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Anatomy & Physiology (Open + Free)
Sy lla bu s
Unit 8:: Digestive System
Introduction
Module 29 /
Digestiv e Sy stem
Structures and Functions
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Digestiv e Sy stem Lev els of
Organization
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Liver
251
Identify and discuss the
functions of the liv er and its
structures.
The liver is the largest gland in the body, as well as one of the most important organs. It plays a number of
major roles in metabolism and regulation. In an adult, the reddish-brown liver weighs about 3 pounds (1.4 kg)
This wedge-shaped organ lies inferior to the diaphragm in the right upper quadrant of the abdominal cavity.
The liver is almost completely surrounded by the rib cage, which offers it some protection.
The Liver’s Anatomical Structures
The liver has different levels of structure and function, and different models have been used to describe the
organizational and functional relationships between hepatocytes, bile canaliculi, and hepatic sinusoids. The
hepatic lobule was considered the functional unit of the liver in years past. In this model, each hexagonal
(six-sided) hepatic lobule has a central vein. Radiating from this vein are rows of hepatocytes and hepatic
sinusoids.
Microscopic structure of the liv er. This work by Cenv eo is licensed under a
Creativ e
Com m ons
Attribution
3 .0
United
States
(http://creativ ecom m ons.org/licenses/by /3 .0/us/).
Microscopic structure of the hepatic acinus. This
work by Cenv eo is licensed under a Creativ e
Com m ons
Attribution
3 .0
United
States
(http://creativ ecom m ons.org/licenses/by /3 .0/us/).
The hepatic acinus has become the preferred model of the metabolic functional unit of the liver in recent
years. Each primarily oval hepatic acinus includes parts of two adjacent hepatic lobules. Branches of the portal
triad that run along the hepatic lobule border delineate the short axis of the hepatic acinus. The long axis is
created by two imaginary curved lines that connect the two central veins nearest the short axis. In the
hepatic acinus, hepatocytes are organized in three zones around the short axis. Zone 1 cells, which are nearest
the portal triad branches, are the first to receive oxygen, nutrients, and toxic substances from incoming
blood. After a meal, zone 1 cells are the first to absorb glucose and store it as glycogen; during fasting, they
are the first to break down glycogen to glucose. If circulation is disrupted, zone 1 cells are the last to die and
the first to regenerate. Zone 3 cells, which are farthest away from the portal triad branches, die first if
circulation is disrupted and are the last to regenerate. They are also the first to demonstrate the
accumulation of fat. Predictably, the structural and functional qualities of zone 2 cells lie between those of
zone 1 and 3 cells.
The popularity of the hepatic acinus model is based on its reasonable description of glycogen storage and
release patterns, as well as the degeneration, regeneration, and toxic effects in the three zones according to
their proximity to portal triad branches.
Liver Gross Anatomy
The liver is divided into two primary lobes, a large right lobe and a smaller left lobe. Separating the right and
left lobes anteriorly is a mesentery called the falciform ligament, which also helps suspend the liver in the
abdominal cavity. A number of anatomists believe that the right lobe also includes an inferior quadrate lobe
and a posterior caudate lobe, which are defined by internal features. Ligaments connect the liver to other
visceral organs, and the visceral peritoneum enshrouds almost the entire liver, except for the part that abuts
the diaphragm. The lesser omentum tethers the liver to the lesser curvature of the stomach. The porta
hepatis ("gateway to the liver") is where the hepatic artery and hepatic portal vein enter the liver on
its inferior surface. These two vessels, along with the common hepatic duct, run through the lesser omentum
on the way to their destinations.
Gross anatom y of the liv er. This work by Cenv eo is licensed under a Creativ e Com m ons Attribution 3 .0 United States
(http://creativ ecom m ons.org/licenses/by /3 .0/us/).
The hepatic artery delivers oxygenated blood from the heart to the liver. The portal system in general has a
direct cconnection between two organs bypassing the systemic circulation. The hepatic portal vein delivers
deoxygenated blood containing newly absorbed nutrients, drugs, and possible bacteria from the alimentary
canal. Branches of both vessels bring blood into liver sinusoids, where hepatocytes take up most nutrients and
some of the toxins. The hepatocytes secrete substances, including nutrients needed by other cells, back into
the blood, which drains into the central vein and then the hepatic vein. In this hepatic portal circulation, blood
from the GI tract passes through the liver before entering the systemic circulation. This is why the liver is a
common site for the metastasis of cancer that originates in the alimentary canal.
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Liver Ducts
The liver has three main components: hepatocytes, bile canaliculi, and hepatic sinusoids. Between adjacent
hepatocytes, grooves in the cell membranes provide room for bile canaliculi ("small canals"). These small
ducts accumulate the bile produced by hepatocytes. From here, bile flows first into bile ductules and then into
bile ducts. The bile ducts unite and form the larger right and left hepatic ducts, which themselves merge
and exit the liver as the common hepatic duct. This duct then joins with the cystic duct from the gall
bladder, forming the common bile duct through which bile flows into the small intestine. Bile production
increases when fat-rich chyme stimulates the release of secretin from intestinal cells. Between meals, when
most absorption is complete, the hepatopancreatic sphincter prevents bile flow into the duodenum, and bile
backs up into the gall bladder, where it is stored until needed.
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Liver Circulation
Hepatic sinusoids are specialized blood vessels, more like dilated channels, located between rows of
hepatocytes. Branches of the hepatic artery deliver oxygenated blood to hepatic sinusoids; while branches of
the hepatic portal vein deliver nutrient-laden blood. The hepatic sinusoids combine and send blood into a
central vein. Blood then flows into the hepatic veins, which drain into the inferior vena cava. This means
that blood and bile flow in opposite directions. The hepatic sinusoids also contain the immune cells called
Kupffer cells. A distinctive arrangement in the liver called the portal triad includes three basic
structures: a bile duct, a hepatic artery branch, and a hepatic portal vein branch.
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Liver Ligaments
The liver is covered with a visceral peritoneum. These ligaments are useful anatomic positions but serve little
physiological purpose aside from attaching the liver to the body wall and other structures. The ligamentum
teres (round ligament) runs along the inferior border of the falciform ligament. Tapered extensions of
the parietal peritoneum, called the right and left coronary ligaments, hang the liver from the diaphragm.
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Open Learning Initiativ e
Unless otherwise noted this work is licensed under a Creativ e Com m ons Attribution 3 .0 Unported License.
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