THE LIVER AND HEPATOBILIARY SYSTEMThe liver and biliary systemMetabli! "#n!tins " the liverIt produces bile; It metabolizes hormones and drugs;Initscapacityformetabolizingdrugsandhormones,theliver servesasanexcretoryorgan.Inthisrespect,thebile,which carries the end-products of substances metabolized by the liver, ismuchliketheurine,whichcarriesthebodywastesfilteredby the kidneys.It synthesizes proteins, glucose, and clotting factors; It stores vitamins and minerals; Itchangesammoniaproducedbydeaminationofaminoacidsto urea; It converts fatty acids to ketones$arbhydrate metablismTheliverisespeciallyimportantinmaintainingglucose homeostasis. Itstoresexcessglucoseasglycogenandreleasesitintothe circulation when blood glucose levels fall. Theliverconvertsgalactoseandfructosetoglucoseandit synthesizesglucosefromaminoacids,glycerol,andlacticacid asameansofmaintainingbloodglucoseduringperiodsof fasting or increased need. Theliveralsoconvertsexcesscarbohydratestotriglyceridesfor storage in adipose tissue.Li%id metablismertain aspects of lipid metabolism occur mainly in the liver. These include!Theoxidationoffattyacidstosupplyenergyforotherbody functions;Toderiveenergyfromneutralfats"triglycerides#,thefatmust firstbesplitintoglycerolandfattyacids,andthenthefatty acidssplitintoacetyl-coenzyme$"acetyl-o$#.$cetyl-o$ canbeusedbythelivertoproduceadenosinetriphosphate "$T%# or it can be converted to acetoacetic acid and released into the bloodstream and transported to other tissues, where it is used for energy.Theacetyl-o$unitsfromfatmetabolismalsoareusedto synthesize cholesterol and bile acids. Thesynthesisoflarge&uantitiesofcholesterol,phospholipids, and most lipoproteins;holesterolcanbeesterifiedandstored,itcanbeexported bound to lipoproteins or it can be converted to bile acids.The formation of triglycerides from carbohydrates and proteins.Prtein metablismTheliverhasthegreatestrateofproteinsynthesispergramof tissue.Itproducestheproteinsforitsowncellularneedsandsecretory proteins that are released into the circulation. The most important of these secretory proteins is albumin.$lbumincontributessignificantlytotheplasmacolloidalosmotic pressureandtothebindingandtransportofnumerous substances, includingsomehormones, fatty acids,bilirubin, and other anions.The liver also produces other important proteins, such as fibrinogen and the blood clotting factors.Bile %rd#!tin and !hlestasisThe secretion of bile, approximately '(( to )*(( m+ daily, is one of the many functions of the liver.,ile functions in the digestion and absorption of fats and fat-soluble vitaminsfromtheintestine,anditservesasavehicleforexcretion ofbilirubin,excesscholesterol,andmetabolicend-productsthat cannot be eliminated in the urine.,ile contains water, electrolytes, bile salts, bilirubin, cholesterol, and certain products of organic metabolism.$pproximately-./ofbilesaltsthatentertheintestineare reabsorbed into the portal circulation by an active transport process thattakesplaceinthedistalileum.0romtheportalcirculation,the bile salts pass into the liver, where they are recycled. 1ormally, bile saltstravelthisentirecircuitapproximately)2timesbeforebeing expelled in the feces.Thissystemforrecirculationofbileiscalledtheenterohepatic circulation.Bilir#bin eliminatin,ilirubinisformedduringthebreakdownofsenescentredblood cells. Intheprocessofdegradation,thehemeportionofthehemoglobin moleculeisoxidizedtoformbiliverdin,whichisthenconvertedto free bilirubin. 0reebilirubin,whichisinsolubleinplasma,istransportedinthe bloodattachedtoplasmaalbumin.3venwhenitisboundto albumin, this bilirubin is still called free bilirubin. $sitpassesthroughtheliver,freebilirubinisreleasedfromthe albumin carrier molecule and moved into the hepatocytes. Inside the hepatocytes,freebilirubinisconvertedtocon4ugatedbilirubin, making it soluble in bile. on4ugatedbilirubin issecretedasaconstituentofbile,and inthis form it passes through the bile ducts into the small intestine.Bilir#bin eliminatinIntheintestine,approximatelyonehalfofthebilirubinisconverted intoahighlysolublesubstancecalledurobilinogenbytheintestinal flora. 5robilinogen is either absorbed into the portal circulation or excreted in the feces. 6ostoftheurobilinogenthatisabsorbedisreturnedtotheliverto be re-excreted into the bile. $ small amount of urobilinogen, approximately 7/, is absorbed into the general circulation and then excreted by the kidneys.5sually, only a small amount of bilirubin "(.) to ).* mg8d+# is found in the blood. +aboratory measurements of bilirubin usually measure thefreeandthecon4ugatedbilirubinaswellasthetotalbilirubin. Thesearereportedasthedirect"con4ugated#bilirubinandthe indirect "uncon4ugated or free# bilirubin.Bilir#bin eliminatin&a#ndi!e 'i!ter#s(9aundice"i.e.,icterus#,whichresultsfromanabnormallyhigh accumulation of bilirubin in the blood, is a yellowish discoloration to the skin and deep tissues. 9aundicebecomesevidentwhentheserumbilirubinlevelsrise above *.( to *.7 mg8d+.,ilirubinhasaspecialaffinityforelastictissue.Thescleraofthe eye, which contains considerable elastic fibers, usually is one of the first structures in which 4aundice can be detected.