The jejunum is the middle segment of the small intestine, positioned between the duodenum and the ileum. This portion of the digestive tract serves as the primary engine for nutrient uptake in the human body. While earlier digestive phases break down food into smaller components, the jejunum is responsible for absorbing the vast majority of these usable molecules. Its specialized structure and intense enzymatic activity make it the central site where digested carbohydrates, proteins, and fats are transferred into the bloodstream.
Anatomical Placement and Unique Structure
The jejunum begins at the duodenojejunal flexure and constitutes approximately two-fifths of the small intestine’s total length. Located in the upper abdomen, it is ideally positioned to receive nutrient-rich contents, known as chyme, from the duodenum. The tissue wall of the jejunum is thicker and a deeper red than the ileum, reflecting its dense network of blood vessels and high metabolic activity.
Internally, the jejunum is architecturally optimized for maximum absorption efficiency. The intestinal lining features large, crescent-shaped folds called plicae circulares, which are particularly tall and numerous in this segment. These folds slow the passage of chyme and significantly increase the surface area available for contact with the intestinal wall.
Projecting from these circular folds are microscopic, finger-like projections known as villi, which are denser and longer in the jejunum than in adjacent segments. Each villus is covered in enterocytes, whose apical membranes possess even smaller projections called microvilli, collectively forming the brush border. This triple-layered specialization—folds, villi, and microvilli—expands the absorptive surface area dramatically, enabling the rapid uptake of nutrients.
Primary Digestive Roles
The final stages of digestion occur directly on the jejunal brush border. This is accomplished by membrane-bound enzymes fixed to the microvilli surface. These enzymes ensure that large molecules are reduced to their smallest, absorbable forms immediately before transport.
Disaccharidases, such as lactase and sucrase, break down double sugars into single sugars like glucose, fructose, and galactose. Similarly, various peptidases finish the digestion of protein fragments, cleaving small oligopeptides into individual amino acids or di- and tripeptides. This final enzymatic action prepares nutrients for immediate transport across the enterocyte membrane, making the jejunum efficient at processing the chyme it receives.
Specialized Nutrient Absorption
The jejunum’s core function is the high-capacity absorption of all three major macronutrients, which requires multiple specialized transport systems.
Carbohydrates
The absorption of monosaccharides relies on membrane transport proteins. Glucose and galactose are absorbed via the sodium-glucose transporter 1 (SGLT1), coupled with sodium ions. Fructose, however, is absorbed by facilitated diffusion through the glucose transporter 5 (GLUT5), which does not require sodium. Once inside the enterocyte, all three monosaccharides exit the cell into the bloodstream via the GLUT2 transporter located on the basolateral membrane.
Proteins
Amino acids are absorbed using various transporters, many of which are sodium-dependent, similar to glucose uptake. Di- and tripeptides are absorbed whole through the peptide transporter 1 (PepT1). Once inside the enterocyte, these small peptides are broken down by intracellular peptidases into individual amino acids before entering the bloodstream.
Fats
Long-chain fatty acids and monoglycerides are packaged into micelles with bile salts, which transport them to the brush border. Upon reaching the enterocyte, these lipid components diffuse across the membrane. Inside the cell, they are re-esterified back into triglycerides and cholesterol esters using enzymes like monoacylglycerol acyltransferase (MGAT) and diacylglycerol acyltransferase (DGAT). These triglycerides are then packaged along with apolipoprotein B-48 into large lipoprotein particles called chylomicrons.
Chylomicrons are too large to enter the small pores of the capillaries, so they are instead secreted into the central lacteal, the lymphatic vessel within the villus, for transport into the lymphatic system. Short- and medium-chain fatty acids bypass this complex packaging and are absorbed directly into the portal vein. The jejunum is also the primary site for absorbing most water-soluble vitamins and various electrolytes.
Differentiation from Adjacent Intestine Segments
The jejunum’s structure and function are distinct from the duodenum and the ileum.
The duodenum, the first segment, focuses on neutralizing the acidic chyme from the stomach. It contains Brunner’s glands, which secrete alkaline mucus, and is the main site for absorbing certain minerals.
In contrast, the ileum, the final segment, has less prominent plicae circulares and shorter villi, reflecting its role in absorbing leftover nutrients. The ileum is specialized for the recovery of specific substances, most notably Vitamin B12 and bile salts. Furthermore, the ileum contains large aggregates of lymphoid tissue called Peyer’s patches, which are much less numerous in the jejunum. This difference highlights the ileum’s stronger association with immune surveillance, whereas the jejunum is dedicated to bulk nutrient absorption.