Liver Function Tests Module 1 PDF

Document Code FM-SNS-018 Revision No. 00 Effectivity JUNE 1, 2021 At the end of this module, the student should be able to: discuss the functions of the liver. determine physiologic and patholog...

Document Code FM-SNS-018 Revision No. 00 Effectivity JUNE 1, 2021 At the end of this module, the student should be able to: discuss the functions of the liver. determine physiologic and pathologic conditions of the liver. determine the appropriate tests for evaluation of liver function and identify different factors that may affect the test results. UNIT 1: BILIRUBIN Unit learning objectives At the end of this unit, the student will be able to: describe the anatomy and physiology of the liver. discuss the physiology of bilirubin formation and associated abnormal conditions. Did you know that the liver is the largest and heaviest internal organ of the body weighing about 1.2 to 1.5 kg? ↓ only organ that can regenerate The LIVER is located beneath and is attached to the diaphragm. It is protected by the lower rib cage and held in place by ligamentous attachments. It is divided into two unequal lobes by the falciform ligament, with the right lobe being larger than the left lobe. Microscopically, each lobe consists of lobules/acini (functional units of the liver) which are typically six sided structures that consist of specialized epithelial cells called HEPATOCYTES. The hepatocytes perform most of the functions associated with ↳ 80 % of the liver the liver including regenerative activities. Approximately 1, 500 ml of These lobules are centered around portal blood passes through the liver triads, which are interconnecting loose bands per minute. that contain branches of portal vein, hepatic artery and bile duct. Property of and for the exclusive use of SLU. Reproduction, storing in a retrieval system, distributing, uploading or posting online, or transmitting in any form or by any means, electronic, mechanical, photocopying, recording, or otherwise of any part of this document, without the prior written permission of SLU, is strictly prohibited. 1  small bile receives Canaliculi ducts to > hepatic portal rein > heart filters hepatocytes > - - hepatic artery sinusoids blood > - - > > - > - - nutrients portal of triads The liver receives blood supply from the HEPATIC ARTERY and the HEPATIC PORTAL VEIN. The hepatic artery supplies oxygen-rich blood from the heart (provides 25 % of the total blood supply of the liver) and the portal vein supplies deoxygenated blood containing nutrients, drugs, and possibly microbes and toxins from the GIT and spleen (both provides 75 % of the total blood supply of the liver). Blood from the portal vein and hepatic artery mix together and flows towards the sinusoids (capillary- like blood vessel lined by fenestrated endothelial cells allowing free filtration of blood) which course between individual hepatocytes. Hepatocytes take up most of the oxygen, nutrients and certain toxic substances. Small grooves are present between adjacent hepatocytes termed canaliculi. These small passages coalesce and eventually enter into small bile ducts within the portal triads. Products manufactured by the hepatocytes and other nutrients needed by other cells are secreted back into the blood which drains into the central vein and passes to the hepatic vein towards the heart. O lobe smaller & - Can you label the parts? minor lobes : other ·caudate · quadrate lobe right ↳ larger Located posteriorly in Gallstones are mostly the surface of the formed from precipitated liver is the cholesterol. gallbladder. The gallbladder stores O and concentrates bile produced by the liver (about 800-1000 ml) until it is needed in the small intestine. Bile is a yellow brownish liquid that has a pH of about ~ waste product of liver 7.6-8.6 and is consists of mostly bile salts, water, cholesterol, lecithin, bile pigments (bilirubin) and gallstones = several ions. As it is secreted by hepatocytes, it enters the bile canaliculi then to the bile duct. Bile duct carries bile from the liver to the gallbladder for storage. The bile ducts merge to form the hepatic ducts which unite and exit the liver as the common hepatic duct. The common hepatic ducts merges with the cystic duct from the gallbladder to form the common bile duct. Bile then passes through the common bile duct towards the small intestines (see Fig. 1). Property of and for the exclusive use of SLU. Reproduction, storing in a retrieval system, distributing, uploading or posting online, or transmitting in any form or by any means, electronic, mechanical, photocopying, recording, or otherwise of any part of this document, without the prior written permission of SLU, is strictly prohibited. 