Hepatic tissue is the functional tissue of the liver, a large, reddish-brown organ in the upper right abdomen that weighs about three pounds in an average adult. The liver is one of the body’s most complex organs, and its tissue is responsible for processes that influence nearly every system in the body. The tissue’s cellular makeup and structural organization allow it to perform its wide-ranging functions.
Cellular Composition of Hepatic Tissue
The majority of hepatic tissue, about 70-80% of its mass, is composed of cells called hepatocytes. These are the primary functional units of the liver, carrying out most of its metabolic and synthetic tasks. Hepatocytes are rich in cellular machinery like mitochondria for energy and endoplasmic reticulum for synthesis, reflecting their high metabolic activity. Their appearance can change with the body’s nutritional state due to the storage of glycogen and lipids.
Supporting the hepatocytes is a group of non-parenchymal cells that are important to the liver’s overall function. Liver sinusoidal endothelial cells (LSECs) form the lining of the liver’s specialized capillaries and are a large component of this cell population. Kupffer cells act as the resident immune cells, functioning as macrophages that filter the blood and respond to pathogens or cellular debris.
Another cell type is the hepatic stellate cell, also known as an Ito cell. In a healthy liver, these cells are responsible for storing vitamin A. Stellate cells reside in the space of Disse, a narrow area between the endothelial cells and hepatocytes. They also help produce the extracellular matrix, which provides structural support.
Structural Organization and Blood Flow
The liver’s cellular components are organized into functional units called hepatic lobules, which are microscopic, hexagonal structures. At the center of each lobule is a central vein that collects filtered blood. Plates of hepatocytes radiate outwards from this central vein in cords that are one cell thick.
At each of the six corners of the lobule is a portal triad. This structure consists of a branch of the hepatic artery, a branch of the hepatic portal vein, and a small bile ductule. This arrangement gives the liver a dual blood supply, allowing it to process blood from two different sources. The hepatic artery delivers oxygen-rich blood from the heart, while the portal vein carries nutrient-rich blood from the digestive organs.
Blood from the hepatic artery and portal vein enters the lobule and mixes as it flows through wide capillaries called sinusoids. These sinusoids run between the plates of hepatocytes, allowing direct contact between the blood and the liver cells. This arrangement facilitates the transfer of substances from the blood into the hepatocytes for processing. After being filtered, the blood collects in the central vein, which drains into the hepatic vein and returns to systemic circulation.
Primary Functions of Hepatic Tissue
Hepatic tissue performs several functions, including metabolism, detoxification, synthesis, and storage.
- Metabolism: The tissue regulates carbohydrate, fat, and protein metabolism. It manages blood glucose by converting excess glucose into glycogen for storage and breaking it down when needed. It also breaks down fats for energy, produces cholesterol, and converts amino acids into usable forms.
- Detoxification: Hepatocytes break down and modify harmful substances from the blood. This includes converting metabolic byproducts like ammonia into urea for excretion and metabolizing drugs and environmental toxins into forms that can be eliminated.
- Synthesis: The tissue manufactures many proteins found in blood plasma, such as albumin, which maintains fluid balance in the circulatory system. It also synthesizes most of the body’s clotting factors required for blood coagulation.
- Production and Storage: The liver produces 800 to 1,000 milliliters of bile daily to aid in fat digestion. It also serves as a storage site for glycogen, vitamins A, D, and B12, and minerals like iron and copper, releasing them as needed.
Tissue Response to Injury and Regeneration
When hepatic tissue is injured by toxins or viruses, it initiates an inflammatory response. This process recruits immune cells to the site of damage to clear away dead or damaged hepatocytes. If an injury is chronic, this healing response can lead to fibrosis. During fibrosis, hepatic stellate cells become activated and produce excessive amounts of extracellular matrix proteins like collagen.
This excessive deposition of matrix proteins leads to scar tissue formation within the liver. The accumulation of scar tissue can alter the liver’s architecture and impede its function, a state known as cirrhosis. In cirrhosis, the organized structure of the lobules is disrupted, which impairs blood flow and hepatocyte function. This advanced scarring diminishes the liver’s capacity to repair itself.
Despite its susceptibility to fibrosis, the liver has a strong capacity for regeneration. Following an acute injury or surgical removal of a portion of the organ, healthy hepatic tissue can regrow to its original mass. This process involves the proliferation of remaining mature hepatocytes, which divide to replenish the lost tissue. The ability to regenerate without significant scarring after acute damage is a distinctive feature of the liver.