29.3 Introduction to the Small and Large Intestines

Anatomy and Physiology of the Small and Large Intestines

The small and large intestines handle the final stages of digestion: breaking down food, absorbing nutrients and water, and eliminating waste. For nurses, understanding how these structures work is essential because so many common GI conditions directly disrupt these processes, affecting medication absorption, fluid balance, and nutritional status.

Small and Large Intestine Structure and Functions

The small intestine is the primary site of nutrient absorption. It's divided into three sections, each with a distinct role:

• Duodenum: The first and shortest section. It receives chyme from the stomach and mixes it with bile (from the gallbladder) and pancreatic enzymes. This is where chemical digestion really ramps up.
• Jejunum: The middle section, heavily lined with blood vessels. Most nutrient absorption happens here, including glucose, amino acids, and fatty acids.
• Ileum: The final section. It absorbs whatever the jejunum didn't catch, particularly bile salts and vitamin B12. The ileum connects to the large intestine through the ileocecal valve, which prevents backflow of colonic contents.

The large intestine picks up where the small intestine leaves off. Its main jobs are absorbing water and electrolytes, forming and storing feces, and hosting gut bacteria that ferment undigested carbohydrates.

• Cecum: A small pouch at the beginning of the large intestine where the appendix is attached.
• Colon: Makes up the majority of the large intestine. It's divided into four regions (ascending, transverse, descending, and sigmoid). The colon absorbs most of the remaining water and electrolytes, concentrating fecal matter as it moves along.
• Rectum and anal canal: Store feces until defecation occurs.

Common Gastrointestinal Conditions Affecting the Intestines

Several conditions can disrupt normal intestinal function. Knowing the key features of each helps you recognize symptoms and anticipate nursing priorities.

Inflammatory bowel disease (IBD) is an umbrella term for two chronic inflammatory conditions:

• Crohn's disease can affect any part of the GI tract, from mouth to anus, and often involves patchy, transmural (full-thickness) inflammation. Symptoms include abdominal pain, diarrhea, weight loss, and malnutrition. Complications like fistulas and strictures are common.
• Ulcerative colitis is limited to the colon and rectum, with continuous inflammation of the mucosal lining. It typically presents with bloody diarrhea, abdominal pain, and urgency.

Celiac disease is an autoimmune disorder triggered by gluten (a protein found in wheat, barley, and rye). Gluten exposure damages the intestinal villi, leading to malabsorption. Patients experience diarrhea, bloating, abdominal pain, and deficiencies in iron, calcium, and fat-soluble vitamins. Treatment is strict, lifelong gluten avoidance.

Irritable bowel syndrome (IBS) is a functional disorder, meaning there's no visible structural damage. It affects bowel motility and visceral sensitivity, causing abdominal pain, bloating, and alternating episodes of diarrhea and constipation. Unlike IBD, IBS does not cause inflammation or tissue damage.

Diverticulitis occurs when diverticula (small outpouchings in the colon wall) become inflamed or infected. The classic presentation is left lower quadrant abdominal pain, fever, and changes in bowel habits. It's most common in older adults and is associated with low-fiber diets.

Colorectal cancer is a malignancy arising in the colon or rectum. Warning signs include changes in bowel habits lasting more than a few weeks, rectal bleeding, unexplained weight loss, and persistent abdominal discomfort. Screening (typically colonoscopy starting at age 45) is critical for early detection.

Intestinal Villi: Role in Nutrient Absorption and Consequences of Disorders

Intestinal villi are tiny, finger-like projections that line the inner wall of the small intestine. Their purpose is to dramatically increase the surface area available for absorption.

Each villus is covered with epithelial cells that have their own even smaller projections called microvilli (together forming the "brush border"). The brush border contains transport proteins and enzymes responsible for the final breakdown and absorption of nutrients. Once absorbed, nutrients follow two routes:

• Glucose and amino acids enter capillaries and travel to the liver via the portal vein.
• Fatty acids enter lacteals (lymphatic vessels within each villus) and reach the bloodstream through the lymphatic system.

When villi are damaged, the absorptive surface area shrinks and nutrient uptake drops. This leads to malnutrition, vitamin and mineral deficiencies, and weight loss. Several conditions cause this kind of damage:

• Celiac disease triggers villous atrophy (flattening of the villi) in response to gluten, significantly reducing absorptive capacity.
• Tropical sprue is a malabsorption disorder, likely related to bacterial overgrowth, that also causes villous atrophy and chronic inflammation. Its effects on absorption are similar to celiac disease, but it's typically seen in tropical regions and responds to antibiotics.
• Intestinal infections such as giardiasis and cryptosporidiosis can directly damage villi. The resulting impaired absorption causes diarrhea, dehydration, and malnutrition, with severity depending on the infection's duration and the patient's baseline health.