Colonic fluid

Gastrointestinal tract Inhibition of most gut hormones, gastric acid, pepsin, bile and colonic fluid secretion... 

Hammer, J., Pruckmayer, M., Bergmann, H., Rletter, K., Gangl, A., The distal colon provides reserve storage capacity during colonic fluid overload, Gut, 1997, 41, 658—663. 

In the colon Na+, Cl , and water are efficiently absorbed. Na+ absorption is regulated by aldosterone (Chapter 32). K+ is secreted into the lumen as a component of mucus but is reabsorbed by passive diffusion. The amount of K+ in the feces is usually far below the daily intake however, in chronic diarrhea the loss of ileal and colonic fluids can cause negative K" " balance. 

In the fasting state, 9 L of fluid enters the proximal small intestine each day. Of this fluid, 2 L are ingested through diet, while the remainder consists of internal secretions. Because of meal content, duodenal chyme is usually hypertonic. When chyme reaches the ileum, the osmolality adjusts to that of plasma, with most dietary fat, carbohydrate, and protein being absorbed. The volume of ileal chyme decreases to about 1 L/day upon entering the colon, which is further reduced by colonic absorption to 100 mL daily. If the small intestine water absorption capacity is exceeded, chyme overloads the colon, resulting in diarrhea. In humans, the colon absorptive capacity is about 5 L daily. Colonic fluid transport is critical to water and electrolyte balance. 

In vitro drug release from uncoated tablet in rat cecal content-free simulated colonic fluid is 60% at 11 hr of dissolution. This increases up to 80% with the use of medium with rat cecal content. 

Fluid redistributes between prone and supine imaging (Fig. 14.19). Polyethylene glycol electrolyte solution bowel preparation typically results in more retained colonic fluid, compared to other bowel preparation regimens, but leaves less particulate stool matter in the colon. Fluid can also be seen in 3D endoluminal images (Fig. 14.20). When there is excessive fluid, intravenous contrast can be used to enhance submerged lesions. 

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