This is how the food flows down from esophagus (gullet) into the stomach. The stomach makes acid which is not essential, but helps to digest food. After being mixed in the stomach, food passes into the duodenum (the first part of the small intestine). In the duodenum and the rest of the small intestine, food mixes with enzymes (chemicals). The enzymes come from the pancreas and from cells lining the intestine. The enzymes break down (digest) the food which is absorbed into the body.
The small intestine is made up of three segments or parts such as Duodenum, Jejunum, and Ileum. The small intestine is 22-foot long muscular tube that breaks down food using enzymes released by the pancreas and bile from the liver. Peristalsis also is at work in this organ, moving food through and mixing it with digestive secretions from the pancreas and liver. The duodenum is largely responsible for the continuous breaking-down process, with the jejunum and ileum mainly responsible for absorption of nutrients into the bloodstream.
The stomach releases food into the duodenum, which is the first segment of the small intestine. Food enters the duodenum through the pyloric sphincter in amounts that the small intestine can digest. When full, the duodenum signals the stomach to stop emptying.
The duodenum receives pancreatic enzymes from the pancreas and bile from the liver and gallbladder. These fluids, which enter the duodenum through an opening called the sphincter of Oddi, are important in aiding digestion and absorption. Peristalsis also aids or helps in digestion and absorption by churning up food and mixing it with intestinal secretions.
The first few inches of the duodenal lining are smooth, but the rest of the lining has folds, small projections (villi), and even smaller projections (microvilli). These villi and microvilli increase the surface area of the duodenal lining, allowing for greater absorption of nutrients.
The rest of the small intestine, located below the duodenum, consists of the jejunum and the ileum. These parts of the small intestine are largely responsible for the absorption of fats and other nutrients. Churning movements facilitate absorption. Absorption is also enhanced by the vast surface area made up of folds, villi, and microvilli.
The intestinal wall is richly supplied with blood vessels that carry the absorbed nutrients to the liver through the portal vein. The intestinal wall releases mucus, which lubricates the intestinal contents, and water, which helps dissolve the digested fragments. Small amounts of enzymes that digest proteins, sugars, and fats are also released.
In duodenum, food is diluted with pancreatic enzymes and bile, which decrease stomach acidity. The contents continue to travel through the lower small intestine, becoming more liquid as they mix with water, mucus, bile, and pancreatic enzymes. Ultimately, the small intestine absorbs most of the nutrients and all but about one liter of fluid before emptying into the large intestine.