When most people think of digestion, the stomach usually comes to mind. Indeed, the start of protein digestion occurs here. Yet, in reality, the chemical digestion of nutrients mainly occurs in the small intestine, another part of the digestive tract. Digestion breaks down foods and liquids into smaller molecules so they can be absorbed and used by the body for nourishment and energy.
Structure
The small intestine, also called the small bowel, is the longest portion of the digestive tract. It is a coiled organ measuring more than 6 meters long and is composed of three segments: the duodenum, jejunum and ileum. The duodenum is C-shaped and only 25 cm long. The jejunum, or mid-section, is 1 to 2 meters long, and the ileum, or final portion, measures 2 to 4 meters in length and leads into the large intestine. Most of the small intestine is suspended in a thin layer of fat, giving it flexibility and mobility. Peristalsis, or wave-like contractions of the small intestine, helps propel the food along the digestive tract.
The Duodenum
The duodenum, or the first segment of the small intestine, is the main site of digestion. Small secretions of the partially digested food mixture, or chyme, enter the small intestine from the stomach through the pyloric sphincter. When it is full, the duodenum signals the stomach to stop emptying its contents. The acid from the stomach is quickly neutralized in the alkaline environment of the duodenum.
Digestion
Bile from the gallbladder and digestive enzymes from the pancreas enter the duodenum through the sphincter of Oddi. Other enzymes involved in the digestive process come from glands in the wall of the intestine itself. The intestinal wall also releases mucus, lubricating the contents to help dissolve the digested parts. These fluids and enzymes mix with the food and aid in the digestion of carbohydrates, fats and proteins into their small molecules, including glucose, amino acids, fatty acids, vitamins, minerals, salt and water. This content is pushed forward to allow for further digestion and absorption.
The Jejunum and Ileum
The rest of the small intestine consisting of the jejunum and the ileum is responsible for the absorption of nutrients. Their inner linings contain folds and villi, or finger-like projections, which enhance absorption by increasing the surface area and trapping food. The villi absorb nutrients and transfer them into the bloodstream and liver. Waste products, including undigested parts of food, move into the large intestine.
Regulation
Hormonal and nerve regulation control digestion in the small intestine. Gastrin, secretin and cholecystokinin, simply known as CCK, are the main hormones that stimulate digestive juices and cause organ movement. Additional hormones, ghrelin and peptide YY, produced in the intestinal tract regulate appetite. Extrinsic, or outside, nerves come from the brain or spinal cord and release acetylcholine and adrenaline. Acetylcholine increases peristalsis and production of digestive juices, while adrenaline has the opposite effect. The intrinsic nerves are embedded in the walls of the intestine. They release different substances that speed up or slow down digestion.
