Food from the stomach is allowed into the duodenum through the pylorus by a muscle called the pyloric sphincter. Digestion The<unk>  is where most chemical digestion takes place. Many of the digestive enzymes that act in the<unk>  are secreted by the pancreas and liver and enter the<unk>  via the pancreatic duct. Pancreatic enzymes and bile from the gallbladder enter the<unk>  in response to the Hormone cholecystokinin, which is produced in the response to the presence of nutrients. Secretin, another hormone produced in the<unk> , causes additional effects on the pancreas, where it promotes the release of bicarbonate into the duodenum in order to neutralize the potentially harmful acid coming from the stomach. The three major classes of nutrients that undergo digestion are proteins, lipids (fats) and carbohydrates: * Proteins are degraded into<unk>  peptides and amino acids before absorption. Chemical breakdown begins in the stomach and continues in the<unk> . Proteolytic enzymes, including trypsin and chymotrypsin, are secreted by the pancreas and cleave proteins into<unk> er peptides. Carboxypeptidase, which is a pancreatic brush border enzyme, splits one amino acid at a time. Aminopeptidase and dipeptidase free the end amino acid products. * Lipids (fats) are degraded into fatty acids and glycerol. Pancreatic lipase breaks down triglycerides into free fatty acids and monoglycerides. Pancreatic lipase works with the help of the salts from the bile secreted by the liver and stored in the gall bladder. Bile salts attach to triglycerides to help emulsify them, which aids access by pancreatic lipase. This occurs because the lipase is water-soluble but the fatty triglycerides are hydrophobic and tend to orient towards each other and away from the watery<unk> l surroundings. The bile salts emulsify the triglycerides in the watery surroundings until the lipase can break them into the<unk> er components that are able to enter the villi for absorption. * Some carbohydrates are degraded into<unk> mple sugars, or monosaccharides (e.g., glucose). Pancreatic amylase breaks down some carbohydrates (notably starch) into oligosaccharides. Other carbohydrates pass undigested into the large<unk>  and further handling by<unk> l bacteria. Brush border enzymes take over from there. The most important brush border enzymes are dextrinase and glucoamylase, which further break down oligosaccharides. Other brush border enzymes are maltase, sucrase and lactase. Lactase is absent in some adult humans and, for them, lactose (a disaccharide), as well as most polysaccharides, is not digested in the<unk> . Some carbohydrates, such as cellulose, are not digested at all, despite being made of multiple glucose units. This is because the cellulose is made out of beta-glucose, making the inter-monosaccharidal bindings different from the ones present in starch, which consists of alpha-glucose. Humans lack the enzyme for splitting the beta-glucose-bonds, something reserved for herbivores and bacteria from the large<unk> . Absorption Digested food is now able to pass into the blood vessels in the wall of the<unk>  through either diffusion or active transport. The<unk>  is the<unk> te where most of the nutrients from ingested food are absorbed. The inner wall, or mucosa, of the<unk> , is lined with<unk> mple columnar epithelial tissue. Structurally, the mucosa is covered in wrinkles or flaps called circular folds, which are considered permanent features in the mucosa. They are distinct from rugae which are considered non-permanent or temporary allowing for distention and contraction. From the circular folds project microscopic finger-like pieces of tissue called villi (Latin for "shaggy hair"). The individual epithelial cells also have finger-like projections known as microvilli. The functions of the circular folds, the villi, and the microvilli are to increase the amount of surface area available for the absorption of nutrients, and to limit the loss of said nutrients to<unk> l fauna. Each villus has a network of capillaries and fine lymphatic vessels called lacteals close to its surface. The epithelial cells of the villi transport nutrients from the lumen of the<unk>  into these capillaries (amino acids and carbohydrates) and lacteals (lipids). The absorbed substances are transported via the blood vessels to different organs of the body where they are used to build complex substances such as the proteins required by our body. The material that remains undigested and unabsorbed passes into the large<unk> . Absorption of the majority of nutrients takes place in the jejunum, with the following notable exceptions: * Iron is absorbed in the duodenum. * Folate (Vitamin B9) is absorbed in the duodenum and jejunum. * Vitamin B12 and bile salts are absorbed in the terminal<unk> . * Water is absorbed by osmosis and lipids by passive diffusion throughout the<unk> . * Sodium bicarbonate is absorbed by active transport and glucose and amino acid co-transport * Fructose is absorbed by facilitated diffusion. Immunological The<unk>  supports the body's immune system. The presence of gut flora appears to contribute positively to the host's immune system. Peyer's patches, located within the<unk>  of the<unk> , are an important part of the digestive tract's local immune system. They are part of the lymphatic system, and provide a<unk> te for antigens from potentially harmful bacteria or other microorganisms in the digestive tract to be sampled, and subsequently presented to the immune system. https://www.medicome.live/2021/08/definition-of-liver.html liver disease
Small intestine
Large intestine
Stomach
Liver
Gastrointestinal tract
Gastric mucosa
Lung
Kidney