liver carries many important physiological functions which include: processing
of absorbed substances; synthesis and secretion of bile acids; bilirubin
production and excretion; participation in metabolism of key nutrients,
including carbohydrates, proteins, and lipids; and detoxification and excretion
of waste products.
liver participates in carbohydrates, proteins, and lipids metabolism. During
carbohydrate metabolism, the liver performs gluconeogenesis, stores glucose as
glycogen, and releases stored glucose into the bloodstream, when glucose level is
low. During protein metabolism, the liver synthesizes non-essential amino acids
and modifies amino acids so that they may enter biosynthetic pathways for
carbohydrates. In addition, the liver also synthesizes almost all plasma
proteins, including albumin and the clotting factors. Patients with liver
failure develop hypoalbuminemia which may lead to edema due to loss of plasma
protein oncotic pressure and clotting disorders. The liver also converts toxic
ammonia which is byproduct of protein catabolism, to urea, which is later excreted
in the urine. When it comes to lipid metabolism, the liver participates in
fatty acid oxidation and synthesizes lipoproteins, cholesterol and
phospholipids. The liver converts a portion of the cholesterol to bile acids,
which participate in lipid digestion and absorption
reticuloendothelial system (RES) processes senescent red blood cells. When
hemoglobin is degraded by the RES, one of the byproducts is biliverdin which later
is converted to yellow-colored bilirubin. Later Bilirubin is bound to albumin
in the circulation and carried to the liver, where it is taken up by the
hepatocytes. In hepatic microsomes, bilirubin is conjugated with glucoronic
acid via the enzyme UDP glucuronyl transferase. (BecauseUDP glucuronyl
transferase is synthesized slowly after birth, some newborn babies develop
“newborn jaundice.”) Conjugated bilirubin is water-soluble and a portion of it easily
is excreted in the urine. The remainder of the conjugated bilirubin is secreted
into bile and then, via bile, into the small intestine. The conjugated
bilirubin travels down to the terminal ileum and colon, where it is
deconjugated by bacterial enzymes and metabolized to urobilinogen, some of
which is absorbed via the enterohepatic circulation and delivered back to the
liver; the remainder is converted to urobilin and stercobilin, which are
excreted in the feces.
is necessary for the digestion and absorption of lipids in the small intestine.
In contrast with carbohydrates and proteins, lipids pose special problems for
digestion and absorption because they are insoluble in water. Bile, a mixture
of bile salts, bile pigments, and cholesterol, solves this problem of
insolubility. Bile is produced and secreted by the liver, stored in the
gallbladder, and ejected into the lumen of the small intestine when stimulated
to contract. In the lumen of the intestine, bile salts emulsify lipids to
prepare them for digestion and then solubilize the products of lipid digestion
in packets called micelles. The hepatocytes of the liver continuously
synthesize and secrete the constituents of bile. The components of bile include
the bile salts, cholesterol, phospholipids, bile pigments, ions, and water.
Bile flows out of the liver through the bile ducts and fills the gallbladder,
where it is stored . The gallbladder then concentrates the bile salts by
absorption of water and ions. When chyme reaches the small intestine, CCK issecreted..
In the small intestine, the bile salts emulsify and solubilize dietary lipids.
When lipid absorption is complete, the bile salts are recirculated to the liver
via the enterohepatic circulation .The steps involved in the enterohepatic
circulation include absorption of bile salts from the ileum into the portal
circulation, delivery back to the liver, and extraction of bile salts from the
portal blood by the hepatocytes .
DETOXIFICATION OF SUBSTANCES
The liver protects the
body from potentially toxic substances that are absorbed from the GI tract.
These substances are presented to the liver via the portal circulation, and the
liver modifies them in so-called “first pass metabolism,” ensuring that little
or none of the substances make it into the systemic circulation. For example,
bacteria absorbed from the colon are phagocytized by hepatic Kupffer cells and
thus never enter the systemic circulation. In another example, liver enzymes
modify both endogenous and exogenous toxins to render them watersoluble and
thus capable of being excreted in either bile or urine. Phase I reactions,
which are catalyzed by cytochrome P-450 enzymes, are followed by phase II
reactions that conjugate the substances with glucuronide, sulfate, amino acids,