Anatomy of Liver
The liver is both the largest and heaviest internal organ in the human body, occupying 2.5% of total body weight and providing a host of functions necessary for maintaining normal physiological homeostasis. It weighs about 1.4 kg in an adult female and about 1.8 kg in the male .
The liver is located in the right upper quadrant of the abdomen, against the inferior surface of the diaphragm. Most part of the liver is behind the rib cage but the lower edge may be palpated along the right costal margin during inspiration. A fibrous membrane known as Glisson’s capsule covers the surface of the liver. The capsule extends to invest all but the smallest the vessels within the liver . The liver is the largest organ in the body. Its domed upper surface relates entirely to the diaphragm while its postero-inferior, or visceral, surface rests against the abdominal oesophagus, stomach, upper duodenum, hepatic flexure of the colon, right kidney and suprarenal gland, as well as carrying the gall bladder (see Fig. 6.10) . To the right, these layers fuse to form the short right triangular ligament. The falciform ligament runs to the liver from the umbilicus carrying the ligamentum teres,
Fig. 6.10 The bed of the liver. The outline of the liver is shaded green. The central bare area is unshaded  (Copyright permission from Elsevier Science Ltd)
Fig. 6.11 The surface features, ligaments and peritoneal attachments of the liver  (Copyright permission from Elsevier Science Ltd)
in its free border. The ligamentum teres passes into its fissure on the inferior surface of the liver, while the falciform ligament passes over the dome of the liver and then divides; its right limb joins the upper layer of the coronary ligament, while its left limb stretches out as the long narrow left triangular ligament, which joins the lesser omentum as this arises from the fissure for the ligamentum venosum. (see Fig. 6.11) .
The connective tissues divide the liver lobes into hexagon-shaped lobules with a portal triad at each corner and a central vein in the center of each lobule. Lobules are the functional units of the liver. Each lobule is made up of millions of hepatocytes. Branches of the hepatic artery, portal vein, and bile duct are arranged at the periphery of each lobule to form the portal triad . Hepatic cords composed with many layers of cuboidal hepatocytes radiate out from the central vein of each lobule. Sinusoids are the spaces between the plates of hepatocytes, which are enlarged capillaries through which blood from the portal triads drains to the central vein.
The liver is primarily composed of five types of specialized cells classifiable into parenchymal and nonparenchymal types. The main cell type of the liver that carries out most hepatic functions is the parenchymal cell, or hepatocyte, which makes up ~80% of hepatic volume or mass. The other 20% comprises extracellular spaces and the non-parenchymal cells including ECs, Kupffer cells, lymphocytes and Stellate cells . The liver parenchyma is arranged in hexagonal lobules as presented before. Each sinusoidal capillary which drains the portal triad toward the central vein is lined with specialized fenestrated ECs, through which the portal blood can communicate with the adjacent hepatocytes . Hepatocytes dominate the liver’s mass and are responsible for most functions of the liver including synthesis and storage functions and filtration of the portal venous blood . Kupffer cells are specialized liver macrophages with immunological and phagocytic functions . Hepatic stellate cells, which are liver fibroblasts, are typically quiescent but are integral to the pathological liver remodeling process associated with hepatic fibrosis . Bile duct epithelial cells transport bile and maintain its pH .
Figure 6.12 is a schematic illustration of the liver microstructure and the fabrication procedure of the hepatic cord-like micro-organoid by using the microfluidic system and the hydrogel microfiber .