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 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.