Basic Science - Gastro 53 Liver anatomy and physiology

• The functional anatomy of the liver is composed of eight segments, each supplied by a single portal triad composed of a portal vein, hepatic artery, and bile duct.
• These segments are further organized into four sectors separated by scissurae containing the three main hepatic veins.
• The main scissura contains the middle hepatic vein, which runs in an anteroposterior direction from the gallbladder fossa to the left side of the vena cava. It divides the liver into right and left hemilivers. The line of the main scissura is also known as Cantlie line.
• The right liver is divided into anterior (segments V and VIII) and posterior (segments VI and VII) sectors by the right scissura, which contains the right hepatic vein.
• The left liver is split into anterior (segments III and IV) and posterior (segment II, the only sector composed of a single segment) sectors by the left scissura.
• At the hilum of the liver, the right portal triad has a short extrahepatic course of approximately 1 to 1.5 cm before entering the substance of the liver and branching into anterior and posterior sectoral branches.
• The left portal triad, however, has a long extrahepatic course of up to 3 to 4 cm and runs transversely along the base of segment IV in a peritoneal sheath.

• First liver - Hemiliver 
• Second order - Section (section and sector are different)
• Third order - segment 

• However, the addendum to the second order division actually makes the terminology more complicated by allowing the use of both ‘sector’ and ‘section’, whiles these two terms actually describe the different combinations of Couinaud segments anatomically. 
• To simplify this matter, I would suggest avoiding the use of the terms ‘section’ or ‘sector’ and naming the Couinaud segments only by Arabic numerals instead.

• The portal vein provides approximately 75% of the hepatic blood inflow.
• Hepatic artery provides 25% of hepatic blood inflow.

• The cystic artery normally arises from the right hepatic artery, which can pass posterior or anterior to the common bile duct to supply the gallbladder.
• The cystic artery may arise from the right hepatic, left hepatic, proper hepatic, common hepatic, gastroduodenal, or superior mesenteric artery.
• Although variable, the cystic artery generally lies superior to the cystic duct and is usually associated with a lymph node, known as Calot node.
• Because this node provides some of the lymphatic drainage of the gallbladder, it can be enlarged in the setting of gallbladder disease, whether it is inflammatory or neoplastic.

• The portal vein forms behind the neck of the pancreas at the confluence of the superior mesenteric vein and the splenic vein.
• The length of the main portal vein ranges from 5.5 to 8 cm, and its diameter is usually approximately 1 cm.
• Cephalad to its formation behind the neck of the pancreas, the portal vein runs behind the first portion of the duodenum and into the hepatoduodenal ligament, where it runs along the right border of the lesser omentum, usually posterior to the common bile duct and proper hepatic artery.
• The portal vein divides into main right and left branches at the hilum of the liver.
• The left branch of the portal vein runs transversely
• along the base of segment IV and into the umbilical fissure, where it gives off branches to segments II and III and feedback branches to segment IV. The left portal vein also gives off posterior branches to the left side of the caudate lobe.
• The right portal vein has a short extrahepatic course; it usually enters the substance of the liver, where it splits into anterior and posterior sectoral branches.
• There is usually a small caudate process branch off the main right portal vein or at the right portal vein bifurcation that comes off posteriorly to supply this portion of liver.

• The hepatic artery provides approximately 25% of the hepatic blood flow and 30% to 50% of its oxygenation.
• The celiac trunk originates directly off the aorta, just below the aortic diaphragmatic hiatus, and gives off three branches—splenic artery, left gastric artery, and common hepatic artery.
• The common hepatic artery passes forward and to the right along the superior border of the pancreas and runs along the right side of the lesser omentum, where it ascends toward the hepatic hilum, lying anterior to the portal vein and to the left of the bile duct. At the point where the common hepatic artery begins to head superiorly toward the hepatic hilum, it gives off the gastroduodenal artery, followed by the supraduodenal artery and right gastric artery.
• The common hepatic artery beyond the takeoff of the gastroduodenal is called the proper hepatic artery; it divides into right and left hepatic arteries at the hilum.
• The left hepatic artery heads vertically toward the umbilical fissure to supply segments II, III, and IV. The left hepatic artery usually also gives off a middle hepatic artery branch that heads toward the right side of the umbilical fissure and supplies segment IV.
• The right hepatic artery usually runs posterior to the common hepatic bile duct and enters Calot triangle, bordered by the cystic duct, common hepatic duct, and liver edge, where it gives off the cystic artery to supply the gall bladder and then continues into the substance of the right liver.

• Replaced common hepatic artery taking origin from superior mesenteric artery.
• Proximal bifurcation of hepatic artery or right and left hepatic arteries originating separately from celiac trunk.
• Replaced right hepatic artery taking origin from superior mesenteric artery
• Replaced left hepatic artery taking origin from left gastric artery.
• Accessory right hepatic artery from superior mesenteric artery
• Accessory left hepatic artery from left gastric artery
• Accessory left hepatic artery from right hepatic artery
• Right hepatic artery crossing anterior to common hepatic duct instead of posterior

Bile duct is formed by union of the cystic and common hepatic ducts near the porta hepatis. It is a 8 cm and has a diameter of about 6mm.

