The liver, an organ with many functions, is organized into smaller units known as hepatic lobules. These lobules are the fundamental functional building blocks of liver tissue, processing blood and performing various metabolic tasks. They play a significant role in maintaining the body’s internal balance by filtering substances, processing nutrients, and producing essential compounds.
Basic Structural Components
Each hepatic lobule is typically shaped like a hexagon in cross-section. At the center of this hexagonal structure lies a central vein. Around the perimeter of this hexagon, at each corner, are structures known as portal triads.
A portal triad is a cluster of three main components: a branch of the hepatic artery, a branch of the hepatic portal vein, and a small bile duct. The hepatic artery branch delivers oxygen-rich blood to the liver cells, while the portal vein branch carries nutrient-rich blood, often low in oxygen, from the digestive tract. The bile duct collects bile produced by liver cells. Radiating outwards from the central vein towards the portal triads are cords of liver cells, called hepatocytes.
Microscopic Architecture and Blood Flow
Hepatocytes, the main liver cells, are organized into plates that extend like spokes from the central vein towards the periphery of the lobule. Between these plates of hepatocytes are specialized capillaries called hepatic sinusoids.
Both oxygenated blood from the hepatic artery and nutrient-rich blood from the hepatic portal vein enter the sinusoids from the portal triads located at the lobule’s edges. This mixed blood then flows inward through the sinusoids, past the hepatocytes, and eventually drains into the central vein. From the central vein, blood collects into larger hepatic veins, which then return it to the general circulation.
Bile, a digestive fluid produced by hepatocytes, follows an opposite path. It is secreted into tiny channels called bile canaliculi, which are situated between adjacent hepatocytes. These canaliculi form a network that drains bile from the center of the lobule outwards, towards the bile ducts within the portal triads.
Key Cellular Roles and Metabolic Functions
Beyond hepatocytes, the hepatic lobule contains other cell types, each contributing to the liver’s diverse functions. Liver sinusoidal endothelial cells (LSECs) line the hepatic sinusoids, forming a porous barrier with small openings called fenestrae. These fenestrae allow for direct exchange of substances between the blood plasma and the hepatocytes, facilitating nutrient uptake and waste removal. LSECs also help regulate blood flow within the sinusoids and act as a filter for larger particles.
Kupffer cells, which are specialized macrophages, are found within the sinusoids and are attached to the LSECs. They act as the liver’s primary immune cells, engulfing foreign particles, bacteria, and cellular debris that enter the liver from the digestive tract. These cells are also involved in recycling hemoglobin from old red blood cells.
Hepatic stellate cells reside in the space between the LSECs and hepatocytes. In a healthy liver, these cells primarily store vitamin A; however, when the liver is injured, they can become activated and contribute to the formation of scar tissue.
The hepatic lobule performs an array of metabolic functions. Hepatocytes are central to nutrient processing, converting excess glucose into glycogen for storage and releasing glucose when needed to maintain blood sugar levels. They are also involved in protein synthesis, producing components like albumin and clotting factors.
The lobule is a primary site for detoxification, converting harmful substances into less toxic forms that can be excreted from the body. This detoxification process includes metabolizing drugs and clearing waste products like ammonia, converting it into urea for elimination. Different metabolic activities are concentrated in specific regions of the lobule, a concept known as metabolic zonation, influenced by gradients of oxygen and nutrients across the lobule.