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The small intestine is made up of three different parts, which are the duodenum, the jejunum and the ileum. The duodenum forms the beginning of the small intestine, and it is attached to the stomach by the pyloric sphincter. Compared to the remainder of the small intestine, the duodenum is short and wide, with a distinct C shape. The anatomy of the duodenum reflects the fact that the majority of digestion occurs there.
The gross anatomy of the duodenum can be broken down into four separate areas. These are not distinct parts, but have been labeled to describe where in the duodenum different actions occur. The four parts of the duodenum are the superior part, the descending part, the horizontal part and the ascending part. Each part refers to a particular area of the C shape of the duodenum.
The superior part of the duodenum is also referred to as the first part and the duodenal bulb. It is about 2 inches (5 cm) long, with the first 3/4 of an inch (2 cm) being mobile. This is unique within the anatomy of the duodenum, as the rest of it is immobile, attached to the abdominal wall. The superior part makes up the top of the C shape and begins at the pyloric sphincter.
The descending part, or second part, is the longest section of the gross anatomy of the duodenum. It is where the digestive juices from the pancreas and liver are emptied into the duodenum. Within this part and the horizontal, or third, part of the duodenum, food is mixed with the digestive juices. The enzymes within the digestive juices break down the food into smaller molecules, which are absorbed into the bloodstream from within the ileum.
The microscopic anatomy of the duodenum shows that it contains four layers within its walls, which are the same as the rest of the small intestine. From the inside of the duodenum the four layers are the mucosa, submucosa, muscularis propria and serosa. Brunner’s glands are found within the mucosal layer of the duodenum. These glands produce mucus to lubricate the internal wall of the duodenum, as well as mineral salts to neutralize the pH of the food that has passed from the stomach into the duodenum.
The walls of the duodenum are highly folded and covered by tiny finger-like projections called villi. These projections contain smooth muscle to help move the food through the duodenum, as well as to mix it thoroughly with the digestive juices. Some digestive enzymes are produced from cells found at the tips of the villi.