17.3:
Accessory Organs
Accessory digestive organs secrete enzymes and other substances into the primary digestive tract to facilitate digestion. The first of these are the salivary glands in the mouth that secrete amylase. This enzyme starts the breakdown of starches and other carbohydrates.
The remaining organs secrete digestive enzymes into the duodenum where most digestion occurs. The liver and gallbladder work in tandem to distribute bile, a substance produced by the liver but stored and released by the gallbladder. Bile is a concoction of water, bile salts, cholesterol, and bilirubin, a waste product of hemoglobin breakdown.
Bile salts are both hydrophobic and hydrophilic, which helps to emulsify large globules of fat into smaller ones.
Lipase from the pancreas can then hydrolyze the dietary fats into fatty acids and glycerides. Additional pancreatic enzymes, trypsin and chymotrypsin, break down proteins into peptides, which are catabolized into amino acids by carboxypeptidase. Amylase, which digests carbohydrates, is also released by the pancreas.
Lastly, the pancreas secretes bicarbonate into the duodenum to neutralize the chyme, a mixture of food and stomach acid, and protect the intestines from the acidity.
17.3:
Accessory Organs
Accessory organs are those that participate in the digestion of food but do not come into direct contact with it like the mouth, stomach, or intestine do. Accessory organs secrete enzymes into the digestive tract to facilitate the breakdown of food.
Salivary Glands
Salivary glands secrete saliva—a complex liquid containing in part water, mucus, and amylase. Amylase is a digestive enzyme that begins breaking down starches and other carbohydrates even before they reach the stomach.
Liver, Gallbladder, and Pancreas
The liver, gallbladder, and pancreas are the other accessory organs involved in digestion. All three secrete enzymes into the duodenum of the small intestine via a series of channels called the biliary tree. The liver and gallbladder work together to release bile into the duodenum. The liver produces bile, but it is stored in the gallbladder for secretion when needed.
Bile is a mixture of water, bile salts, cholesterol, and bilirubin. Bile salts contain hydrophobic areas and hydrophilic areas which allows it to engage with both fats and water. Thus it breaks down large fat globules into smaller ones—a process called emulsification. Bilirubin is a waste product that accumulates when the liver breaks hemoglobin from red blood cells. The globin is recycled and the heme, which contains iron, is excreted in the bile. The presence of bilirubin is what gives feces its brown color.
Gallstones are bile aggregations that form in the gallbladder or connecting bile ducts. Cholesterol stones are made from mostly the cholesterol in bile. Pigment stones are formed from bilirubin. Most stones form without notice; however, if a stone becomes lodged in the bile duct severe symptoms can emerge such as inflammation of the gallbladder or pancreas, fever, vomiting, and jaundice. A cholecystectomy—the removal of the gallbladder—is the primary treatment and there are minimal long-term side effects of its removal.
The pancreas secretes several enzymes. Lipase breaks down the smaller fat globules into fatty acids and triglycerides. Trypsin and chymotrypsin catabolize proteins into peptides. Carboxypeptidase then breaks those peptides into individual amino acids. The pancreas also releases amylase to digest any remaining carbohydrates.
Suggested Reading
Njeze, Gabriel E. “Gallstones.” Nigerian Journal of Surgery : Official Publication of the Nigerian Surgical Research Society 19, no. 2 (2013): 49–55. [Source]
Ianiro, Gianluca, Silvia Pecere, Valentina Giorgio, Antonio Gasbarrini, and Giovanni Cammarota. “Digestive Enzyme Supplementation in Gastrointestinal Diseases.” Current Drug Metabolism 17, no. 2 (February 2016): 187–93. [Source]