Liver

LIVER

The liver is the largest gland of the body, weighing 1200 -1600 g, it is wedge-shaped, and covered by a network of connective tissue (Glisson's capsule).

The liver takes up the majority of the right upper abdominal cavity and extends from the right lateral aspect of the abdomen 15 to 20 cm transversely toward the xiphoid. The weight of the adult liver varies from 1200 to 1800 g, dependent on the overall body size, and constitutes about 1.8% to 3.1% of the total body weight; however, at birth, the liver is larger compared to adjacent thoracic and abdominal viscera and constitutes about 5% to 6% of the body weight. The liver anatomically has four lobes: right, left, caudate, and quadrate. The right lobe accounts for one half to two thirds the total liver volume; however, functionally, the right and left lobes are of about equal size and are divided by a line extending from the inferior vena cava superiorly to the middle of the gallbladder fossa inferiorly. A total of eight functional segments are present, each demarcated by the vascular and biliary drainage: the lateral (segments VI and VII) and medial (segments V and VIII) divisions of the right lobe, the medial (segment IV) and lateral (segments II and III) divisions of the left lobe, and the caudate lobe (segment I), the latter a “watershed” area of the right and left lobe vasculature.

The hepatoduodenal ligament connects the liver to the superior aspect of the duodenum and supports the hilar vessels and duct structures. The transverse fissure separates anteriorly the right lobe from the caudate lobe, whereas the umbilical fissure is located to the left of the quadrate lobe, which itself is bordered on the right by the gallbladder. The peritoneal layers forming the falciform ligament, which extends between the liver and the anterior abdominal wall, separate to form the superior layer of the coronary ligament and the left triangular ligament. The ligamentum teres is located along the lower edge of the falciform ligament and contains the obliterated umbilical vein remnant. The total surface area of the liver is structured by direct continuity with the surrounding abdominal organs, ligaments, and fascia.

Situated in the upper right portion of the abdominal cavity, the liver is divided by fissures (fossae) into four lobes: the right (the largest lobe), left, quadrate and caudate lobes.

It is connected to the diaphragm and abdomainal walls by five ligaments: the membranous falciform (also separates the right and left lobes), coronary, right andleft triangular ligaments, and the fibrous round ligament (which is derived from the embryonic umbilical vein).

The liver is the only human organ that has the remarkable property of self-regeneration. If a part of the liver is removed, the remaining parts can grow back to its original size and shape.

Blood flow

Venous blood from the entire gastrointestinal tract (containing nutrients from the intestines) is brought to the liver by the hepatic portal vein. Branches of this vein pass in between the lobules and terminate in the sinusoids. Oxygenated blood is supplied in the hepatic artery.  The blood leaves the liver via a central vein in each lobule, which drains in the hepatic vein.

• Hepatic vein - one of several short veins originating within the lobes of the liver as small branches, which unite to form the hepatic veins. These lead directly to the inferior vena cava, draining blood from the liver.
• Inferior vena cava - formed by the union of the right and left common iliac veins, collects blood from parts of the body below the diaphragm and conveys it to the right atrium of the heart.
• Hepatic artery - a blood vessel which supplies the liver with oxygenated blood. It supplies 20% of the liver's blood.
• Hepatic portal vein - a blood vessel which drains venous blood into the liver from the entire gastrointestinal tract. It supplies the remaining 80% of the liver's blood.

Microscopic structure

• Lobules - hexagonally shaped functional units of the liver, made up of liver cells arranged in one-cell-thick platelike layers that radiate from the central vein to the edge of the lobule.

• Hepatocytes - the liver cells.
• Sinusoids - small blood vessels between the radiating rows of hepatocytes. They receive oxygen-rich blood from the hepatic artery and nutrients from the intestines via the portal vein. Oxygen and nutrients diffuse through the capillary walls into the liver cells.
• Portal area - situated at he corner of each lobule, it is a complex composed of branches of the hepatic portal vein, hepatic artery, bile duct, and nerve.
• Bile ducts - any of the ducts that convey bile from the liver. Bile is drained from the liver cells by many small ducts that unite to form the main bile duct of the liver, the hepatic duct. This joins the cystic duct, which leads from the gallblader, to form the common bile duct, which drains into the duodenum.
• Central vein - a blood vessel in the middle of each lobule which receives blood from the hepatic portal vein and hepatic artery via the sinusoids and drains the blood into the hepatic vein.