liver

The liver

The liver is composed of a single type of parenchymal cell, the hepatocyte, it is surrounded by a dense, irregular collagenous connective tissue known as Glisson capsule, which gives rise to septa that subdivide the liver into lobes and lobules.

Function.
The liver produces bile and plasma proteins and has a myriad of other functions.

Liver lobules

1. The classic liver lobule is a hexagonal mass of tissue primarily composed of plates of hepatocytes, which radiate from the region of the central vein toward the periphery.


a. Portal areas (portal canals or portal triads)

(1) The portal areas are regions of the connective tissue between lobules that contain branches of the portal vein, hepatic artery, lymph vessel, and bile duct.

(2) They are present at alternate corners of a classic liver lobule.

b. Liver sinusoids

(1) Liver sinusoids are sinusoidal capillaries that arise at the periphery of a lobule and run between adjacent plates of hepatocytes.
(2) They receive blood from the vessels in the portal areas and deliver it to the central vein.

(3) They are lined by sinusoidal lining cells (endothelial cells) that have large discontinuities between them, display fenestrations, and lack basal laminae.

(4) They also contain phagocytic cells (Kupffer cells) derived from monocytes; these cells remove debris, old erythrocytes, and cellular fragments from the bloodstream.

c. Space of Disse

(1) The space of Disse is the subendothelial space between hepatocytes and sinusoidal lining cells.

(2) It contains the short microvilli of hepatocytes, reticular fibers (which maintain the architecture of the sinusoids).

(3) It also contains stellate fat-storing cells (Ito cells, perisinusoidal stellate cells), which preferentially store vitamin A. However, when the liver is compromised, hepatocytes release tumor growth factor _ and, in response, these fat-storing cells can divide, change their phenotype, and begin to synthesize collagen, leading to fibrosis and, if necessary, differentiate into myofibroblasts to control blood flow into the sinusoids.

(4) Function. The space of Disse functions in the exchange of material between the bloodstream and hepatocytes. Hepatocytes do not directly contact the bloodstream.

Blood and bile flow

1. Blood flow into the liver is derived from two sources and is directed from the portal triads at the periphery of each classic liver lobule toward the central vein.

a. The hepatic artery brings oxygen-rich blood from the abdominal aorta and supplies 20% to 30% of the liver’s blood.

b. The portal vein brings nutrient-rich blood from the alimentary canal and spleen; it supplies 70% to 80% of the liver’s blood.

2. Blood flow out of the liver occurs via the hepatic vein, formed by the union of numerous sublobular veins, which collect blood from the central veins.

3. Bile flow is directed toward the periphery of the classic liver lobule (in the opposite direction of blood flow). Bile is carried in a system of ducts that culminate in the left and right hepatic ducts, which leave the liver and carry bile to the gallbladder.

a. Bile canaliculi

(1) The bile canaliculi are expanded intercellular spaces between adjacent hepatocytes that form tiny canals for the initial flow of bile.

(2) They receive the liver’s exocrine secretion (bile) and carry it to the canals of Hering (bile ductules) at the very periphery of classic liver lobules.

b. Bile ducts

(1) Bile ducts are located in the portal areas.

(2) They receive bile from the canals of Hering.

(3) They enlarge and fuse to form the hepatic ducts, which leave the liver at the porta hepatis.

Hepatocytes