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Describe absorption of fluids in different segments of the gut

About 9 L of water enters the gut each day, of which only approximately 2 L is dietary. The remainder is secreted by the mouth, stomach, biliary tree, pancreas, and jejunum. Close to 4.5 L is absorbed by the jejunum, and the ileum takes up all but about 1 L of the remaining water (~3.5L). The colon absorbs nearly 900 ml, so that only about 100 to 200 ml is normally excreted in stool


What are the main groups of laxatives?

•Dietary fiber and Bulk-forming laxatives.

•Surfactant laxatives

•Osmotic laxatives

•Stimulant laxatives.

•Miscellaneous laxatives.


What are some common bulk-forming laxatives?

-Psyllium husk

-Semisynthetic celluloses (Carboxymethylcellulose, methylcellulose)



How does psyllium husk work (Metamucil; Effer-syllium)?

This is a hydrophilic muciloid that forms gelatinous mass when mixed with water and bulks up stool to activate stretch receptors in the GI


What are some AEs of psyllium husk?

Allergic reactions,



intestinal obstruction;

May inhibit coumarin absorption


How do Semisynthetic celluloses work (Citrucel and Cologel)?

Hydrophilic and digestible; forms a colloid mass with water

May bind and impede drug absorption.


How do Polycarbophils work (Mitrolan)?

Hydrophilic polyacrylic resins; absorb 60-100x their weight in water to bulk stool 


Ca+ polycarbophils are contraindicated in which pts? 

They release Ca+ that is contraindicated with tetracycline usage.


How does fiber promote defecation?

*Increase delivery of water to the colon.

*Increase bulk by consisting of non-digestable carbs

*Reduce pressure in sigmoid Colon

*Net result: more formed stools.


What are the main surfactant laxatives?



-Castor oil


How do Docusates work (Colace, Surfak)?

They are anionic surfactants primarily used as a stool softener to reduce the strain of defecation (in e.g. HA pts.) by reducing water tension with epithelium 

*Has no effect on intestinal peristalsis.


What are some AEs of Docusates?

Not for use during abdominal pain or vomiting;

*Can irritate the intestinal mucosa and increase absorption of other drugs. 

*Recommended for short-term use ONLY


How do Poloxamers work (Poloxamer 188)?

Non-ionic surfactant similar to docusates that acts as a Stool softener.


What are the AEs of poloxamers?

Diarrhea; Not for use during abdominal pain, nausea, or vomiting.

dont use for a long time, they irriatate the mucosa- give 2-4 weeks max; 


How does castor oil work?

Rapid-acting and effective anionic surfactant that produces catharsis i.e. complete evacuation of the bowels. DOES stimulate intestinal peristalsis (unlike docusates)


What are some AEs of castor oil?

Colic, dehydration, and electrolyte imbalance with overdose; 

Can induce uterine contraction in pregnant women.

This is an emulsion that irritates the mucosa and produces a cathartic effect.


How do stimulant laxatives work?

These predominantly act on the large bowel and increase the permeability of intestinal mucosa by weakening tight junctions to increase back diffusion of water and electrolytes.

they also increase propulsive contractility of the colon by stimulating colonic mucosal myenteric plexus and stimulate prostaglandin synthesis and increase intestinal secretions.


What is the most potent class of laxatives?



What are some types of stimulant laxatives?

-Diphenylmethanes (Bisacodyl)



How do Diphenylmethanes like bisacodyl work (Modane, Dulcolax)?

This is a prodrug, converted by enteric bacteria into the desacetyl active form.

*Administered in enteric coated tablets- want it to be released in large intestine.


What are the AEs of Diphenylmethanes?

Overdosing can cause excessive fluid and electrolyte loss and/or intestinal enterocyte damage leading to colonic inflammatory response.


How do Anthraquinones (Senokot) work?

These are natural derivatives of Lilliaceae plants (senna, cascara) that are stimulants that act by promoting colonic motility

•More gentle than synthetic drugs.



What are the AEs of Anthraquinones?

May cause melanotic (dark)  pigmentation of the colonic mucosa, abnormal urine coloration (brownish)


What are saline/osmotic laxatives?

Saline laxatives are laxatives containing magnesium cations or other nonabsorbable molecules. They exert an osmotic effect which retains water in the lumen of the G. I. tract.


How do Mg2+ containing laxatives work?

Produce laxation by their osmotic effect and through the release of cholecystokinin. CCK increases intestinal motility and secretion


What are some examples of magnesium containing laxatives?

i.   Magnesium sulfate: Epsom salt.

ii.  Magnesium hydroxide: milk of magnesia (dose = 30 ml) Take in evening= diarrhea in morning.

iii.  Magnesium citrate:  Citroma™, is a cathartic (dose 4-8 oz)


What is another type of osmotic laxative?

Phosphate containing laxatives

Given as enema or oral Sodium Phosphate tablets (Visicol, OsmoPrep etc.)


Nondigestible sugars and alcohols are also used as osmotic laxatives. What is the most common type?

lactulose (cephulac)


What is lactulose?

a semisynthetic disaccharide which is not absorbed and produces an osmotic laxative effect.

It is metabolized by enteric bacteria to organic acids such as lactic, formic, and acetic acid.Therefore, it is a fecal acidifier, that is used in management of symptoms of liver failure [portal systemic encephalopathy]).