LIVER, PANCREAS AND GALL BLADDER- ENGLISH
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PANCREAS, LIVER AND GALL BLADDER-
• The
digestive system is also known as the gastrointestinal system or the alimentary
canal or GIT. This vast system is
approximately 10 m long. It travels the length of the body from the mouth
through the thoracic, abdominal and pelvic cavities, where it ends at the anus.
We are covering it in multiple lectures. Today we are going to discuss the
anatomy and physiology of accessary organs of GIT that are pancrease, liver and
gall bladder.
PANCREAS-
• The
pancreas is a pale grey gland weighing about 60 grams. It is about 12 to 15 cm
long and is situated in the epigastric and left hypochondriac regions of the
abdominal cavity. It consists of a head, body and tail
• The
head lies in the curve of the duodenum, the body behind the stomach and the
tail lies in front of the left kidney and just reaches the spleen. The
abdominal aorta and the inferior vena cava lie behind the gland. The pancreas
is both as :
• Exocrine
gland and
• Endocrine
gland.
EXOCRINE PANCREAS-
• This
consists of a large number of lobules made up of small acini, the walls
of which consist of secretory cells. Each lobule is drained by a tiny duct and
these unite eventually to form the pancreatic duct, which extends the
whole length of the gland and opens into the duodenum. Just before entering the
duodenum the pancreatic duct joins the common bile duct to form the
hepatopancreatic ampulla. The function of the exocrine pancreas is to produce pancreatic
juice containing enzymes that digest carbohydrates, proteins and fats
ENDOCRINE PANCREAS-
• There
are groups of specialised cells called the pancreatic islets (islets of
Langerhans). These islets are distributed throughout the gland. The islets have
no ducts so the hormones diffuse directly into the blood. The endocrine
pancreas secretes the hormones insulin and glucagon, which are
principally concerned with control of blood glucose levels
LIVER-
• The
liver is the largest gland in the body, weighing between 1 and 2.3 kg. It is
situated in the upper part of the abdominal cavity occupying the greater part
of the right hypochondriac region, part of the epigastric region and extending
into the left hypochondriac region. Its upper and anterior surfaces are smooth
and curved to fit the under surface of the diaphragm. its posterior surface is
irregular in outline
• The
liver is enclosed in a thin inelastic capsule and incompletely covered by a
layer of peritoneum. Folds of peritoneum form supporting ligaments attaching
the liver to the inferior surface of the diaphragm. It is held in position
partly by these ligaments and partly by the pressure of the organs in the
abdominal cavity. The liver has four lobes. The two most obvious are the large right
lobe and the smaller, wedge shaped, left lobe. The other two, the caudate
and quadrate lobes, are areas on the posterior surface
• The
portal fissure- This is the name given to the region on the posterior
surface of the liver where various structures enter and leave the gland.
• The
portal vein enters, carrying blood from the stomach, spleen, pancreas
and the small and large intestines.
• The
hepatic artery enters, carrying arterial blood. It is a branch from the
coeliac artery, which is a branch from the abdominal aorta.
• Nerve
fibres, sympathetic and parasympathetic, enter here.
• The
right and left hepatic ducts leave, carrying bile from the liver
to the gall bladder.
• Lymph
vessels leave the liver, draining some lymph to abdominal and some to
thoracic nodes.
• 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
STRUCTURE OF LIVER-
• The
lobes of the liver are made up of tiny functional units, called lobules,
which are just visible to the naked eye. Liver lobules are hexagonal in outline
and are formed by cubical-shaped cells, the hepatocytes, arranged in
pairs of columns radiating from a central vein.
• Blood
drains from the sinusoids into central or centrilobular veins.
These then join with veins from other lobules, forming larger veins, until
eventually they become the hepatic veins, which leave the liver and empty into
the inferior vena cava.
• One
of the functions of the liver is to secrete bile. Bile canaliculi run between
the columns of liver cells. The
canaliculi join up to form larger bile canals until eventually they form the
right and left hepatic ducts, which drain bile from the liver.
FUNCTIONS OF LIVER
• Carbohydrate
metabolism
The liver has an important role
in maintaining plasma glucose levels. After a meal when levels rise, glucose is
converted to glycogen for storage under the influence of the hormone insulin.
Later, when glucose levels fall, the hormone glucagon stimulates conversion of
glycogen into glucose again, keeping levels within the normal range
• Fat
metabolism
Hepatocytes store some
triglycerides; break down fatty acids to generate ATP; synthesize lipoproteins,
which transport fatty acids, triglycerides, and cholesterol to and from body
cells; synthesize cholesterol; and use cholesterol to make bile salts
• Protein
metabolism- Hepatocytes deaminate (remove the amino group, NH2,
from) amino acids so that the amino acids can be used for ATP production or
converted to carbohydrates or fats. The resulting toxic ammonia (NH3) is then
converted into the much less toxic urea, which is excreted in urine.
