The small intestine is made up of three distinct parts, which are the duodenum, jejunum and ileum. The duodenum is much shorter than the other two parts, but it is where the most digestion occurs. It is only about 10 inches (25 cm) long, and is made up four parts, which are the superior part, descending part, horizontal part and ascending part.
The duodenum is connected directly to the stomach by the pyloric sphincter, which controls the movement of food from the stomach into the duodenum. It has a distinct C shape that is made up by the four parts of the duodenum, and allows it to wrap around the head of the pancreas. Once food has passed through the final part of the duodenum, it moves into the jejunum.
Although parts of the duodenum are referred to, they are not physically distinct. The parts have been labeled as a way to discuss a particular place within the C shaped organ. As well as being named by where the part falls along the C shape, the parts of the duodenum are also referred to as the first part, second part, third part and fourth part.
The superior part, or first part, starts at the pyloric sphincter. It is about 2 inches (5 cm) long, and the first 3/4 inch (2 cm) is the only part of the duodenum that is movable. Unlike the rest of the parts of the duodenum, it is not attached to the posterior abdominal wall. The first part stretches from the pyloric sphincter towards the gall bladder and the right kidney, in a direction that is slightly towards the back of the body and slightly upwards.
The second part of the duodenum, or the descending part, is about 3 - 4 inches (8 – 10 cm) long. It travels down the length of the body before making sharp turn in towards the middle of the body. Pancreatic juices and bile are emptied into this part of the duodenum through the pancreatic duct and the common bile duct. This is necessary to aid in the digestion of food.
The horizontal part, or third part, of the duodenum is about 4 inches (10 cm) long and passes across the body to the left side. It is found at about the same level as the L3 vertebra of the spine. It attaches to the ascending, or fourth, part of the duodenum. This last part is about 1 inch long (2.5 cm) and travels upwards to where it joins with the jejunum at about the level of the L2 vertebra.