Liver and Hepatobiliary Tract

• The liver is an accessory gland of the digestive system, and the largest of the abdominal organs. The liver is located in the upper right quadrant of the abdomen, just under the right dome of the diaphragm.

  • The liver is even bigger in an embryo, as it also functions as a haemopoietic organ (it formed red blood cells) during its development.
  • During development, the liver grows at a faster rate than the rest of the body
• The lower border of the liver corresponds approximately to the right costal margin while nearer the midline, it crosses the epigastrium below the costal margin.

  The upper surface of the liver lies beneath the diaphragm and rises approximately to the level of the xiphisternal joint. (The liver provides a resistance to the descent of the diaphragm during breathing, particularly when the patient is lying down).
• The liver is the only human organ that has the property of self-regeneration. If a part of the liver is removed, the remaining parts can grow back to their original size and shape.
• The liver is not normally palpable, however, if the liver is enlarged and hard, then it can be felt moving downwards during deep inhalation. A soft, enlarged liver is not palpable.

  The left lobe of the liver, despite crossing into the epigastric region, is not palpable because the rectus abdominis muscle lies in front of it.
• The liver is an intraperitoneal organ attached anteriorly to the anterior abdominal wall by the falciform ligament and posteriorly to the lesser curvature of the stomach by the lesser omentum.

  Superiorly, there is an area devoid of peritoneum, under the diaphragm. This is known as the bare area of the liver. Around this bare area are reflections of peritoneum called the coronary ligament. The coronary ligament has an anterior and posterior leaflet which join at the right and left ends of the liver. These are known as the right and left triangular ligaments respectively.
• The liver has a diaphragmatic surface and a visceral surface. The liver is in direct contact with the diaphragm at the bare area.

  The visceral surface is closely related to other organs, most notable being the gall bladder, right kidney, hepatic flexure of the colon/right colic flexure, the oesophagus, stomach, and duodenum. These organs are vulnerable to the spread of infection from abscesses (swelling) of the liver.
• The liver is bounded above and below by reflections of peritoneum. These reflections form the sub-phrenic space above, and sub-hepatic (or hepatorenal) pouch below.
• The hepato-renal pouch/recess is a space of peritoneum bounded by the liver, the right kidney, the colon and the duodenum. This space is the most dependent part of the abdominal cavity in the supine position, and as such is prone to collection of pus from the abdominal cavity in patients with abdominal infections. Such infections can spread to and from the liver here.
• The sub-phrenic space is a recess on each side of the falciform ligament, between the underside of the diaphragm and the upper side of the liver. If an infection reaches this space, it may cause irritation of the diaphragm, and hence an interruption of breathing. It will also irritate the parietal layer of peritoneum and this will be felt as pain over the shoulder. This is because the peritoneum here is supplied by the phrenic nerves, which originate from C3-C5.
• The liver is composed of one type of cell called the hepatocyte, but has a great range of different functions. The liver has very little connective tissue except for a fine reticular network of fibres which make it soft and susceptible to tearing in abdominal trauma.
• Functions of the liver are:

  • produces bile to aid in the breakdown of fat in the small intestine
  • coverts excess glucose in the blood into glycogen for storage
  • produces cholesterol to help carry fats through the body
  • regulates production of fat and amino acids
  • stares iron to be used in haem
  • detoxification of the blood
  • produces blood plasma proteins
  • produces immune factors
• The liver is a gland of the alimentary tract, and one of its main roles is to produce bile. The liver produces bile continuously. The hepatocytes secrete bile acids, bile pigments and cholesterol, which are used to help digest fat in the small intestine. These substances enter the small intestine through bile ducts which drain into the duodenum.

  Bile also contains the pigments, biliverdin and bilirubin, which are breakdown products of haemoglobin. These are excreted from the liver and ultimately leave the body in the faeces. These secreted pigments are what gives faeces their brown colour.
• Glucose in high concentrations in the blood is harmful to body tissues, so the liver converts this excess glucose into glycogen and stores the glycogen (the blood enters the liver through the hepatic portal vein). The liver can convert the glycogen back into glucose when blood glucose concentration is low and needs to be increased.
• The liver produces cholesterol, which helps to carry fats through the body. It is a key component of bile.
• The liver regulates most chemical levels in the blood including fats and proteins. It produces certain proteins for blood plasma, and regulates the concentration amino acids in the blood.
• The liver breaks down old and dysfunctional red blood cells; Macrophages called Kupffer cells are involved in this process. The liver processes the haemoglobin, strips it of its iron content, and then stores this iron as ferritin. This makes it available for haemopoietic tissue to use when making new red blood cells.
• The liver removes many toxins from the blood, or neutralises them so they can be excreted in the urine or faeces safely. This includes removing drugs or poisonous substances from the blood stream, preventing them from reaching harmful levels.

  For example, the liver converts excess ammonia (which is poisonous at these levels) into urea. Urea is one of the end products of protein metabolism, and is excreted in the urine.
• The liver produces many of the plasma proteins and it helps regulate blood clotting.
• The liver forms an important part of the bodies defence mechanism by producing immune factors which lower infections. The liver also removes bacteria from the blood stream.
• The liver is divided anatomically into two lobes by the falciform ligament; these are the right and left lobes, and the right lobe being bigger than the left lobe.

  In the lower edge of the falciform ligament, there is the round ligament of the liver. This is the obliterated remnant of the umbilical vein.
• The liver can also be divided into 8 segments by its vascular divisions. These are the functional sub-divisions of the liver. These segments are known as Couinaud’s segments.
• Couinaud's segments are numbered 1-8, although number 4 is divided into 2 parts: 4a and 4b.