Hepatocytes also synthesize most plasma proteins, such as alpha and beta
globulins, albumin, prothrombin, and fibrinogen.
• Processing
of drugs and hormones.
The liver can detoxify
substances such as alcohol and excrete drugs such as penicillin, erythromycin,
and sulfonamides into bile. It can also chemically alter or excrete thyroid
hormones and steroid hormones such as estrogens and aldosterone.
• Excretion
of bilirubin.
Bilirubin, derived from the heme
of aged/dead red blood cells, is absorbed by the liver from the blood and
secreted into bile. Most of the bilirubin in bile is metabolized in the small
intestine by bacteria and eliminated in feces.
• Synthesis
of bile salts- Bile salts are used in the small intestinefor the
emulsification and absorption of lipids.
• Storage-In
addition to glycogen, the liver is a prime storage site for certain vitamins
(A, B12, D, E, and K) and minerals (iron and copper), which are released from
the liver when needed elsewhere in the body.
• Inactivation
of hormones- hormones are inactivated/regulated by liver. These include
insulin, glucagon, cortisol, aldosterone, thyroid and sex hormones.
• Production
of heat-The liver uses a considerable amount of energy, has a high
metabolic rate and produces a great deal of heat. It is the main heat-producing
organ of the body.
• Phagocytosis-
The stellate reticuloendothelial (Kupffer) cells of the liver phagocytize
aged red blood cells, white blood cells, and some bacteria.
• Activation
of vitamin D- The skin, liver, and kidneys participate in synthesizing the
active form of vitamin D.
BILE JUICE
• Between
500 and 1000 ml of bile are secreted by the liver daily. Bile consists of:
• Water,
mineral salts, mucus, bile pigments, mainly bilirubin, bile salts, which are
derived from the primary bile acids, cholic acid and chenodeoxycholic acid
• cholesterol.
• The
principal bile pigment is bilirubin. The
phagocytosis of aged red blood cells liberates iron, globin, and
bilirubin (derived from heme) . The iron
and globin are recycled; the bilirubin is secreted into the bile and is
eventually broken down in the intestine. One of its breakdown products—stercobilin—gives
feces their normal brown color. A
small amount is reabsorbed and excreted in urine as urobilinogen
• Bile
is partially an excretory product and partially a digestive secretion. Bile
salts, which are sodium salts and potassium salts of bile acids (mostly
chenodeoxycholic acid and cholic acid), play a role in emulsification, the
breakdown of large lipid globules into a suspension of small lipid globules
GALL BLADDER-
• The
gall bladder is a pear-shaped sac attached to the posterior surface of the
liver by connective tissue. It has a fundus or expanded end, a body or main
part and a neck, which is continuous with the cystic duct.
The wall of the gall bladder has the same layers of
tissue as those described in the basic structure of the alimentary canal, with
some modifications.
• Peritoneum-This
covers only the inferior surface because the upper surface of the gall bladder
is in direct contact with the liver
• Muscle
layer-There is an additional layer of oblique muscle fibres.
• Mucous
membrane-This displays small rugae when the gall bladder is empty that
disappear when it is distended with bile.
BILE DUCTS-
• The
right and left hepatic ducts join to form the common hepatic
duct just outside the portal fissure. The hepatic duct passes downwards for
about 3 cm where it is joined by the cystic duct from the gall bladder.
The cystic and hepatic ducts merge forming the common bile duct, which
passes downwards behind the head of the pancreas. This is joined by the main
pancreatic duct at the hepatopancreatic ampulla and the opening into the
duodenum, at the duodenal papilla, is controlled by the hepatopancreatic
sphincter (of Oddi). The common bile duct is about 7.5 cm long and has a diameter of about 6 mm.
FUNCTIONS OF GALL BLADDER-
• Functions
include:
• Reservoir
for bile- concentration of the bile by up to 10- or 15-fold, by absorption
of water through the walls of the gall bladder
• Release
of stored bile- When the muscle wall of the gall bladder contracts, bile
passes through the bile ducts to the duodenum. Contraction is stimulated by:
the hormone cholecystokinin (CCK), secreted by the duodenum the presence of fat
and acid chyme in the duodenum.
• Relaxation
of the hepatopancreatic sphincter (of Oddi) is caused by CCK and is a
reflex response to contraction of the gall bladder.
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