Histology - Liver
Hepatic Portal Triad
- The liver drains bile via the common hepatic duct
- Bile enters the cystic duct for storage in the gallbladder (and is later secreted via the common bile duct into the duodenum)
- Blood is supplied to the liver by two vessels:
- The proper hepatic artery carries high oxygen blood
- The hepatic portal vein carries mixed oxygen blood from the digestive tract
- Collectively, these vessels and the common hepatic duct are referred to as the portal triad.
- Filtered blood exits the liver via hepatic veins that drain to the inferior vena cava.
Pause to consider the blood arriving at the liver via the hepatic portal vein: coming from the digestive tract, it is rich in nutrients, vitamins, minerals, and, when ingested, drugs and toxins. Recall that the liver is responsible for their metabolism, detoxification, and storage.
Classical Lobule Model
Historically, hepatic tissue is modeled as hexagon-shaped lobules surrounding central venules (aka, centralobular venules). The area of the lobule is bound by the peripheral portal triads.
- The liver tissue comprises rows, aka, plates, of hepatocytes radiating from the central venule
- Between the plates, sinusoids provide passage for blood and bile.
- Portal triads, aka, portal tracts, lie at the periphery, with the smaller vessels connecting them.
- Hepatocytes are epithelial cells with round nuclei; they are often binucleate, and, in some histological samples, glycogen granules are visible in the cytoplasm. The hepatocytes directly encircling the central venule are the limiting plate hepatocytes.
- Bile canaliculi facilitate bile passage between adjacent hepatocytes.
- Sinusoids run between plates of hepatocytes; are lined by fenestrated endothelial cells, which allow molecules, including bile, to pass from the tissue to the lumen;
- Kupffer cells, which are macrophages that remove expired red blood cells and other debris, are also present in sinusoid lining.
- Reticular fibers also line the sinusoids and form a supporting network for the liver tissues.
- Bile flows towards the peripheral portal triad, enter hepatic ducts and exit the liver.
- Arterial and venous blood flows towards the central venule; Filtered, deoxygenated blood drains to the hepatic veins and to the inferior vena cava to re-enter general circulation.