Hepatocytes are the main cell type of the liver, representing approximately 70-85% of its mass. These specialized cells perform a vast array of functions, central to maintaining bodily balance. Characterized by high metabolic activity and unique adaptability, hepatocytes carry out many of the liver’s estimated 500 distinct functions.
Orchestrators of Metabolism
Hepatocytes play a central role in regulating the body’s metabolism, processing nutrients absorbed from digestion. They manage carbohydrate, protein, and lipid metabolism to ensure a stable supply of energy and building blocks, dynamically adjusting to the body’s fed or fasted states.
For glucose metabolism, hepatocytes are key regulators of blood sugar levels. They store excess glucose from the bloodstream as glycogen through a process called glycogenesis, with the liver capable of storing around 100g of glycogen. When blood glucose levels drop, hepatocytes can break down this stored glycogen back into glucose via glycogenolysis, releasing it into the blood. Furthermore, they can synthesize new glucose from non-carbohydrate sources like certain amino acids, lactate, or glycerol, a process known as gluconeogenesis, important during prolonged fasting.
They are also involved in lipid metabolism. Hepatocytes synthesize cholesterol, phospholipids, and a significant portion of the body’s lipoproteins, which are important for transporting fats throughout the body. Additionally, these cells break down fatty acids for energy through a process called beta-oxidation, and can convert excess carbohydrates and amino acids into fatty acids for storage.
Hepatocytes also significantly influence amino acid metabolism. Most amino acids absorbed from dietary proteins are metabolized in the liver. Hepatocytes are responsible for synthesizing many amino acids and processing excess amino acids, including deamination, which removes the amino group. This deamination process is important for converting amino acids into forms that can be used for energy or glucose production.
The Body’s Detoxification Center
Hepatocytes are central to the body’s detoxification processes, neutralizing and removing harmful substances. They act as a protective barrier, processing compounds before they can impact other organs.
These cells metabolize a wide range of drugs, alcohol, and environmental toxins through various enzymatic reactions. This detoxification often occurs in two phases: Phase I reactions modify the toxins, while Phase II reactions conjugate them with other molecules to make them more water-soluble and easier to excrete. The liver metabolizes a significant portion of circulatory ethanol, with about 80-90% processed through the alcohol dehydrogenase system.
Hepatocytes also convert toxic waste products generated by the body’s own metabolic activities. For example, ammonia, a byproduct of protein metabolism, is converted into urea through the urea cycle (also known as the ornithine cycle). Urea is a much less toxic compound that can be safely excreted by the kidneys in urine.
Another important detoxification function involves the processing of bilirubin, a yellowish waste product resulting from the breakdown of old red blood cells. Hepatocytes take up unconjugated bilirubin, convert it into a conjugated form, and then secrete it into bile for excretion from the body via stool. Impairment in this process can lead to the accumulation of bilirubin, indicating liver dysfunction.
Producers of Vital Substances
Beyond their metabolic and detoxification roles, hepatocytes produce and secrete numerous substances important for the body’s proper functioning. These include a variety of proteins, digestive fluids, and hormones.
Hepatocytes synthesize nearly all plasma proteins found in the blood, with the exception of gamma-globulins. This includes albumin, the most abundant plasma protein, important for maintaining osmotic pressure and transporting substances like fatty acids and hormones. They also produce important clotting factors, such as fibrinogen and prothrombin, important for blood coagulation and preventing excessive bleeding.
Hepatocytes are responsible for synthesizing bile, a yellowish fluid composed of water, electrolytes, bile salts, cholesterol, and bilirubin. Bile is secreted into the bile canaliculi and eventually enters the small intestine, where bile salts emulsify dietary fats, aiding in their digestion and absorption. Bile also serves as a route for the excretion of waste products like bilirubin.
Furthermore, hepatocytes contribute to the production of certain hormones and enzymes. For instance, they synthesize angiotensinogen, a hormone involved in blood pressure regulation, and thrombopoietin, which regulates platelet production. These synthetic capabilities highlight the liver’s broad influence on systemic physiology.
Storage and Release Hub
Hepatocytes serve as a storage and release center for various important nutrients, ensuring their availability when the body requires them. This function helps maintain stable internal conditions over time.
Hepatocytes store glucose in the form of glycogen, serving as the body’s readily available reserve. When blood sugar levels decrease, these cells can rapidly release stored glucose into the bloodstream to restore normal levels.
Hepatocytes also store several fat-soluble vitamins, including vitamins A, D, E, and K, as well as some water-soluble vitamins like vitamin B12. The liver holds a significant portion of the body’s total vitamin A (50-80%) and about 50% of its vitamin B12. In addition to vitamins, they store minerals such as iron and copper, releasing them as needed for various bodily processes. This storage ensures a steady supply of micronutrients, even during inconsistent dietary intake.