• Course -
• The bile duct runs downwards and backwards, first in the free margin of the lesser omentum, supraduodenal part; then behind the first part of the duodenum the retroduodenal part; and lastly behind, or embedded in, the head of the pancreas infra-duodenal part.
• Near the middle of the left side of the second part of the duodenum it comes in contact with the pancreatic duct and accompanies it through the wall of the duodenum, the intraduodenal part.
• The course of the duct through the duodenal wall is very oblique)
• Within the wall the two ducts usually unite to form the hepatopancreatic ampulla, or ampulla of Vater.
• The distal constricted end of the ampulla opens at the summit of the major duodenal papilla 8 to 10 cm distal to the pylorus. Quite often the bile and pancreatic duct may open independently on the papilla.

Relations

• A. Supraduodenal part in the free margin of lesser omentum.
• 1. Anteriorly : Liver.
• 2. Posteriorly : Portal vein and epiploic foramen.
• 3. To the left : Hepatic artery. 

• B. Retroduodenal part
• 1. Anteriorly : First part of duodenum 
• 2. Posteriorly : Inferior vena cava.
• 3. To the left : Gastroduodenal artery.

• C. Infraduodenal part
• 1. Anteriorly : A groove in the upper and lateral parts of the posterior surface of the head of the pancreas.
• 2. Posteriorly : Inferior vena cava.

D. Intraduodenal part

The supraduodenal common bile duct, from the duodenal bulb to the cystic duct, and common hepatic ducts receive their blood supply from the right hepatic and cystic arteries. 

The inferior bile duct, below the level of the duodenal bulb, receives its perfusion from tributaries of the posterosuperior pancreaticoduodenal and gastroduodenal arteries. The small branches coalesce to form the two vessels that run along the common bile duct at the 3- and 9-o’clock positions.

The middle segment : is vascularized by an axial network of a varying number of arterial anastomoses between the cranial and caudal supplies.

• 60% by the distal vessels, 38% by the cranial ones, 2% by a nonaxial supply from common hepatic artery.
• This arterial pattern predisposes the supraduodenal segment of the common bile duct to ischemic damage and resulting in strictures.

?

• Liver, bone, prostate, placenta
• Heat labile - ALP from liver
• Heat stable - from other sources

heme -heme oxidase -  biliverdin - biliverdin reductase - biliruibin - bind with albumin - goes to liver - becomes conjugated by UDP glucoronyl transferase

• Arterial supply of pancreas
• Head and neck supplied by
• - Superior pancreaticoduodenal (anterior and posterior branches) - the branch from GDA
• - Inferior pancreaticoduodenal (anterior and posterior brances) - the branch from SMA

Neck, Body and tail - Branches from Splenic artery

Dorsal Bud - appears first, forms head, neck, body and tail

Ventral Bud - Form inferior part of head and uncinate proces, at 4-8 weeks, ventral bud rotates posteriorly and gets fused with dorsal bud. The ventral bud will become the inferior part of the head and uncinate process of the gland.

• Main duct - Wirsung
• Accessory duct - Santorini

• Pancreas Divisum
• Annular pancreas
• Ectopic pancreas

• During normal organogenesis, the dorsal and ventral buds most commonly fuse to form a common duct, which enters the duodenum along with the common bile duct through the ampulla of Vater.
• Failure of the dorsal and ventral ducts to fuse during embryogenesis leads to pancreas divisum, a condition identified by a ventral pancreatic duct and common bile duct that enter the duodenum through a major papilla, whereas a dorsal pancreatic duct enters through a minor papilla that is slightly proximal.
• Because most pancreatic exocrine secretions exit through the dorsal duct, pancreas divisum can lead to a condition of partial obstruction caused by a small minor papilla, leading to chronic back pressure in the duct.
• This relative outflow obstruction has been implicated in the development of relapsing acute or chronic pancreatitis.

• Annular pancreas results from aberrant migration of the ventral pancreas bud, which leads to circumferential or near-circumferential pancreas tissue surrounding the second portion of the duodenum.
• This abnormality may be associated with other congenital defects, including Down syndrome, malrotation, intestinal atresia, and cardiac malformations.
• If symptoms of obstruction occur, surgical bypass through duodenojejunostomy is performed

Child–Pugh score is used to assess the prognosis of chronic liver disease, mainly cirrhosis. Although it was originally used to predict mortality during surgery, it is now used to determine the prognosis, as well as the required strength of treatment and the necessity of liver transplantation.



[@ ABCDE - Albumin, Bilirubin, Clotting, Distension (ascitis), Encephalopathy]

Bile salts are primarily produced in the liver and secreted to be used in the biliary tree and intestine. The primary bile salts cholic acid and chenodeoxycholic acid are produced in the liver from cholesterol and subsequently conjugated with glycine or taurine in the hepatocyte. Once secreted in the gut, the primary bile acids are modified by intestinal bacteria to form the secondary bile acids deoxycholic acid and lithocholic acid. Bile acids are reabsorbed passively into the jejunum and actively into the ileum. Thus, the bile acids reenter the portal venous system, and up to 90% of the bile acids are extracted by hepatocytes. Only a small fraction spills over into the systemic circulation because of efficient hepatic extraction, which accounts for low levels of plasma bile acids.

After hepatic extraction, bile acids are recirculated into the canaliculi and back into the biliary tree, completing the circuit. A small amount of intestinal bile acids is not absorbed by the portal system and is excreted in the stool. Thus, the active secretion of bile salts from hepatocytes into bile and from ileal enterocytes into the portal vein is the engine behind the enterohepatic circulation.