1625915643-5d53d156c9525bd62bd0d3434ecdc231 (843955), страница 76
Текст из файла (страница 76)
Bile is polar, allowing it toincorporate lipids into a micelle, and then taxi the lipidsthrough the unstirred water layer adjacent to the enterocytes (see Chapter 25). Without bile, the bulk of thehydrophobic lipids cannot get near the brush border.Endocrine functions: The hepatocytes produce andsecrete hormones into blood, including insulin-likegrowth factor-1 (IGF-1), hepatocyte growth factor,angiotensinogen, and cytokines.
It converts thyroxine(T4) to active triiodothyronine (T3) and participates inactivation of vitamin D.Detoxification: The liver contains reticuloendothelialcells, the Kupffer cells, which are fixed macrophages inthe endothelial lining of hepatic sinusoids. As bloodpasses through the liver, old and damaged erythrocytesundergo phagocytosis by the Kupffer cells. Hormones,drugs, and other chemicals are metabolized by thehepatocytes.Vitamin and iron storage: The liver stores several elements crucial to normal body functions (B12, folic acid,iron). Vitamins can be stored for weeks to months, andin the case of essential vitamins like B12 (cobalamin),this provides a source of the vitamin if the dietary supplyis depleted. The liver is also the site of large iron stores,bound to the protein ferritin (containing 25% of thebody’s iron).
When needed, the iron (bound to transferrin) is released into the blood and enters the bonemarrow for production of hemoglobin.This great variety of functions makes the liver a crucial organfor maintaining immune cell function and proper bloodglucose levels, excreting waste products, processing proteins, and even indirectly regulating blood pressure andfluid homeostasis (through synthesis of plasma proteins)(Fig.
24.1). When part of the liver is damaged, it has the abilityto regenerate functional hepatocytes, and this compensatoryaction allows it to maintain adequate metabolic function. Inaddition, the ability of the liver to achieve a high level ofmetabolic activity is dependent on the blood flow.LIVER STRUCTURE AND BLOOD FLOWThe liver is like a sponge. The hepatocytes are surroundedby sinusoidal capillaries and lymph vessels (Fig. 24.2).
Thisallows for the free flow of blood to the cells, and the cellularproducts secreted into the perisinusoidal space of Disse canbe released back into blood, or into the lymphatic vessels.Kupffer cells are affixed to the sinusoidal membranes and arepresent throughout the liver. These cells phagocytose bacteriaand damaged red blood cells. The hepatocytes metabolizehormones and drugs and clear the waste, “detoxifying” theblood. The heptocytes also produce bile and electrolyte solutions that drain into the bile canaliculi, which lead to the bileduct.Under normal conditions, the liver contains ∼400 milliliters(mL) of blood, which is ∼8% of the total blood volume.
Thisillustrates the “blood storage” aspect of the liver. Two keyfactors that make the system work properly are that hepaticvein pressure is very low, near 1 mm Hg, and that there is noobstruction of blood flow through the liver sinusoids. Thisallows the free flow of blood through and out of the liver intothe vena cava. If there is obstruction of flow through the liver(cirrhosis, hepatitis) or increased hepatic vein pressure (congestive heart failure), blood will back up and pressure will rise,causing portal hypertension. The volume of blood associatedwith the liver can also increase to 1 liter (L), causing hepatomegaly (enlarged liver).284Gastrointestinal PhysiologyEpithelial CellsKupffer CellsStoragePhagocytosisMetabolicpoolSecretion:Glucose,proteins,coagulationfactors,enzymesBileLiver Asa WholeBloodpigmentbreakdownFilter actionDetoxificationSponge action(blood volumeregulation)Sphinctericblood flowregulationBile drainageBile Duct SystemSinusoidal permeabilityVascular SystemFigure 24.1 Overview of Liver Function The liver performs many functions, including metabolismof carbohydrates, proteins, and lipids; phagocytosis and removal of waste and bacteria; detoxification;synthesis of bile; blood volume control; and vitamin storage.The blood supply to the liver comes from the hepatic arteryand portal vein.
The systemic blood from the hepatic arteryenters the liver at a rate of ∼450 mL per minute. The portalvein carries the blood from the intestines (∼1 L per minute),and the arterial and venous blood intermingles in the sinusoidal capillaries. This amount represents about 30% of thecardiac output. As the blood from the portal vein enters theliver, nutrients, bacteria, and foreign bodies are processed—this is the “first pass” effect, which allows the absorbed materials to be “cleared” by the liver before the blood leaves throughthe hepatic vein into the systemic circulation. Although notall such substances will be cleared or metabolized in one pass,the bulk of the substances will be handled.Ascites is fluid in the peritoneal cavity of the abdomen.This can be caused by increased vascular pressure withinthe liver.
