Why the hepatic blood carried to liver before it enters systemic circulation?
The hepatic portal system is a unique vascular system that ensures that blood from the gastrointestinal tract, spleen, and pancreas passes through the liver before it enters the systemic circulation. This arrangement serves several important functions:
1. Nutrient Processing and Metabolism: The liver plays a central role in processing and metabolizing nutrients absorbed from the gastrointestinal tract. When blood from the gut enters the liver via the hepatic portal vein, it brings with it various nutrients, such as glucose, amino acids, vitamins, and minerals. The liver cells (hepatocytes) take up these nutrients and perform various metabolic processes, including:
- Glycogenesis and Gluconeogenesis: The liver converts excess glucose into glycogen for storage or breaks down stored glycogen into glucose when needed (glycogenolysis). It also converts other nutrients, such as amino acids, into glucose through a process called gluconeogenesis.
- Protein Synthesis: The liver synthesizes various proteins essential for bodily functions, including plasma proteins (e.g., albumin) and enzymes.
- Lipid Metabolism: The liver plays a key role in lipid metabolism. It synthesizes cholesterol, phospholipids, and triglycerides. It also breaks down fatty acids and produces ketones during fasting or starvation.
- Bile Production: The liver produces bile, which helps in the emulsification and digestion of fats in the small intestine. Bile components are synthesized in the liver and secreted into the bile canaliculi, which converge to form the hepatic ducts.
2. Detoxification and Waste Removal: The liver serves as a detoxification center for the body. It processes and removes various toxins, metabolic waste products, and harmful substances from the blood. These include:
- Drug Metabolism: The liver metabolizes drugs and medications, converting them into inactive forms that can be excreted.
- Ammonia Detoxification: The liver converts toxic ammonia, produced by the breakdown of proteins, into urea, which is less harmful and excreted in the urine.
- Removal of Bilirubin: Bilirubin, a yellow pigment produced from the breakdown of heme (the iron-containing component of hemoglobin), is transported to the liver and excreted as a component of bile.
3. Storage: The liver acts as a storage organ for various nutrients and substances, including:
- Glycogen: The liver stores excess glucose as glycogen for later use when blood glucose levels drop.
- Vitamins and Minerals: The liver stores vitamins (e.g., vitamins A, D, and B12) and minerals (e.g., iron and copper) for future use.
- Iron Recycling: The liver plays a role in iron recycling by storing excess iron and releasing it when needed for hemoglobin synthesis.
By ensuring that blood from the gut passes through the liver before entering the systemic circulation, the hepatic portal system allows for efficient nutrient processing, metabolism, detoxification, and storage, contributing to overall homeostasis and well-being.