肝臓生理学

The liver maintains blood glucose between 70–110 mg/dL through two key mechanisms. When glucose is low, the liver breaks down glycogen stores and performs gluconeogenesis, synthesizing glucose from other carbohydrates or amino acids. When glucose is high, the liver removes glucose from the bloodstream for storage as glycogen or conversion to lipids, responding to insulin signals from the pancreas.

Hepatocytes break down amino acids through deamination, removing the amino group to produce ammonia and energy. Since ammonia is toxic, the liver immediately converts it into less toxic urea, which is then excreted in urine. Amino acids can also be converted into carbohydrates or fats for energy storage and metabolic use.

The liver synthesizes bile, which emulsifies dietary fats to aid mechanical and chemical digestion in the small intestine and absorption. Hepatocytes also store triglycerides, synthesize lipoproteins for lipid transport, and produce cholesterol. Additionally, the liver oxidizes fatty acids to generate ATP, extracting energy from stored lipids.

The liver eliminates drugs like penicillin and water-soluble toxins by excreting them in bile. It absorbs lipid-soluble toxins from the diet and stores them in lipid deposits, preventing cellular disruption. The liver also breaks down other bloodborne toxins and chemically modifies hormones to regulate their activity and maintain homeostasis.

Kupffer cells, specialized liver cells, phagocytize aged red blood cells, white blood cells, and some bacteria. These cells recycle iron and other hemoglobin components for reuse. The liver also excretes bilirubin, a byproduct of red blood cell breakdown, in bile, regulating blood composition and supporting immune function.

The liver synthesizes insulin-like growth factor 1 (IGF-1) in response to growth hormone, promoting cell growth and repair. It also secretes angiotensinogen, a precursor to angiotensin for blood pressure regulation, and produces thrombopoietin to stimulate platelet production. Additionally, the liver metabolizes steroid hormones and converts vitamin D into its primary circulating form for calcium regulation.

The liver synthesizes and secretes bile into the duodenum, where it emulsifies dietary fats to facilitate absorption of nutrients. Bile contains cholesterol synthesized by hepatocytes and enables the absorption of fat-soluble vitamins and lipids. This bile production is essential for efficient nutrient absorption and overall digestive function.