Jejunum Function – Role of the Middle Small Intestine

What Is the Jejunum?

The jejunum is the middle section of the small intestine, located between the duodenum and the ileum. It accounts for approximately two-fifths of the total length of the small intestine, measuring roughly 1.5 to 2.5 meters in an adult. The name is derived from the Latin word jejunus, meaning empty, as this section of the bowel is typically found empty during autopsy.

Main Functions of the Jejunum

The jejunum performs several vital roles within the human digestive tract:

• Nutrient absorption: The jejunum is the primary site for the uptake of macronutrients such as carbohydrates, proteins, and fats, as well as vitamins and minerals.
• Water absorption: A significant proportion of ingested water is absorbed in the jejunum.
• Transport function: Coordinated muscle contractions known as peristalsis propel the digested food mixture (chyme) forward through the intestine.
• Secretion of digestive enzymes: The mucosal lining of the jejunum produces enzymes such as disaccharidases and peptidases, which chemically break down food components.

Anatomical Features and Their Functional Significance

The inner surface of the jejunum is greatly enlarged by several anatomical structures, which substantially increases its absorptive capacity:

• Circular folds (plicae circulares): Ring-shaped mucosal folds that multiply the surface area.
• Villi (intestinal villi): Finger-like projections of the mucosa that further increase the absorptive surface.
• Microvilli (brush border): Microscopically small projections on enterocytes (intestinal cells) that dramatically amplify the surface area further.

Through this threefold surface expansion, the effective absorptive area of the entire small intestine is estimated at approximately 200 to 300 square meters.

Absorption of Individual Nutrients in the Jejunum

Carbohydrates

Carbohydrates are absorbed in the jejunum after enzymatic breakdown into simple sugars (monosaccharides) such as glucose, fructose, and galactose. Uptake occurs via specific transport proteins in the enterocytes.

Proteins

Following pre-digestion in the stomach and duodenum, proteins are absorbed in the jejunum as amino acids and short peptide chains. Specialized transporters carry them into the intestinal cells and subsequently into the bloodstream.

Fats

Fats are broken down into fatty acids and monoglycerides with the help of bile salts and pancreatic lipase. These are taken up by enterocytes, assembled into chylomicrons, and transported into the body via the lymphatic system.

Vitamins and Minerals

The jejunum is a key site for the absorption of water-soluble vitamins (e.g., vitamin C and B vitamins) and minerals such as calcium, iron, magnesium, and zinc. The fat-soluble vitamins A, D, E, and K are absorbed together with dietary fats.

Clinical Relevance of Jejunum Function

Disruptions to jejunal function can have wide-reaching health consequences:

• Coeliac disease: An autoimmune condition in which gluten triggers inflammation and villous atrophy in the small intestine -- primarily the jejunum -- leading to malabsorption.
• Short bowel syndrome: Surgical removal of large portions of the jejunum can result in severe malnutrition.
• Crohn's disease: A chronic inflammatory bowel disease that can affect the jejunum and impair nutrient absorption.
• Jejunal malabsorption: Various conditions may reduce nutrient uptake, resulting in weight loss, vitamin deficiencies, and diarrhea.

Diagnostic Methods

Several diagnostic procedures are available to assess jejunal function and structure:

• Upper endoscopy (oesophagogastroduodenoscopy): Allows direct visualization and biopsy of the upper small intestine.
• Capsule endoscopy: A swallowed miniature camera captures images of the entire small intestine.
• Laboratory tests: Measurement of absorption markers, vitamins, and minerals in the blood provides an indirect assessment of jejunal function.
• Imaging procedures: MRI or CT enterography to visualize wall changes or inflammation in the jejunum.

References

1. Schünke M., Schulte E., Schumacher U. – Prometheus Atlas of Anatomy: Internal Organs. Georg Thieme Verlag, Stuttgart, 2018.
2. Kasper D. L. et al. – Harrison's Principles of Internal Medicine, 20th Edition. McGraw-Hill Education, 2018.
3. World Gastroenterology Organisation (WGO) – Global Guidelines: Small Intestinal Bacterial Overgrowth (SIBO). WGO, 2020. Available at: https://www.worldgastroenterology.org