The small intestine is one of the most vital yet often overlooked organs in the human body. Sitting quietly in your abdomen, this long, winding tube connects the stomach to the large intestine and serves as the primary hub for digestion and nutrient absorption. Measuring close to 20 feet in length, the small intestine not only processes the food you eat but also determines how well your body receives essential vitamins, minerals, and energy. Without its continuous work, daily life and overall health would be impossible to maintain.

Given its importance, understanding the anatomy, functions, and common disorders of the small intestine can help you recognize warning signs early and adopt habits that protect your digestive health.

Anatomy of the Small Intestine

The small intestine is unique in that it is both long and narrow. Despite stretching around 20 feet in length, its diameter is typically less than an inch. To fit into the abdominal cavity, it coils and folds in tight curves, forming a compact but highly efficient digestive structure.

The organ is divided into three major segments:

the duodenum
This initial section is about 10 inches long and shaped like a “C.” It connects to the stomach through the pyloric sphincter. Here, food mixes with pancreatic enzymes, bile from the liver, and gastric juices. This chemical blend is crucial for breaking down proteins, fats, and carbohydrates into smaller, digestible units.

the jejunum
Located in the middle portion of the small intestine, the jejunum stretches over 6 feet. Its walls are lined with muscular folds and tiny projections known as villi, which significantly increase the surface area for nutrient absorption. This is where much of the body’s essential energy sources—such as glucose, amino acids, and fatty acids—enter the bloodstream.

the ileum
At approximately 10 feet long, the ileum makes up the final segment before food passes into the large intestine. It plays a critical role in absorbing vitamin B12, bile salts, and other nutrients that remain after digestion in earlier sections. The ileum ends at the cecum, a pouch-like entry into the colon.

Together, these three regions create a seamless system for breaking down food and harvesting the nutrients that sustain life. Surrounding blood vessels, nerves, and muscle layers coordinate movement and function throughout the small intestine.

Location and Developmental Variations

The small intestine sits in the lower abdominal region, nestled between the stomach and the large intestine, which frames it in a loop-like structure. While this is the typical location, certain birth defects can alter its position or shape.

For example, gastroschisis is a congenital condition where a baby is born with part of the intestines protruding outside the body through a small hole near the umbilical cord. Another defect, known as malrotation, occurs when the small intestine does not rotate properly during fetal development, potentially leading to dangerous twists or blockages. These rare but serious conditions highlight how precise the positioning of this organ needs to be for proper function.

Functions of the Small Intestine

The small intestine is responsible for some of the most complex and essential digestive processes.

nutrient absorption
The organ absorbs the bulk of carbohydrates, proteins, and fats from food. Vitamins such as folate and B12, along with minerals like calcium, magnesium, and iron, also pass through its walls into circulation. Out of roughly nine liters of fluid that move through the gastrointestinal system daily, the small intestine absorbs nearly 80%.

digestion and breakdown
Digestive enzymes, bile, and stomach acids are all funneled into the small intestine to chemically dismantle food into absorbable particles. Mechanical movements known as peristalsis help push and mix food through the tube.

immune defense
The small intestine is a key part of gut immunity. Specialized cells monitor harmful bacteria and prevent them from entering the bloodstream, ensuring the body absorbs only safe and beneficial substances.

Without these functions, essential nutrients would pass through the body unused, leading to malnutrition and serious illness.

Common Disorders Affecting the Small Intestine

A variety of conditions can impair small intestine function. Some are genetic, while others develop due to lifestyle, infections, or autoimmune triggers.

celiac disease
This autoimmune disorder is triggered by gluten, a protein found in wheat, barley, and rye. When consumed, gluten damages the intestinal lining, preventing nutrient absorption. Left untreated, it can cause malnutrition, osteoporosis, and other complications.

crohn’s disease
Crohn’s disease is a chronic inflammatory condition that often targets the small intestine, particularly the ileum. It causes pain, diarrhea, and malabsorption. The cause remains unclear, but autoimmune reactions and genetics are considered contributors.

