The Colon

The human large intestine (colon) is about 5 ft long and its largest diameter (2.4-3.6 in) is in the cecum. Unlike the small intestine, its mucosa does not carry villi, but it does have crypts. It is characterized by extensive lymphatic tissue and plasma cells, especially in the area surrounding the appendix.

Colonic contractions also differ from those of the small intestine. Circular musculature contracts in an irregular fashion and in several places at once, without any net movement of chyme. True peristaltic waves that move the contents along about 8 in at a time are rare. Actual transport occurs through so-called mass movements, powerful contractile waves that occur once or twice daily. Like the other types of contractions, they are triggered by food intake—the so-called gastrocolic reflex. Chyme from low-fiber foods takes 23 days to transit the colon (stool weight 100-150 g), chyme from extremely high-fiber food 1-2 days, with a stool weight of up to 500 g. Larger particles may appear in the stools much faster if transported with the central stream of materials.

The colon reabsorbs 1-1.5 l of water/d. The structure of the pores and tight junctions prevents reflux of Na+ and water into the intestinal lumen. This allows the colon to absorb Na+ even from highly hypotonic solutions. The basolateral Na+/K+ pump is the driving force here, producing low Na+ concentrations in the mucosa cells, on the one hand, and on the other, building an electrical gradient (0 mV in lumen, -30 mV inside cells, 20 mV in the interstitium [A]). Cell membrane permeability is increased by aldosterone on the lumen side. K+ travels back through the tight junctions passively, driven by the electrical gradient. As a net result, the colon secretes K+ but absorbs Na+ almost completely. Whereas the stomach and small intestine are almost completely sterile, the colon contains a dense population of ~1012 bacteria/ml chyme. Stool dry matter is 30-75% bacteria, predominantly anaerobic Bifidus and Bacteroides. Aerobes like Escherichia coli or enterococci make up less than 1 %. Attempts are being made to achieve immunomod-ulation by altering intestinal flora through oral administration of certain lactic acid bacteria (so-called probiotics, see p. 288). The anaerobes are able to use ~50% of the nutrients that come down the pipeline (B). They produce useful metabolites, which are absorbed by the colon mucosa—the most well-known example is vitamin K (see p. 160). Resulting gases, like hydrogen, for example, are also partially absorbed and end up in the air we exhale. Short-chain fatty acids, which bacteria produce during fiber breakdown (see p. 80), are used as fuel by mucosa cells and thereby have an impact on cell proliferation. They activate colon motility and improve blood flow inside the mucosa.

The Colon 47

  • A. Electrolyte Absorption in the Colon
  • A. Electrolyte Absorption in the Colon
Names Colon Bacteria

i- B. The Role of Colon Bacteria -

Vitamin K2

Cell division

Oligo-polysaccharides Polypeptides Glycoproteins i- B. The Role of Colon Bacteria -

Vitamin K2

Oligo-polysaccharides Polypeptides Glycoproteins

Names Colon Bacteria

Cell division

Bacterial mass Non-fermentable components

Blood supply to colon

Bacterial mass Non-fermentable components

Blood supply to colon

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