What is the first section of the small intestine called?

The first section of the small intestine is called the duodenum.

It is a short, C-shaped tube that connects the stomach to the jejunum, the middle section of the small intestine. In humans, the duodenum is about 25–38 centimeters long and is divided into four parts: superior, descending, horizontal, and ascending. Its name comes from Medieval Latin for “twelve finger-widths,” reflecting an ancient estimate of its length.

The duodenum is where much of the early chemical digestion in the small intestine begins. Acidic food leaving the stomach mixes there with bile from the liver and gallbladder, plus bicarbonate and digestive enzymes from the pancreas. Hormones such as secretin and cholecystokinin help coordinate these secretions and regulate how quickly the stomach empties.

The usual mix-up is confusing the duodenum with the ileum or jejunum. The order is duodenum first, jejunum second, and ileum third. The cecum is not part of the small intestine at all; it is the pouch-like beginning of the large intestine. The duodenum also plays an important role in nutrient sensing and is a major site for the beginning of intestinal absorption.