Anatomy of the Intestines

Villi and microvilli are finger-like extensions that dramatically increase the small intestine's surface area for nutrient absorption. These structures, along with mucosal folds, amplify the absorptive surface 60 to 120 times, ensuring optimal absorption of nutrients from chyme as it passes through the duodenum, jejunum, and ileum.

The large intestine absorbs remaining water and electrolytes from chyme, converting it into feces. The transverse colon performs most of this absorption, while the descending colon, sigmoid colon, and rectum store feces until elimination. The large intestine also hosts a bacterial ecosystem that completes digestion by breaking down cellulose and fiber.

Chyme first enters the duodenum through the pyloric sphincter, then travels to the jejunum and ileum. The ileocecal valve marks where the small intestine attaches to the large intestine at the cecum. This approximately 22-foot-long organ is compacted into the abdominal cavity to fit within the body.

The large intestine begins at the cecum, where the appendix, a small lymphatic structure, dangles from its bottom. Above the cecum are the ascending colon and transverse colon. The descending colon, sigmoid colon, and rectum follow, with the rectum connecting to the anus for fecal elimination.

Bacterial diversity in the large intestine is associated with metabolic health. Obese individuals have fewer bacterial strains than non-obese counterparts, and low bacterial diversity correlates with insulin resistance and Type 2 diabetes development. The gut biome is established after birth and shaped by genetics, diet, age, sex, immune system, medications, and geographical location.

Although protein digestion, carbohydrate digestion, and lipid digestion may begin in the stomach, nutrient absorption is completed in the small intestine. The highly convoluted inner lining with villi and microvilli provides the optimal surface area for absorbing nutrients, water, and electrolytes from food and drink.

The small intestine is approximately 7 meters long with a 2.5 cm inner diameter and features mucosal folds, villi, and microvilli for nutrient absorption. The large intestine is about 1.5 meters long with a 4.8 cm inner diameter and lacks folds and villi, though its epithelial cells retain microvilli for water and electrolyte absorption.