Figure 1.

Anatomy and physiology of the small intestine (SI) versus colon

(A) Overview of the gastrointestinal tract displaying spatial distribution of key processes influencing microbial colonization. In the healthy individual, liquid volume presented to the lumen of the SI is 8.5 L per day, for which pancreatic and bile secretion accounts for 1.5 and 0.5 L, respectively; ∼6.5 L are reabsorbed in the SI, while ∼2 L enters the colon. Here ∼1.9 L are reabsorbed and ∼0.1 L are lost in fecal secretions. Both secretion and absorption of liquids are affected by diet and diseases. Acidic ventricle secretion is neutralized by natrium hydrogen carbonate-buffered pancreatic juice in the upper duodenum. Proximal duodenum is also the site of bile acid (BA) secretion from the gall bladder. Nominal amounts of bile are passively absorbed through the entire length of the intestine, while the majority is recirculated by active transport in the distal ileum. Simple carbohydrates and protein are absorbed in the SI with a gradually decreasing absorption from proximal duodenum to distal ileum. Complex carbohydrates and proteins escaping digestion in the SI are converted into short-chain fatty acids (SCFAs) by microbial fermentation in the large intestine.

(B) Inner mucus layer (colon): green. Loosely attached mucus layer (outer layer in colon and only layer in SI): blue. Villus: in healthy individuals, most dietary lipids are absorbed in the first 60 cm of the SI, where BA concentrations are highest. As chylomicrons are too large to pass through the fenestrae of villus blood capillaries, they instead enter the lymph through larger inter-endothelial channels of the lacteals. Dietary proteins and carbohydrates are digested by pancreatic and brush border enzymes, respectively. Crypt: crypt-residing Paneth cells secreting abundant amounts of HDPs further protect and nurture the neighboring stem cells. In the transit-amplifying zone, lineage-committed progenitor cells swiftly divide to fuel the rapid intestinal cell turnover. Colon: the outer mucus layer provides a niche for mucolytic bacteria, many of which can ferment complex carbohydrates/dietary fibers, thereby generating SCFAs fortifying barrier function, including goblet cell-mediated mucus secretion. IgA, immunoglobulin A; TA-zone, transit-amplifying zone.