17.2
Anatomy of the Intestines
Although digestion of proteins, carbohydrates, and lipids may begin in the stomach, it is completed in the intestine. The absorption of nutrients, wat…
The small and large intestines absorb nutrients and water from digested food.
The small intestine is narrow and stretches about 22 feet long, fitting compactly within the abdominal cavity.
Acidic chyme from the stomach enters the duodenum, the first section of the small intestine, and then moves into the jejunum, the next section.
To increase absorption, the inner lining of the small intestine forms mucosal folds known as plicae circulares. These folds are covered with finger-like projections called villi. The epithelial cells that line the villi have tiny extensions on their apical surfaces called microvilli.
Together, these three structures greatly increase the surface area of the small intestine and help it absorb nutrients efficiently.
The last and longest part of the small intestine is the ileum. It connects to the large intestine, which is about 5 feet long, through the ileocecal valve.
The first section of the large intestine is the cecum, which has a smooth inner surface without folds or villi. Hanging from the bottom of the cecum is the appendix, a small structure that is linked to immune function.
Above the cecum, the ascending colon begins, followed by the transverse colon.
Throughout the large intestine, water and electrolytes are absorbed, and the remaining material gradually turns into feces.
The final sections—the descending colon, sigmoid colon, and rectum—store feces until it leaves the body through the anus.
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Q1: Why does the small intestine have villi and microvilli?
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.
Q2: What is the role of the large intestine in digestion?
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.
Q3: How does chyme move through the small intestine?
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.
Q4: What structures make up the large intestine?
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.
Q5: How does bacterial diversity in the gut affect health?
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.
Q6: Where does nutrient absorption primarily occur in the digestive system?
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.
Q7: What is the difference in structure between the small and large intestines?
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.