Liver

Main functions

The liver performs many interrelated tasks to ensure homeostasis:

• Exocrine: continuous production of bile, necessary for emulsifying fats in the intestinal lumen. Bile contains pigments (heme breakdown products from the spleen), bile acids synthesized from cholesterol, and other metabolites.
• Synthetic: production of essential plasma proteins (albumins, fibrinogen, alpha and beta globulins, except antibodies), lipids, urea, and glucose.
• Protective (barrier): chemical detoxification of ammonia, hormones, and drugs via conjugation and microsomal hydroxylation, as well as 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 structure in the shape of a pentagonal or hexagonal prism up to 2 mm in diameter. In humans, connective tissue layers between them are normally poorly developed (unlike the pronounced septa in pigs), and the lobules visually merge.

In the geometric center of the lobule lies the muscle-free vena centralis (central vein). At the corners of the lobules within the loose stroma run portal triads, consisting of three tubular structures:

1. Interlobular vein (branch of the portal vein, v. portae).
2. Interlobular artery of the muscular type (branch of a. hepatica).
3. Interlobular bile duct (ductuli biliferi)—easily recognized by its cuboidal epithelium with dark round nuclei.

The lobule parenchyma itself is formed by hepatic cords—double rows of epithelial cells (hepatocytes) radiating from the center.

Hemodynamics and bile flow

Fluid circulation in the liver follows a strict rule of opposing flows.

Inflow and microcirculation system: Blood moves from the periphery (from the triads) into the lobule. Arterial (oxygen-rich) and venous (containing nutrients and toxins from the intestine) blood merge. This mixed blood flows through sinusoidal capillaries located between the hepatic cords toward the central vein. Due to this gradient, hepatocytes at the periphery receive more oxygen but are the first to encounter toxins, whereas central cells are more susceptible to hypoxia during heart failure.

Blood outflow: From the central vein, blood collects into sublobular veins (v. sublobularis), then into the hepatic trunks 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 portal bile ducts.

Cellular microenvironment

Liver morphology is determined not only by hepatocytes, but also by a complex of sinusoidal cells.

• Hepatocytes: Large polygonal cells, often binucleated and polyploid. They are polarized: the vascular side (with microvilli) faces the blood, the contact side is connected to neighbors by tight junctions, and the biliary side forms the wall of the bile canaliculus (the bile canaliculus itself lacks its own epithelial wall).
• Endotheliocytes: Form the wall of the sinusoid, featuring pores and fenestrae. 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 plasma and reticular fibers.
• Kupffer cells (stellate macrophages): Embedded in the endothelium predominantly at the periphery of the lobules, responsible for phagocytosis.
• Pit cells: Natural killer cells (NK lymphocytes) that destroy altered or irreversibly damaged cells of the organ.
• Ito cells (perisinusoidal lipocytes): Located in the Space of Disse, they store fat-soluble vitamins. In pathology, they become activated, synthesize collagen, and initiate the sclerosing process (cirrhosis).

Briefly: answers to topical questions

Class and exam questions

What else to study on this topic

• Alternative functional units: portal lobule and hepatic acinus
• Hepatocyte ultrastructure: distribution of smooth and rough ER
• Mechanisms of fibrosis and cirrhosis formation upon Ito cell activation
• Biochemistry of bile formation: synthesis and conjugation of bile acids
• Extrahepatic biliary tracts and histology of the gallbladder

Other topics in the section "Digestive System: Large Intestine, Liver, and Pancreas"

Prepared by WhiteCoat Pro faculty in accordance with the medical school curriculum. For educational purposes only.