Metabolism And Excretion Lecture 2- Structure And Function Of The Liver Part 1

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Microstructure of the liver

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Uniform histology across species

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Capsule and stroma

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The liver is entirely covered by a thin connective tissue capsule
Gives rise to extensive framework to support parenchyma of liver

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Hepatocytes

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Polyhedral shaped cells with a centrally places spherical nucleus with prominent nuclei
Mitochondria and other organelles abundant
Cytoplasm varies with the functional state of the cell

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Sinusoids

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Specialized blood capillaries that extend between cords of liver cells
Lined by endothelial cells and kupffer cells

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Kupffer cells
.

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Phagocytic cells (macrophages) that are extremely efficient at removing micro particles and abnormal molecules and old red blood cells from the sinusoidal blood

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Stellate cells

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Pericytes found in the perisinusoidal space
Produce collagen fibers

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Hepatic or classical lobule

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The basic structural unit of the liver
In cross section it is somewhat hexagonal in shape
CENTRAL VEIN (HEPATIC VEIN) is present in the center of the lobule
PORTAL TRIAD- bile duct, hepatic artery and portal vein- are at the periphery of the lobule in 3-4 of the 6 corners of the hexagon
Lymphatic vessels are also present in portal triad
HEPATIC CHORDS extend outward in a radial pattern from central vein
Formed by hepatocytes

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Sinusoids extend between

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liver cells cords

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Sinusoidal blood flows in

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centripetal direction- toward central vein

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Portal Lobule

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Defined as a FUNCTIONAL SECRETORY UNIT OF THE LIVER- DRAIGANGE OF BILE
Bile salts- function to emulsify fats in the small intestine- resulting in micelle formation to facilitate lipid absorption and active intestinal lipases
Portal lobule is triangular in shape, formed by parts of THREE ADJACENT CALSSICAL LOBULES adjoining a portal canal with the bile duct as its central axis
Boundary is defined by drawing lines between the central veins of three classical lobules

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Liver acinus

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Defined as FUNCTIONAL UNIT of liver- SUPPLY BOOD to liver tissue
Diamond shaped, formed by the contiguous parts of two adjacent classical lobules
Boundary is defined by drawing lines between two adjacent central veins via their common portal triads

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Biliary system =

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liver + gallbladder

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Between hepatocytes are

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fine channels known as bile canaliculi

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Bile flows in a

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centrifugal direction towards portal triads

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Emptying of gallbladder occurs in response....This causes

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...to food in the duodenum...rhythmic contraction of the wall of the gallbladder, simultaneous relaxation of the sphincter of oddi and peristalsis in the duodenum aids sphincter relaxation.

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CHOLECYSTOKININ

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Released in response to fat in duodenum
Causes contraction of gallbladder and relaxation of sphincter

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VAGAL STIMULATION is

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therefore also a stimulus to the gall bladder emptying

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Bile consists of:

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Aqueous alkaline fluid= added by duct cells
Plus organic constitutes from hepatocytes activity
Bile salts
Cholesterol
Lecithin
Bilirubin

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Bile functions in the

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absorption and digestion of fats.

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Bile salts are derivates of ... and are synthesized by... from... and then conjugated with one to other of two amino acids:

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cholesterol
hepatocytes
cholesterol
Taurine
Glycine

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Bile salts also come from the

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portal vein after they are reabsorbed in the ileum

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bile salts are highly polar, the steroid backbone is ... and the amino acid conjugate is ...

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lipophilic
hydrophilic

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Conjugated bile salts

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Act as detergents
Keep fat solutions in aqueous environment
Fats are then accessible to lipase

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Bile pigments

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Formed from the breakdown of haem portion of haemaglobin.
Effete red blood cells are engulfed and broken down by macrophages of the reticuloendothilial system and the haem moiety is converted to biliverdin and then Bilirubin
Bilirubin, bound to albumin in the plasma is taken up by a specific carrier mechanism into the hepatocytes
It is them conjugated with glucuronic acid and transported into bile by a specific carrier mechanism.

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Enterohepatic circulation

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98% of secreted bile salts are reabsorbed.
Small loss of bile salts into the colon may help is colonic water absorption
Bilirubin is converted by bacteria to urobilinogen- water soluble reabsorbed and secreted in urine

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