2  CAN YOU TRACE THE FLOW OF BILE FORMATION? Aside from the hepatocytes, other cells such as Kupffer, stellate and oval cells contribute to the physiologic function of the liver. Kupffer cells are derived from monocytes and are found in the sinusoids. They contain lysosomes with hydrolytic enzymes that can breakdown phagocytized worn out blood cells, bacteria and other foreign matters. They also contain immunoglobulin and complement receptors that serves as the main site for clearance of antigen-antibody complexes in the venous blood from the gastro-intestinal tract. They also secrete interleukins, tumor necrosis factor, collagenase, prostaglandins and other factors involved in the inflammatory response. Stellate cells/Ito cells lie in the space between the endothelial lining of the sinusoids and the hepatocytes within a small cleft referred to as the space of Disse. They are capable of synthesizing nitric oxide which serves to regulate blood flow through the liver. In their normal state, they can store fat soluble vitamins (Vitamin A). When they are stimulated by injury, stellate cells transform into collagen forming cells which are the source of fibrous tissue leading to fibrosis then cirrhosis. Oval cells, found near the portal areas around small-bile ↳ regeneration Property of and for the exclusive use of SLU. Reproduction, storing in a retrieval system, distributing, uploading or posting online, or transmitting in any form or by any means, electronic, mechanical, photocopying, recording, or otherwise of any part of this document, without the prior written permission of SLU, is strictly prohibited. 3  passages are believed to be liver stem cells involved in regeneration of hepatocytes and bile ducts after liver injury. The liver can perform several physiologic functions. These include metabolic, synthetic, conjugation, excretory and storage functions. ▪ METABOLIC FUNCTIONS: The liver can metabolize several substances such as carbohydrates, ammonia and bilirubin. METABOLISM OF BILIRUBIN BILIRUBIN is a major metabolite of heme, the iron binding component found in hemoglobin, myoglobin and cytochromes (electron transporters). In normal individuals, production is about 250-350 mg daily in which 85 % is derived lifespanoedays from turnover of senescent red blood cells (red cell that has lived its life span) and 15 % is produced from RBC precursors destroyed in the bone marrow and from the catabolism of other heme-containing proteins such as myoglobin, cytochromes and peroxidases. So how is bilirubin formed and cleared? IProtoporphyrin + BILIRUBIN 4 Heme + 46LOBIN - > recycled In the spleen, senescent red blood cells are destroyed through phagocytosis by the reticuloendothelial cells (macrophages), thus releasing methemoglobin (this is a type of hemoglobin where the ferrous iron is oxidized to ferric state and thus cannot transport oxygen or carbon dioxide). Some senescent red blood cells are also destroyed by macrophages in the liver and bone marrow. Methemoglobin splits into globin and heme. The porphyrin ring of heme is oxidized by microsomal heme oxygenase, producing biliverdin and releasing iron. Biliverdin is then reduced to bilirubin by biliverdin For each mole of heme reductase in the presence of NADPH. The iron is bound catabolized, one mole of to transferrin and is returned to iron stores in the liver or bilirubin is produced. in the bone marrow for reuse. The globin is degraded to amino acids which will be reused by the body. Bilirubin is transported into the circulation and bound mainly to albumin, forming unconjugated bilirubin. The unconjugated bilirubin enters the liver via a receptor mediated endocytosis by binding to a transport protein present in the membrane surface of hepatocytes. Once in the hepatocytes, unconjugated bilirubin flows into the sinusoidal spaces (Space of Disse) and is released from albumin. It forms a complex with Z proteins and then binds to a soluble carrier protein called ligandin or Y protein which will be responsible in transporting it to the smooth endoplasmic reticulum (SER) and at the same time prevent Property of and for the exclusive use of SLU. Reproduction, storing in a retrieval system, distributing, uploading or posting online, or transmitting in any form or by any means, electronic, mechanical, photocopying, recording, or otherwise of any part of this document, without the prior written permission of SLU, is strictly prohibited. 4  > Transferrin - recycled / heme oxygenase , BILIVERDIN protoporphyrin # heme BILIVERDIN & oxygenase 4 Gobin _, degraded e in acids NADPH · => mayched in NADH Biliverin reductase the body & = · hemolysis forms a complex fed - ↑ un conjuga with z protein & -n bilimbi F binds to Y protein Smooth ER ↓ acid side- propionic chain of uncon j. bili is estrified w/ glucoronic if deficient ated, ↑ uncominsgeated - acid > secreted to the bile by ATP binding cassette transporter protein conversion - ↳ normal flora in the intestines ↳ 20 % is reabsorbed to the * for newborns : yellow stool liver then same process 4 sterzobilinogen > STERCOBILIN responsible for normal/ brown color of the stool - -  reflux of bilirubin into the plasma. Ligandins are cytosolic proteins of the glutathione-S- transferase gene family and constitute about 5% of the total protein of human liver cytosol. In the SER, the propionic acid side chain of unconjugated bilirubin is esterified with glucuronic acid in the presence of the enzyme uridine diphosphate glucuronyl transferase (UDPGT) to produce bilirubin monoglucuronide and diglucuronide. From the canaliculi, the conjugated bilirubin is then secreted into the bile by the ATP binding cassette transporter protein then to the hepatic duct then to the common bile duct and into the intestines. Bilirubin glucuronides are not substantially reabsorbed in the intestines and thus they are readily hydrolyzed by the catalytic action of beta-glucuronidase and microbial flora in the intestines into unconjugated bilirubin, in which some are reabsorbed through the intestinal mucosa and returns to enterohepatic circulation. Unabsorbed unconjugated bilirubin are further metabolized by the intestinal microbial flora to form a group of three colorless tetrapyrroles compound generally termed as urobilinogens (stercobilinogen, mesobilinogen, urobilinogen) which