Large intestine absorption
The large intestine primarily absorbs water and electrolytes, especially sodium, from the indigestible material that leaves the small intestine. It also absorbs vitamins produced by gut bacteria, including vitamin K and some B vitamins, while compacting waste into feces.
The large intestine (colon) receives chyme from the small intestine after nearly all nutrients — sugars, amino acids, fatty acids — have already been absorbed. Its main absorptive job is recovering water and electrolytes. Colonic epithelial cells actively transport sodium out of the lumen, and water follows osmotically, converting liquid chyme into formed, semi-solid feces.
Electrolyte handling in the colon is selective. Sodium and chloride are absorbed, while potassium and bicarbonate are generally secreted into the lumen. This exchange helps regulate the body's fluid and acid-base balance, and it explains why severe or prolonged diarrhea causes dehydration along with losses of potassium and bicarbonate.
The colon also hosts a dense population of commensal bacteria that ferment indigestible carbohydrates like dietary fiber. This fermentation yields short-chain fatty acids, which colonocytes absorb and use as an energy source, and it produces vitamins the colon can absorb — most notably vitamin K and several B vitamins, including biotin. What the large intestine does not do is absorb significant amounts of dietary protein, carbohydrate, or fat; those are jobs of the small intestine, a distinction exams frequently probe.
The MCAT tests large intestine absorption within the digestive system content of its biology section, and a classic question format asks candidates to contrast the small intestine's nutrient absorption with the colon's water, electrolyte, and bacterial-vitamin absorption.
Key takeaways
- The large intestine chiefly absorbs water and sodium, compacting chyme into feces.
- It absorbs sodium and chloride while secreting potassium and bicarbonate.
- Gut bacteria in the colon produce vitamin K and some B vitamins, which the colon absorbs.
- Colonocytes use bacterial short-chain fatty acids from fiber fermentation as fuel.
- Nutrient absorption (sugars, amino acids, fats) happens in the small intestine — a contrast the MCAT tests.