Core Functions
The liver performs numerous interrelated tasks to maintain homeostasis:
- Exocrine: Continuous production of bile, essential for lipid emulsification in the intestinal lumen. Bile contains pigments (heme breakdown products from the spleen), cholesterol-derived bile acids, and other metabolites.
- Synthetic: Production of vital plasma proteins (albumins, fibrinogen, alpha- and beta-globulins, excluding antibodies), lipids, urea, and glucose.
- Protective (barrier): Chemical detoxification of ammonia, hormones, and drugs via conjugation and microsomal hydroxylation, alongside cellular phagocytosis.
- Storage: Accumulation of carbohydrates as glycogen granules and fat-soluble vitamins (A, D, E, K).
- Hematopoietic: Functions exclusively during the embryonic period.
Structural and Functional Unit
The classical structural unit is the hepatic lobule — a 5- or 6-sided prismatic structure up to 2 mm in diameter. In humans, connective tissue septa between lobules are normally poorly developed (unlike the prominent septa in pigs), causing lobules to visually merge.
- Vena centralis (central vein), which lacks a muscular coat, lies at the geometric center of the lobule. Portal triads are located in the loose stroma at the periphery of the lobules and consist of three tubular structures:
- Interlobular vein (a branch of the portal vein, v. portae).
- Interlobular artery of muscular type (a branch of the hepatic artery, a. hepatica).
- Interlobular bile duct (ductuli biliferi) — readily identified by its cuboidal epithelium with dark, round nuclei.
The lobular parenchyma itself is formed by hepatic cords (plates) — double rows of epithelial cells (hepatocytes) radiating outward from the center.
Hemodynamics and Bile Flow
Fluid circulation in the liver follows a strict countercurrent principle.
Inflow and Microcirculation: Blood flows from the periphery (the triads) toward the center of the lobule. Arterial blood (oxygen-rich) and venous blood (containing nutrients and toxins from the intestine) merge. This mixed blood flows through sinusoidal capillaries located between the hepatic cords toward the central vein. Consequently, peripheral hepatocytes receive more oxygen but are the first to encounter toxins, whereas central cells are more vulnerable to hypoxia during heart failure.
Blood Outflow: From the central vein, blood collects into sublobular veins (v. sublobularis), then into the hepatic veins and inferior vena cava. Collecting veins run independently of arteries and bile ducts.
Bile Movement: Bile is produced by hepatocytes and secreted into bile canaliculi, which begin blindly in the center of the lobule. Bile flow is directed centrifugally — from the center to the periphery, where canaliculi transition into canals of Hering and further into the interlobular bile ducts of the triads.
Cellular Microenvironment
Liver morphology is shaped not only by hepatocytes but also by a complex set of sinusoidal cells.
- Hepatocytes: Large polygonal cells, frequently binucleated and polyploid. They are polarized: the vascular surface (with microvilli) faces the blood, the lateral surfaces connect to neighbors via tight junctions, and the biliary surface forms the wall of the bile canaliculus (the canaliculus itself lacks a proprietary epithelial wall).
- Endothelial cells: Form the sinusoidal wall, featuring pores and fenestrations. The basement membrane in capillaries is discontinuous or absent to facilitate exchange.
- Space of Disse: The perisinusoidal space between the endothelium and hepatocytes, filled with blood plasma and reticular fibers.
- Kupffer cells (stellate macrophages): Embedded in the endothelium primarily at the lobular periphery, responsible for phagocytosis.
- Pit cells: Natural killer (NK) cells that destroy altered or irreversibly damaged organ cells.
- Ito cells (hepatic stellate cells / perisinusoidal lipocytes): Located in the Space of Disse, they store fat-soluble vitamins. In pathology, they become activated, synthesize collagen, and drive the fibrotic process (cirrhosis).