The jejunum is the middle section of the small intestine, a part of the digestive system responsible for breaking down food and absorbing nutrients. It is located between the first section, the duodenum, and the final section, the ileum. Making up about two-fifths of the small intestine’s length, its primary role is the absorption of most nutrients from partially digested food.
The Jejunum’s Specialized Structure for Absorption
The jejunum’s effectiveness stems from its unique internal architecture, which is designed to maximize the surface area available for nutrient absorption. Its walls are lined with circular folds, known as plicae circulares, which are ridges that increase the surface area and help slow the passage of food. These folds are covered in millions of finger-like projections called villi, each about one millimeter long. This structural feature multiplies the contact area between the intestinal lining and the digested food.
Further magnifying this surface area are microvilli, which are microscopic projections on the surface of the individual cells lining the villi. This “brush border” appearance at a microscopic level creates a large absorptive surface. To support this function, the jejunum has a rich blood supply. A dense network of blood vessels, called capillaries, runs through each villus, ready to transport absorbed nutrients away from the intestine and into the general circulation for distribution throughout the body.
Nutrient Absorption: The Jejunum’s Main Task
After being partially broken down in the stomach and duodenum, food, now in a semi-liquid form called chyme, enters the jejunum. Here, the simplest forms of carbohydrates, such as glucose, are transported from the intestine into the bloodstream.
Proteins, which have been broken down into their constituent amino acids and small peptides, are also absorbed in this section. Fats, in the form of fatty acids and monoglycerides, are absorbed as well. Beyond these energy-providing nutrients, the jejunum is also the primary site for the absorption of most vitamins and minerals from digested food.
Completing Digestion in the Jejunum
While significant digestion occurs before food reaches the jejunum, this section plays a part in the final stages of chemical digestion. The process is carried out by enzymes located on the surface of the intestinal lining cells. These enzymes are embedded in the microvilli, forming what is known as the brush border.
These brush border enzymes finalize the breakdown of certain nutrients. For example, disaccharidases act on complex sugars, breaking them down into simple sugars like glucose that can be easily absorbed. Similarly, peptidases complete the digestion of proteins by breaking small peptide chains into individual amino acids. This enzymatic action ensures that nutrients are in their most basic forms for absorption.
Moving Food Through: Jejunal Motility
The movement of chyme through the jejunum is controlled by coordinated muscle contractions. This process, known as peristalsis, involves wave-like contractions of the smooth muscles in the intestinal wall, which propel the contents forward toward the ileum. These movements are quick in the jejunum.
In addition to peristalsis, another type of muscular action called segmentation occurs. Segmentation involves localized, mixing contractions that churn the chyme back and forth. This motion ensures that the chyme is thoroughly mixed with digestive enzymes. It also brings the nutrient-rich chyme into continuous contact with the absorptive surface of the intestinal wall, allowing for efficient nutrient uptake.