irritable bowel syndrome (IBS)
Unlike Crohn’s or celiac disease, IBS does not physically damage the intestine. Instead, it results from disrupted communication between the gut and the brain. This miscommunication leads to hypersensitivity, causing bloating, abdominal pain, and irregular bowel habits.

small intestine cancer
Although rare, cancer can develop within the small intestine, especially in the duodenum. Risk factors include Crohn’s disease, celiac disease, and intestinal polyps.

intestinal obstruction
A blockage prevents food or stool from moving forward. Obstructions may result from hernias, scar tissue, tumors, or medications. Complete obstructions are medical emergencies requiring immediate intervention.

peptic ulcers
Ulcers are painful sores in the lining of the stomach or duodenum, often caused by Helicobacter pylori infections or excessive use of anti-inflammatory drugs. They produce burning abdominal pain and, if untreated, may bleed or perforate the intestine.

SIBO (small intestinal bacterial overgrowth)
Normally, the small intestine contains relatively few bacteria compared to the large intestine. In SIBO, abnormal bacterial growth interferes with nutrient absorption, causing bloating, diarrhea, and malnutrition.

short bowel syndrome
This occurs when a significant portion of the small intestine is missing or damaged. Causes include surgical removal, congenital defects, or severe disease. Patients may struggle to absorb enough nutrients, requiring specialized diets, supplements, or even intravenous nutrition.

Diagnostic Testing for Small Intestine Conditions

Because many small intestine conditions share similar symptoms, accurate testing is critical for diagnosis. Common methods include:

  • Blood tests: Used to identify anemia, inflammation, or antibodies linked to conditions like celiac disease.

  • Genetic testing: Helpful for confirming genetic predispositions, particularly in autoimmune disorders.

  • Upper GI endoscopy: A flexible tube with a camera is inserted through the mouth to visualize the stomach and upper small intestine. Biopsies can also be taken.

  • Capsule endoscopy: Patients swallow a small, pill-sized camera that captures images as it passes through the digestive tract.

  • Stool tests: Used to detect hidden blood, infections, or other abnormalities.

  • Enteroclysis: A contrast material such as barium is introduced into the small intestine and tracked via X-ray.

  • Double-balloon enteroscopy: Allows detailed inspection of long segments of the intestine by advancing two specialized tubes.

  • Breath tests: Measure hydrogen and methane gases to diagnose conditions such as SIBO.

In some cases, such as IBS, there is no definitive test. Diagnosis requires ruling out other disorders using a combination of these methods.

Supporting the Health of Your Small Intestine

Just as diet and lifestyle influence heart or brain health, they play a crucial role in protecting the small intestine.

manage stress
Stress impacts gut function by disrupting brain–gut communication. It can worsen IBS and contribute to flare-ups in Crohn’s disease. Mindfulness practices, yoga, and stress-reduction techniques can significantly improve gut health.

practice mindful eating
Eating slowly and paying attention to flavors, textures, and satiety cues reduces digestive strain. Research suggests mindful eating helps people with IBS experience fewer symptoms.

stay hydrated
Adequate hydration aids digestion, enhances nutrient transport, and supports intestinal function. Aim for sufficient water intake daily, particularly if physically active or living in hot climates.

exercise regularly
Physical activity stimulates intestinal motility, reducing constipation and bloating. Light to moderate exercise like walking also lowers inflammation, supporting those with digestive disorders.

Together, these habits foster a balanced digestive environment that protects the small intestine over the long term.

Conclusion

The small intestine is far more than a simple digestive tube—it is a powerhouse that fuels every aspect of human health. From breaking down food to absorbing essential nutrients and supporting immune defenses, it plays a role in nearly every system of the body. However, conditions like celiac disease, Crohn’s disease, and SIBO can compromise its performance, making awareness and proactive care essential.

If you experience persistent digestive discomfort, unexplained weight loss, or nutrient deficiencies, consulting a healthcare professional is the best step. With accurate testing, effective treatment, and mindful lifestyle choices, you can protect the health of your small intestine and ensure your body receives the nourishment it needs.