5.1.2(b)(i)-(ii) - Liver structure, histology and functions

Gross Structure And Blood Supply

The mammalian liver is a large, lobed organ with a very high blood supply. Its position in the circulation is the key to its functions: blood from the gut reaches the liver before returning fully to the rest of the body.

Three gross structures matter most here:

The main working cells of the liver are hepatocytes. They sit next to blood spaces, so substances can move between the blood and the cells quickly.

The liver also contains bile ducts. Bile is useful as a structural clue in histology because bile duct branches occur with blood-vessel branches in portal triads. Detailed bile function is not the focus of this lesson.

Liver Lobule Histology

Under the microscope, liver tissue is organised into repeating units called lobules. A classic liver lobule is often drawn as roughly hexagonal. A central vein lies in the middle, while portal triads lie near the edges.

[DIAGRAM: liver_lobule_histology_map: Lesson 097: Liver structure, histology and functions - diagram 01; asset_slug: 097_m05_1_2_liver_structure_histology_and_functions__diagram_01; recommended_method: drawn_biology; description: Labelled 16:9 line diagram of a classic mammalian liver lobule, showing portal triads with hepatic portal vein branch, hepatic artery branch and bile duct branch, plus hepatocyte plates, sinusoids, central vein, blood-flow direction towards the central vein and bile-flow direction towards the bile duct.]

Each portal triad contains:

• a branch of the hepatic portal vein, usually the largest lumen;
• a branch of the hepatic artery, usually smaller with a thicker wall;
• a bile duct branch.

Blood from the hepatic portal vein and hepatic artery enters sinusoids. The portal supply is less oxygenated because it has already passed through respiring gut tissues; the arterial supply delivers oxygen at higher pressure. The thicker muscular wall of an arterial branch distinguishes it from the larger, thinner-walled portal venule. These are blood spaces between plates of hepatocytes. The blood flows through the sinusoids towards the central vein, giving hepatocytes direct access to dissolved nutrients, toxins and nitrogenous waste products.

Blood runs from portal vessels through sinusoids to the central vein. Bile takes a different route: hepatocytes secrete it into tiny canaliculi between adjacent cells, which carry it towards bile ducts in the portal triads. A duct has a lining of epithelial cells; it is not another blood vessel. Central veins drain towards the hepatic vein.

Storage And Detoxification

The first specified function is storage of glycogen. After a meal, blood arriving from the gut may contain a high concentration of glucose. Hepatocytes can convert glucose into glycogen and store it. Glycogen is a compact, insoluble storage carbohydrate, so storing glucose in this form avoids making the cell contents too concentrated.

Later, glycogen can be broken back down to glucose when the body needs it. This lesson is about the liver role itself; detailed hormonal control of blood glucose is taught later.

The second specified function is detoxification. Detoxification means changing a harmful substance into a less harmful substance, or into a form that can be excreted more easily. Because blood from the gut passes through the liver, hepatocytes are well placed to metabolise ingested toxins, including some drugs and alcohol.