are eventually converted to their oxidation products (urobilinogen to urobilin, stercobilinogen to stercobilin and mesobilinogen to mesobilin). These are pigments which give an orange-brown color of the stool. Up to 20% of the urobilinogen produced daily is reabsorbed from the intestine and undergoes enterohepatic recirculation. Most of this urobilinogen is taken up by the liver (via the portal vein) and is re-excreted into bile, whereas a small fraction (2–5%) is excreted into the general circulation and filtered by the kidneys, being detectable in urine. What is the expected color of the stool when there is impairment in bilirubin metabolism? _____________________________________ In adults, almost all bilirubin excreted in the bile is in the form of glycosidic conjugates: glucuronides account for 95 % of them (diglucuronide as the major fraction and monoglucuronide as the minor fraction) and glucosides and xylosides constitute the remainder. ▪ SYNTHETIC FUNCTION: The liver can synthesize many biological compounds including bile salts, lipids, carbohydrates and proteins (albumin, globulin, total protein, alpha-1 antitrypsin, ceruloplasmin and coagulation factors). ▪ STORAGE FUNCTION: Substances such as glycogen, iron, amino acids and some lipids can be stored in the hepatocytes. maintaining stable glucose concentration * ▪ DETOXIFICATION FUNCTION: The liver can convert/detoxify high concentration of toxic substances (e.g., xenobiotics) in the circulation into non-toxic substances. Every substance that is absorbed in the gastrointestinal tract (GIT) must pass through the liver first (first pass mechanism) to prevent harmful substances from reaching the circulation. Property of and for the exclusive use of SLU. Reproduction, storing in a retrieval system, distributing, uploading or posting online, or transmitting in any form or by any means, electronic, mechanical, photocopying, recording, or otherwise of any part of this document, without the prior written permission of SLU, is strictly prohibited. 5  ↳ Oxidation reduction,hydrolysis, , hydroxylat one The liver plays several roles in detoxification. It filters the blood to remove large toxins, synthesizes and secretes bile full of cholesterol and other fat-soluble toxins, and enzymatically disassembles unwanted chemicals. takes place in the liver's microsomes via> - the cytochrome P-450 isoenzymes ▪ EXCRETORY FUNCTION: Organic compounds, both endogenous and exogenous, are removed from the sinusoidal blood, then bio-transformed and excreted into the bile or urine. This includes excretion of bilirubin, bile acids (cholic acid and chenodeoxycholic acid) and ammonia (a toxic waste product from amino acid metabolism which is converted to urea for excretion). Through HPLC, four fractions of bilirubin are identified: (a) alpha bilirubin (unconjugated bilirubin), beta bilirubin (monoconjugated bilirubin), (c) gamma bilirubin (diconjugated bilirubin), and (d) delta bilirubin (conjugated bilirubin bound to protein). ↳ main types TYPES OF BILIRUBIN cis dire (direct) Conjugated bilirubin is a polar and water- Photoisomerization may occur when bilirubin is soluble compound that is found in exposed to light wherein trans form becomes cis plasma in the free state (not bound form. to any protein). It is readily filtered Kidneyem by the glomerulus and is actively reabsorbed by the tubules, thus, may appear in urine. It has prob ↓ a half-life of less than 24 hours in the circulation and is soluble in plasma but cannot cross the no reabsorption of bilimbin blood brain barrier. In terms of stereoisomeric configuration, this type of bilirubin occurs in the O cis form and directly reacts with diazotized sulfanilic acid solution (diazo reagent) without an accelerator (solubilizer). It could be monoconjugated (15%), diconjugated (85 %) and rarely, triconjugated. measured total bilimbin-direct bilimbin indirect ↑ indirectly = = Trans Unconjugated (indirect) bilirubin is a nonpolar and water-insoluble substance that is found in plasma bound to albumin. Because of these characteristics, its reaction with diazotized sulfanilic acid is slow and can be hastened in the Half-life refers to the time required for a substance presence of an accelerator. This to be reduced into half from its initial type of bilirubin is insoluble in quantity/concentration. plasma, not filtered by the glomerulus (large molecular size) and not actively reabsorbed by the tubules; hence it is not usually detected in urine. It has a very short half-life (lesser than half-life of conjugated bilirubin) and occurs in the G trans form. It is soluble in lipids which makes it capable to penetrate the neuronal membrane (can cross the blood brain barrier). ↳ Kernicteus = brain = sever malformation > - often happen in neonates Delta bilirubin (biliprotein) is a conjugated bilirubin that is covalently bound to also Au O albumin. This fraction of bilirubin is seen only when there is significant hepatic obstruction. wasoluble Because the molecule is attached to albumin, it is too large to be filtered by the glomerulus and thus, not excreted in the urine. It has a half-life of about 17-20 days, owing to the half- Property of and for the exclusive use of SLU. Reproduction, storing in a retrieval system, distributing, uploading or posting online, or transmitting in any form or by any means, electronic, mechanical, photocopying, recording, or otherwise of any part of this document, without the prior written permission of SLU, is strictly prohibited. 6