Because the liver capillaries have a high permeability,when the pressure increases, fluid is forced out of the space ofDisse and lymphatics, into the peritoneal space. Increasedvenous pressure, as in congestive heart failure, can cause ascitesas well as peripheral edema. Edema results from elevated capillary hydrostatic pressure, increasing the Starling forces pushingfluid out of the capillaries into the interstitial space (in the lowerextremities).Hepatobiliary FunctionCLINICAL CORRELATEBilirubin and JaundiceRed blood cells (RBCs) are viable for about 120 days, and newcells are continually being produced to replace damaged old cells.Phagocytic mononuclear cells throughout the body (especiallyKupffer cells in the liver) remove and break down the old redblood cells, so that the iron can be recycled and waste eliminated.Bilirubin is a by-product of RBC degradation and is eliminatedfrom the body by incorporation into bile (and subsequent loss infeces) and by urinary excretion as urobilinogen.
The addition ofbilirubin to bile, and its excretion in feces, contributes to the pigmentation of the feces. The bile pathway of excretion is importantfor ridding the body of excess bilirubin.BilecanaliculiLiverJaundice occurs when plasma bilirubin is elevated. In patientsit is observed as a yellowing tinge in the whites of the eyes and inskin tone and nail beds; stool is pale. Primary jaundice arises fromhepatic dysfunction, as seen with obstructive liver disease (cirrhosis), blockage of bile ducts (by tumor or gallstones), or inflammation (hepatitis C).
Secondary jaundice occurs from extrahepaticsources, such as abnormal lysis of RBCs (hemolytic disease). Inmost cases, when the primary cause of the jaundice is treated, thejaundice will abate as the excess bilirubin eventually clears (throughbile and urinary excretion). An important distinction is that withsecondary jaundice liver function is normal.BloodstreamLivercellsKidneyConjugationLiversinusoidKupffercellBileductulesIntrahepaticbile ductsExtrahepaticbile ductsMPScellUrineurobilinogenEnterohepaticcirculation ofbile acidPortal veinKEYHemoglobinIndirect-reacting bilirubin(unconjugated)Direct-reacting bilirubin(conjugated) and bile acidsUrobilinogen285RedcellBowelBilirubin Production and ExcretionStool286Gastrointestinal PhysiologyIntralobular bile ductuleCentral veinsPerisinusoidal spaces (of Disse)SinusoidsSublobularveinLymph vesselConnective tissueBile ductPortal vein branchHepatic arterybranchCentral veinPortal arteriolePeriportal arterioleIntralobular arterioleDistributing veinFigure 24.2 Structure of the Liver The liver is highly vascularized to accomplish its primary functionof filtering portal and systemic blood.
The capillary sinusoids surround the liver cells (hepatocytes), allowingefficient access of blood to the cells and transfer of products back into the blood. The sinusoids also containthe Kupffer cells, which are fixed macrophages that phagocytose damaged or aged red blood cells.BASIC METABOLISM OF CARBOHYDRATES,LIPIDS, AND PROTEINSCarbohydratesThe liver acts as a blood glucose monitor, storing glucose asglycogen and releasing it when blood levels are low. Carbohydrates are absorbed in the intestine as monosaccharidesand are carried in the portal blood to the liver. Most of theglucose passes through the liver rapidly and is released intothe systemic blood, where elevated insulin will facilitate itsentry into tissues. In the liver, excess monosaccharides arehandled by:■■Conversion of other monosaccharides to glucose: Fructose and galactose can be converted into glucose.Glycogen synthesis and storage: Excess glucose is polymerized and stored as glycogen. Stored glycogen canprovide glucose for 17 to 24 hours during fasting.
Whenblood glucose levels are low, glucagon (and other hyperglycemic hormones like epinephrine and growthhormone) stimulates glycolysis to break down glycogenThe ingestion of carbohydrates raises blood glucoselevels; the glycemic index of a specific carbohydraterefers to the degree to which it raises blood glucose.
Diets thatproduce high glycemic indices are believed to be associated withdevelopment of type II diabetes. High-fructose corn syrup isused in many foods, and because of its low glycemic index, hasbeen recommended for people with diabetes. However, somestudies suggest that elevated blood fructose levels may actuallycontribute to insulin resistance and hyperlipidemia, which,along with elevated blood pressure, are components of metabolic syndrome.■and release glucose into the blood.
![](https://s.studizba.com/z.php?f=/templates/si/i/noavatar.png&w=100&h=100&t=1"){kind=avatar}
