and does not contract unless a laxative stimulates
it. Laxatives, and particularly suppositories and
enemas, also can irritate the intestinal mucosa
enough to cause chronic INFLAMMATION.
See also BOWEL MOVEMENT; DIARRHEA; FIBER AND
GASTROINTESTINAL HEALTH; NUTRIENTS; NUTRITIONAL
NEEDS; ROUTES OF ADMINISTRATION.
liver The largest internal organ in the body. Its
soft, spongy tissue spreads like a flattened football
between the DIAPHRAGMand the STOMACH, tucked
protectively beneath the lower ribs on the right
side of the abdomen. Weighing about three and a
half pounds, the liver contains 15 percent of the
body’s blood (about a pint). About 60 percent of
this blood is venous and comes from the gastroin-
testinal tract and SPLEEN, entering the liver via the
portalVEIN. The venous blood delivers nutrients
that the liver further metabolizes, the products of
which it then sends back into the bloodstream.
The hepaticARTERY, which branches directly from
the abdominal AORTA, delivers oxygenated blood to
the liver to fuel the functions of its cells. On the
underside of the liver is the GALLBLADDER, which
concentrates and stores the BILEthe liver produces.
The liver’s two main lobes, the small left lobe
and the large right lobe, support an intricate net-
work of lobules, thousands of tiny communities of
hepatocytes (the cells that carry out the liver’s
functions) that filter NUTRIENTS, wastes, BACTERIA,
and toxins from the blood. The microscopic spaces
between the lobules are the sinusoids, into which
the blood from the portal vein drains. Each lobule
is a hexagonal structure two layers of cells deep
and several cells horizontally and vertically in a
platelike configuration. At the vertical junctions of
the lobules are the portal triads, each containing
three microscopic structures: a venule, an arteri-
ole, and a bile duct. The portal triads collect the
substances the lobules produce and convey them
to the larger vessels that will carry them out to the
structures of the body. The membranous connec-
tive tissue that envelopes the liver also extends
like a web through the liver, providing a support-
ive structure for the lobules, sinusoids, and portal
triads.
The lobules are the work stations of the liver.
They metabolize nutrients and toxins, and synthe-
size (manufacture) numerous substances includ-
ing amino acids, proteins essential for PLASMApro-
duction, lipoproteins, cholesterol, immune factors,
CLOTTING FACTORS,LY M P H, and bile. The lobules con-
vert GLUCOSEto glycogen, a storage form of glucose
the body can draw from when blood levels of glu-
cose fall, and glycogen back to glucose, processes
that integrate closely with the balance of glucose
and INSULINin the blood. The lobules also decon-
struct old erythrocytes (red blood cells) to recycle
the iron and BILIRUBINthey contain. Specialized
phagocytic (“cell eating”) cells, called Kupffer
cells, reside in the sinusoids to consume bacteria
and cellular waste. The liver stores iron, glycogen,
vitamin A and vitamin B 12 , and other chemicals
the body needs for cellular activities. The liver is
unique among the body’s organs in its ability to
regenerate itself. This extraordinary capacity
speaks to the significant extent of damage that
must take place to permanently destroy liver tis-
sue. Even so, the liver can meet the needs of the
body as long as 25 percent of its cells remain func-
tional.COMMON CONDITIONS THAT CAN AFFECT THE LIVER
BILIARY ATRESIA CHOLESTASIS
CIRRHOSIS DIABETES
HEPATIC ABSCESS HEPATIC CYST
HEPATITIS LIVER CANCER
LIVER DISEASE OF ALCOHOLISM PANCREATITIS
PORTAL HYPERTENSION PRIMARY BILIARY CIRRHOSIS
PRIMARY SCLEROSING CHOLANGITIS STEATOHEPATITISFor further discussion of the liver within the
context of gastrointestinal structure and function,
please see the overview section “The Gastrointesti-
nal System.”
See also BILE DUCTS; ESOPHAGEAL VARICES;
HEPATOMEGALY; HEPATOTOXINS; JAUNDICE; LIVER FAILURE;
LIVER FUNCTION TESTS; LIVER TRANSPLANTATION; NUTRI-
TIONAL NEEDS; VITAMINS AND HEALTH.liver cancer Malignant growths in the LIVER.
Liver CANCERmay be primary (originates in the
liver) or secondary (metastasizes, or spreads, from
other locations in the body). Primary liver cancer
is less common than metastatic liver cancer. Most
primary liver cancer develops as a complication of
chronic HEPATITISB or hepatitis C INFECTION, condi-
tions that repeatedly damage liver tissue, andliver cancer 69