Accessory organs and chemical digestion
The liver, gallbladder, and pancreas never touch food directly, but their secretions are essential to digestion.
The liver
The liver is the largest internal organ and has hundreds of functions. It makes bile, processes nutrients arriving from the intestines through the hepatic portal vein, stores glycogen, vitamins, and iron, makes plasma proteins such as albumin and clotting factors, and detoxifies drugs and alcohol.
Bile and the gallbladder
Bile is made continuously by the liver and stored and concentrated in the gallbladder. After a fatty meal, the hormone cholecystokinin (CCK) makes the gallbladder contract, releasing bile into the duodenum. Bile salts emulsify fats, breaking large droplets into tiny ones so enzymes can work on them faster. Hardened deposits in the gallbladder are gallstones.
The pancreas
The pancreas secretes about 1.5 litres of pancreatic juice a day into the duodenum. It contains bicarbonate, which neutralises acidic chyme, and enzymes for all major nutrients: pancreatic amylase (starch), lipase (fats), proteases such as trypsin (proteins), and nucleases (nucleic acids). The hormone secretin triggers bicarbonate release.
Carbohydrate digestion
Starch is broken into maltose by amylases, then enzymes on the microvilli (the brush border) break disaccharides into monosaccharides: maltase, sucrase, and lactase. Lactose intolerance results from low lactase; undigested lactose is fermented by bacteria, causing bloating and gas.
Protein digestion
Proteins are broken into smaller peptides by pepsin in the stomach and by trypsin and chymotrypsin in the small intestine. Brush border enzymes finish the job, yielding amino acids that are absorbed into the blood. Proteases are released in inactive forms so they do not digest the cells that make them.
Fat digestion and absorption
Bile emulsifies fats and pancreatic lipase breaks triglycerides into fatty acids and monoglycerides. These enter intestinal cells, are reassembled into triglycerides, and packaged into particles called chylomicrons, which enter lacteals (lymph vessels in the villi) rather than blood capillaries, reaching the blood later.
Absorption of other nutrients
Glucose and amino acids are absorbed into capillaries, often by active transport coupled to sodium. Water is absorbed by osmosis, mostly in the small intestine. Vitamin B12 needs intrinsic factor, a protein made in the stomach, for absorption in the ileum; lack of intrinsic factor causes pernicious anaemia.
Liver disease
Hepatitis is inflammation of the liver, often caused by viruses (hepatitis A, B, C) or alcohol. Long-term damage causes cirrhosis, in which scar tissue replaces liver cells, impairing function. Jaundice, yellowing of the skin and eyes, occurs when the liver cannot clear bilirubin, a breakdown product of haemoglobin.
Key takeaways
The liver makes bile, processes nutrients, stores glycogen, and detoxifies. The gallbladder stores bile, released by CCK to emulsify fats. Pancreatic juice neutralises acid and digests all nutrient types. Carbohydrates become monosaccharides, proteins become amino acids, and fats become fatty acids absorbed into lacteals as chylomicrons. Hepatitis and cirrhosis impair liver function.
Flashcard
Makes bile, processes nutrients, detoxifies
Check your understanding
Why do people with gallbladder problems often have trouble digesting fatty meals?