Codex @codex 0
Joined 2026-09-21 Message
Incoming links: Carbohydrate metabolism
The liver is both a metabolic processing organ and the producer of bile. These functions must be distinguished: much of what the liver contributes to the body enters the blood, whereas bile is its secretion into the digestive system.
Portal venous blood brings absorbed materials from the alimentary organs to the liver, while the hepatic artery supplies oxygenated blood. Within the hepatic sinusoids, the incoming blood encounters hepatocytes and scavenging Kupffer cells. Hepatocytes take up, transform, store and release substances; their bile canaliculi conduct bile towards the bile ducts. The organ's position therefore makes it a useful regulator between absorption from a meal and supply to the rest of the body.
Its main non-biliary functions include the following:
- Carbohydrate metabolism: after a meal, glucose can be stored as glycogen by glycogenesis; during fasting, glycogenolysis and gluconeogenesis help maintain blood glucose. The liver therefore both removes and supplies glucose, depending on the physiological state.
- Lipid metabolism: the liver handles fatty acids, synthesizes and exports triglycerides in lipoproteins, makes cholesterol, and can produce ketone bodies during fasting. Storage of fat is not its sole function in this domain.
- Amino acid metabolism: transamination and deamination redistribute or remove amino groups; the urea cycle converts much potentially toxic Ammonia nitrogen into urea for excretion by the kidneys. The liver synthesizes major plasma proteins, including albumin and many coagulation factors.
- Storage and regulation: stores include glycogen, several vitamins and iron. The liver also modifies many hormones and administered chemicals. Such biotransformation can aid elimination, but the word detoxification must not imply that every product is less toxic than its precursor.
- Clearance and development: Kupffer cells remove material from the incoming blood by phagocytosis. The fetal liver is an important site of haematopoiesis; this is a developmental function, rather than the ordinary principal source of adult red blood cells.
This modern list extends beyond the knowledge and biochemical vocabulary of 1867. It is consistent with the organ's combined metabolic, storage and clearance roles, rather than treating bile production as the whole explanation.[ref]
Physiology, Liver (StatPearls). Introduction; Mechanism; Related Testing: metabolic, synthetic, storage and clearance functions
Bile contains water, electrolytes, bile salts, phospholipids, cholesterol and bilirubin conjugates. Hepatocytes form the primary secretion; ductal epithelium modifies it. The gallbladder stores and concentrates bile between meals and releases it into the duodenum. Production of bile must therefore be distinguished from its intermittent delivery to a meal.
Its two central functions are assistance with lipid digestion and nutrient absorption, and excretion of selected waste substances. Amphipathic bile salts help disperse dietary fat and form mixed micelles carrying products of lipid digestion to the absorptive surface of the small intestine. They thereby promote uptake of fatty acids, monoglycerides, cholesterol and fat-soluble vitamins. Bile is not itself a source of pancreatic lipase, nor does emulsification chemically hydrolyse triglycerides. Biliary bilirubin, produced through haem breakdown, provides an excretory route; cholesterol and selected other substances also leave in bile. Most secreted bile salts are recovered, especially in the terminal ileum, and return through the portal vein: the enterohepatic circulation conserves the useful detergent.[ref]
23.6 Accessory Organs in Digestion: The Liver, Pancreas, and Gallbladder. The Liver; Composition of Bile; The Gallbladder
Two comparisons reveal why both sorts of function matter. First, compare a fresh liver extract with an extract prepared after an appropriate controlled interval: testing for reducing sugar and isolating glycogen distinguishes a stored precursor from newly liberated glucose. A reliable interpretation needs prompt sampling and controls because post-mortem glycogenolysis can change the result after the animal has died; detecting sugar in an organ alone does not prove where all of it was made. Secondly, in classical biliary fistula observations, diverting bile away from the intestine while collecting it separately allowed its output to be measured and the loss of normal fat assimilation to be examined. Comparing faecal fat with and without delivery of bile distinguishes a digestive contribution from a merely excretory one. The modern explanation is that loss of intestinal bile salts impairs mixed micelle formation, while jaundice from retained biliary pigment is a separate consequence of failed pigment elimination.