  Segments 1 and 4-8 belong to the anatomical right lobe (segment 1 is at the back of the liver), and segments 2 and 3 belong to the anatomical left lobe.

  The segmentation is based on the pattern of venous drainage and co-incidentally on the arrangement of the bile ductal system.
• The vascular input to the liver is from below. About 75% of blood entering the liver is from the hepatic portal vein and the remaining 25% from the proper hepatic artery. These divide into right and left branches and run horizontally within the liver substance. They give branches to each segment.

  The venous drainage of the liver is via the hepatic veins, of which there are three. The right hepatic vein runs between segments 6&7 and 5&8, the middle hepatic vein is to the right of segment 4, and the left hepatic vein runs between segment 4 and segments 2&3.
• The right, left, and middle hepatic veins are arranged vertically and all drain into the inferior vena cava posteriorly.
• The liver is almost completely covered by peritoneum except for the bare area, in which the inferior vena cava is embedded. The bare area is surrounded by the coronary ligament. Anteriorly, the left and right leaflets of the coronary ligament form the falciform ligament. Posteriorly, the left and right leaflets of the coronary ligament form the lesser omentum.

  At the free edge of the lesser omentum there is the porta hepatis. The porta hepatis is the point of entry and exit for the hepatic portal vein, the left and right hepatic arteries and the left and right hepatic bile ducts.
• • The gall bladder stores bile and is closely related to the right lobe of the liver.
  • Venous drainage of the gall bladder is directly to the liver.
  • The gall bladder sits in almost vertical alignment with the inferior vena cava.
• The round ligament is a remnant of the umbilical vein. This vein joins to the left portal vein of the liver in development. However, there is little point in the blood travelling through the poorly developed liver, so there is a bypass channel that connects the left portal vein to the left hepatic vein. This channel is known as the ductus venosus, and it is located in the base of the lesser omentum. Both the umbilical vein and the ductus venosus fibrose and close at birth. The umbilical vein becomes the round ligament, and the ductus venosus becomes the ligamentum venosum.
• Drawing vertical lines between the gall bladder and inferior vena cava, and the ligamentum venosum and the round ligament, and then connecting these two lines with a horizontal line through the hepatis porta, an 'H' is formed.

  There are two lobes that are contained within this H-division: the caudate lobe above the hepatis porta, and the quadrate lobe below.
• The caudate and quadrate lobes are part of the anatomical right lobe. However, these lobes are supplied by the left hepatic artery and drained by the left hepatic vein. They also drain via the left hepatic duct. As a result, while the caudate and quadrate lobes are part of the anatomical right lobe, they are also part of the functional left lobe of the liver.

  The right lobe is the larger because it includes the caudate and quadrate lobes.
• The division of the liver into left and right functional lobes, is roughly marked by a line through the gall bladder and the inferior vena cava. This makes for a more equal division of the liver into two halves. This also explains why the right and left hepatic arteries are of equal size.
• The hepatic portal vein receives tributaries from all three embryonic sections of the gut. The foregut is largely drained via the splenic vein, the midgut via the superior mesenteric vein and the hindgut via the inferior mesenteric vein.

  However, in cases of liver disease, this route may become blocked. This will cause varicosities of the veins, and oedema in the tissue beds. This condition is known as portal hypertension.
• If the blood is unable to get through the hepatic portal system, it will try to get into the systemic system. This is either in the superior vena cava via the azygos system, or into the inferior vena cava via the rectal veins.
• There are four possible sites of portosystemic anastomoses. These are:

  • in the lower oesophagus
  • in the anal canal
  • via a recanalized umbilical vein to the anterior abdominal wall, or via a recanalized ductus venosus to the inferior vena cava
  • via anastomoses on the posterior abdominal wall.
• In the lower oesophagus, veins dilate, and become thin-walled, bulging into the lumen of the gullet. They are known as oesophageal varices and are prone to injury.
• Varicose veins in the anal canal are similar to the varicose veins in the lower oesophagus in their basic structure, and are known as haemorrhoids, or piles. Although these are called rectal veins, the are actually in the anal canal.
• There are some anastomoses on the posterior abdominal wall involving the superior and inferior mesenteric veins, and veins of the duodenum and pancreas. These anastomose with tributaries from the posterior abdominal wall which drain to the inferior vena cava.
• The shunt through the re-canalized umbilical vein reverses the direction of flow found in the fetus: blood travels from the hepatic portal vein to the abdominal wall. Here, through multiple anastomoses, blood enters the veins of the anterior abdominal wall.

  A patient with portal hypertension will hence have enlarged veins around the umbilicus and blood will flow away from the paraumbilical region. This will either travel upwards in the thoraco-abdominal space via thoraco-epigastric veins, or downwards in the superficial epigastric veins.
• Blockage of the inferior vena cava will cause a caput medusae (swelling of veins) as blood will try to head upwards along the superficial veins in the anterior abdominal in order to reach tributaries of the superior vena cava.

  However, the direction of flow of blood in the superficial epigastric veins is reversed from that seen in portal hypertension. This is one way to differentiate inferior vena cava blockage from portal vein blockage.
• One possible cause of portal hypertension is cirrhosis of the liver. In cirrhosis, scar tissue replaces the normal healthy tissue, blocking the flow of blood through the organ and preventing it from working as it should. Hence cirrhosis is a common cause of portal blood re-direction. Cirrhosis has many causes. Examples are chronic alcoholism and hepatitis (these are the most common in the UK).
• The porta hepatis contains the hepatic portal vein, the left and right hepatic arteries, and the left and right hepatic ducts.

  The proper hepatic artery, the hepatic portal vein, and the common bile duct are found in the free border of the lesser omentum.