The intestines form the lower portion of the gastrointestinal tract, bridging the stomach to the anus. This continuous muscular tube measures roughly seven to nine meters in an adult, packed compactly within the abdominal cavity. Anatomists divide this vast tract into two primary sections based on diameter and function: the small intestine and the large intestine.
The small intestine occupies the central and lower abdomen. It consists of three continuous segments:
The large intestine frames the small bowel on three sides. It comprises the cecum, appendix, colon, rectum, and anal canal. The colon itself further sub-divides into the ascending, transverse, descending, and sigmoid sections.
The small intestine serves as the primary site for chemical digestion and nutrient absorption. Its internal surface area is massively amplified to maximize absorption efficiency through three distinct structural modifications:
Enzymes embedded in the brush border complete the breakdown of carbohydrates and proteins, while contractions mix and propel the contents via segmentation and peristalsis.
The large intestine spans about 1.5 meters in length. While it lacks the villi found in the small intestine, its mucosa features numerous straight, tubular glands called crypts of Lieberkühn that secrete mucus.
Distinct structural features distinguish the colon from the small bowel:
The large intestine absorbs remaining water and electrolytes, converts liquid chyme into semisolid feces, and houses a vast community of resident bacteria that synthesize vital vitamins like vitamin K.
The wall of the gastrointestinal tract shares a consistent four-layer plan from the esophagus to the anal canal:
Digestion breaks down complex food macromolecules into absorbable monomers. Carbohydrates reduce to monosaccharides, proteins to amino acids, and fats to monoglycerides and fatty acids.
Absorption mechanisms vary by nutrient type. Monosaccharides and amino acids enter the enterocyte via active transport or facilitated diffusion, then pass directly into capillaries. Hydrophobic lipids require a different route. Bile salts emulsify fat globules into micelles, allowing them to diffuse across the brush border. Inside the cell, lipids repackage into triglycerides, combine with proteins to form chylomicrons, and enter the lacteals because they are too large to pass directly through capillary walls.
The vascular supply to the intestines stems directly from the abdominal aorta via the superior and inferior mesenteric arteries. The superior mesenteric artery feeds the small intestine and proximal colon, while the inferior mesenteric artery supplies the distal colon and rectum. Venous blood from these regions drains into the hepatic portal system, routing nutrient-rich blood through the liver first before systemic circulation.
Neural control relies on the enteric nervous system, an independent network of over 100 million neurons embedded in the bowel wall. The autonomic nervous system modulates this intrinsic system: parasympathetic stimulation via the vagus nerve and sacral spinal nerves increases motility and secretions, whereas sympathetic activation inhibits digestive activity.
Intestinal disorders present frequently in clinical practice, requiring vigilant nursing assessment. Bowel obstruction—whether mechanical from adhesions or functional from paralytic ileus—halts normal transit, causing abdominal distension, colicky pain, and vomiting. Nurses must auscultate bowel sounds routinely; absent sounds often indicate a paralytic ileus post-abdominal surgery.
Inflammatory conditions like Crohn disease and ulcerative colitis disrupt mucosal integrity, leading to chronic diarrhea, malabsorption, and electrolyte imbalances. Nurses monitor patient hydration status, track strict intake and output, and administer prescribed fluid replacements or anti-inflammatory therapies while watching for signs of bowel perforation.