17.2:
Anatomy of the Intestines
The small and large intestines are responsible for absorbing nutrients and water.
While the small intestine is rather narrow, the organ is approximately 22 feet long and compacted into the abdominal cavity. Chyme first reaches the duodenum and then travels to the middle part, the jejunum.
Here, the inner lining contains mucosal folds, which are covered in finger-like villi. The overlying layer of epithelial cells have cytoplasmic extensions called microvilli. These three structures increase the surface area of the small intestine to ensure that nutrients are optimally absorbed.
At the end of the small intestine is the ileum, which attaches to the large intestine by the ileocecal valve. In the first part of the large intestine, the cecum, there are no folds or villi.
Dangling at the bottom of the cecum, is the appendix, a small lymphatic structure. Above it, starts the ascending colon, followed by the transverse colon, which absorbs most of the remaining water and electrolytes, turning chyme into feces. The last regions, the descending colon, sigmoid colon, and rectum, store feces until elimination through the anus.
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, water, and electrolytes from food and drink also occurs in the intestine. The intestines can be divided into two structurally distinct organs—the small and large intestines.
Small Intestines
The small intestine is an ~7 meter-long tube with an inner diameter of just 2.5 cm. Since most nutrients are absorbed here, the inner lining of the small intestine is highly convoluted and covered in finger-like extensions called villi, each containing hundreds of microvilli. The folds, villi, and microvilli of the small intestine amplify the surface area of absorption 60 to 120 times. The increased surface area provides ample opportunity for nutrients to be absorbed.
The small intestine connects to the stomach by the pyloric sphincter, which closes off when chyme moves into the duodenum—the beginning of the small intestine. The middle part of the small intestine is the jejunum. The ileum ends the small intestine, where it attaches to the large intestine by the ileocecal valve.
The Large Intestines
The large intestine starts at the cecum. The appendix, a small lymphatic structure, dangles from the bottom of the cecum. Above the cecum, starts the ascending colon followed by the transverse colon. They absorb most of the remaining water and electrolytes from the chyme, turning it into feces. The descending colon, sigmoid colon, and rectum store feces until elimination through the anus.
Overall the large intestine is about 1.5 m long with an inner diameter of 4.8 cm. It does not contain folds and villi, but the absorptive epithelial cells have microvilli. The large intestine is home to a bacterial ecosystem, which performs the final stages of digestion—breaking down cellulose and fiber, compounds which the stomach and small intestine are ill-equipped to process.
Bacterial Diversity
The large intestine contains over 700 different species of bacteria. Bacterial diversity is thought to be related to obesity and the development of Type 2 diabetes. Obese individuals have fewer bacterial strains than their non-obese counterparts. Low bacterial diversity has also been associated with insulin resistance. The intestinal bacterial ecosystem—the gut biome—is established shortly after birth, with those babies being breastfed developing their gut biome from bacteria present in breast milk. After birth, the biome is shaped by an individual’s genetics, diet, age, sex, and immune system. External factors like diet, health status, medications, and geographical location also significantly influence the complexity of the gut biome.
Suggested Reading
Javitt, Norman B. “History of Hepatic Bile Formation: Old Problems, New Approaches.” Advances in Physiology Education 38, no. 4 (December 1, 2014): 279–85. [Source]
Shetty, Sudarshan A., Floor Hugenholtz, Leo Lahti, Hauke Smidt, and Willem M. de Vos. “Intestinal Microbiome Landscaping: Insight in Community Assemblage and Implications for Microbial Modulation Strategies.” FEMS Microbiology Reviews 41, no. 2 (March 2017): 182–99. [Source]
Wen, Li, and Andrew Duffy. “Factors Influencing the Gut Microbiota, Inflammation, and Type 2 Diabetes.” The Journal of Nutrition 147, no. 7 (July 2017): 1468S-1475S. [Source]