferior to the diaphragm and occupies most of the right hypochondriac and part of the epigastric regions of the abdominopelvic cavity 

• The gallbladder (gall- -bile) is a pear-shaped sac that is located in a depression of the posterior surface of the liver. 

• It is 7–10 cm (3–4 in.) long and typically hangs from the anterior infe-

rior margin of the liver 

Anatomy of the Liver and Gallbladder

The liver is almost completely covered by visceral peritoneum and is completely covered by a dense irregular connective tissue layer that lies deep to the peritoneum. 

The liver is divided into two principal lobes—a large right lobe and a smaller left lobe—by

the falciform ligament, a fold of the mesentery .

Although the right lobe is considered by many anatomists to include an inferior quadrate lobe (kwa-DRAˉT) and a posterior caudate lobe (KAW-daˉt), based on internal morphology (primar-

ily the distribution of blood vessels), the quadrate and caudate lobes more appropriately belong to the left lobe.

 The falciform ligament extends from the undersurface of the diaphragm between the two principal lobes of the liver to the superior surface

of the liver, helping to suspend the liver in the abdominal cavity.

In the free border of the falciform ligament is the ligamentum

teres (round ligament), a remnant of the umbilical vein of the fetus ;

 this fibrous cord extends from the liver to the umbilicus. The right and left coronary ligaments are narrow extensions of the parietal peritoneum that suspend the

liver from the diaphragm.

Gallbladder:

 The parts of the gallbladder include the broad fundus, which projects inferiorly beyond the inferior border of the liver;

 the body, the central portion; and the neck, the tapered portion. The

body and neck project superiorly.

Histology of the Liver and Gallbladder / Define Histology of Liver And Gallbladder

Histologically, the liver is composed of several components

1. Hepatocytes. Hepatocytes (hepat- -

 liver; -cytes -cells)

are the major functional cells of the liver and perform a wide array of metabolic, secretory, and endocrine functions. These are specialized epithelial cells with 5 to 12 sides that make up about 80% of the volume of the liver. Hepatocytes form complex three dimensional arrangements called hepatic laminae(LAM-i-ne¯). 

The hepatic laminae are plates of hepatocytes one cell thick bordered on either side by the endothelial-lined

vascular spaces called hepatic sinusoids.

 The hepatic laminae

are highly branched, irregular structures. Grooves in the cell

membranes between neighboring hepatocytes provide spaces

for canaliculi (described next) into which the hepatocytes

secrete bile. Bile, a yellow, brownish, or olive-green liquid

secreted by hepatocytes, serves as both an excretory product and a digestive secretion.

2. Bile canaliculi. Bile canaliculi (kan-a-LIK-uˉ-li -small canals) are small ducts between hepatocytes that collect bile

produced by the hepatocytes. From bile canaliculi, bile passes

into bile ductules and then bile ducts. The bile ducts merge

and eventually form the larger right and left hepatic ducts,

which unite and exit the liver as the common hepatic duct.

 The common hepatic duct joins the cystic duct (cystic -bladder) from the gallbladder to form the common bile duct. 

From here, bile enters the duodenum of the small intestine to participate in digestion.

3. Hepatic sinusoids. Hepatic sinusoids are highly permeable

blood capillaries between rows of hepatocytes that receive

oxygenated blood from branches of the hepatic artery and

nutrient-rich deoxygenated blood from branches of the hepatic

portal vein. Recall that the hepatic portal vein brings venous

blood from the gastrointestinal organs and spleen into the

liver. Hepatic sinusoids converge and deliver blood into a cen-

tral vein. From central veins the blood flows into the hepatic

veins, which drain into the inferior vena cava .

In contrast to blood, which flows toward a central vein, bile

flows in the opposite direction. Also present in the hepatic sinu-

soids are fixed phagocytes called stellate reticuloendothelial

cells (STEL-aˉt re-tik-uˉ-loˉ-en-doˉ-THE¯-le¯-al) or hepatic mac-

rophages, which destroy worn-out white and red blood cells,

bacteria, and other foreign matter in the venous blood draining

from the gastrointestinal tract.

 Together, a bile duct, branch of the hepatic artery, and branch

of the hepatic vein are referred to as a portal triad (tri- -

 three).

The hepatocytes, bile duct system, and hepatic sinusoids can

be organized into anatomical and functional units in three differ-

ent ways:

1. Hepatic lobule. For years, anatomists described the hepatic

lobule as the functional unit of the liver. According to this

model, each hepatic lobule is shaped like a hexagon (six-sided

structure) . At its center is the central vein,

and radiating out from it are rows of hepatocytes and hepatic

sinusoids. Located at three corners of the hexagon is a portal

triad. This model is based on a description of the liver of adult

pigs. In the human liver it is difficult to find such well-defined

hepatic lobules surrounded by thick layers of connective tissue.

2. Portal lobule. This model emphasizes the exocrine function

of the liver, that is, bile secretion. Accordingly, the bile duct of

a portal triad is taken as the center of the portal lobule. The

portal lobule is triangular in shape and is defined by three

imaginary straight lines that connect three central veins that

are closest to the portal triad . This

model has not gained widespread acceptance.

3. Hepatic acinus. In recent years, the preferred structural and

functional unit of the liver is the hepatic acinus (AS-i-nus).

Each hepatic acinus is an approximately oval mass that in-

cludes portions of two neighboring hepatic lobules. The short

axis of the hepatic acinus is defined by branches of the portal

triad—branches of the hepatic artery, vein, and bile ducts—

that run along the border of the hepatic lobules. The long axis

of the acinus is defined by two imaginary curved lines, which

connect the two central veins closest to the short axis . Hepatocytes in the hepatic acinus are ar-

ranged in three zones around the short axis, with no sharp

boundaries between them . Cells in zone 1 are closest to the branches of the portal triad and the first to re-

ceive incoming oxygen, nutrients, and toxins from incoming

blood. These cells are the first ones to take up glucose and

store it as glycogen after a meal and break down glycogen to

glucose during fasting. 

They are also the first to show morpho-

logical changes following bile duct obstruction or exposure to

toxic substances. 

Zone 1 cells are the last ones to die if circula-

tion is impaired and the first ones to regenerate. Cells in zone

3 are farthest from branches of the portal triad and are the last

to show the effects of bile obstruction or exposure to toxins,

the first ones to show the effects of impaired circulation, and

the last ones to regenerate. 

Zone 3 cells also are the first to

show evidence of fat accumulation.

 Cells in zone 2 have struc-

tural and functional characteristics intermediate between the

cells in zones 1 and 3.

 The hepatic acinus is the smallest structural and functional unit

of the liver. 

Its popularity and appeal are based on the fact that it

provides a logical description and interpretation of (1) patterns of

glycogen storage and release and (2) toxic effects, degeneration,

and regeneration relative to the proximity of the acinar zones to

branches of the portal triad.

 The mucosa of the gallbladder consists of simple columnar

epithelium arranged in rugae resembling those of the stomach.

The wall of the gallbladder lacks a submucosa. The middle, mus-

cular coat of the wall consists of smooth muscle fibers. Contrac-

tion of the smooth muscle fibers ejects the contents of the gall-

bladder into the cystic duct. The gallbladder’s outer coat is the

visceral peritoneum. The functions of the gallbladder are to store

and concentrate the bile produced by the liver (up to tenfold)

until it is needed in the small intestine. In the concentration pro-

cess, water and ions are absorbed by the gallbladder mucosa.