Absorption of Water and Electrolytes

Fluid turnover of about 9 L occurs daily in the GI tract. Ingested water contributes about 2 L, and the remainder arises from secretions of the GI tract mucosa and associated glands. Nearly all of this water is reabsorbed, and about 200 mL (2%) or less is excreted (Table 12-8). If the amount of water excreted in feces exceeds 500 mL, diarrhea results. Similarly, only 2% of Na+ and 10% of K+ in gastrointestinal fluids appear in the feces. Most absorption of fluid and electrolytes occurs in the small intestine. 

Na is also absorbed separately from organic solutes through two coupled transport systems, of which one absorbs Na in exchange for H+, while the other absorbs Cl in exchange for HCOj . In the lumen, H+ and HCO3 combine to give rise to CO2 and H2O, which enter the mucosal cell or pass through to the plasma. The Na+ absorbed is pumped out by Na , K+-ATPase in the basolateral membrane. Cl follows Na+ passively, with transfer of NaCl to plasma. 

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. 

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Castor oil is a cathartic only after Hpolysis in the small intestine Hberating ricinoleic acid. Ricinoleic acid inhibits the absorption of water and electrolytes. It is commonly used for preparation of the large bowel for diagnostic procedures. 

Stimulant laxatives increase intestinal motility thereby decreasing absorption of water and electrolytes. Included in this group are diphenylmethane derivatives and anthraquinones. 

Loperamide, 4 mg initially, then 2 mg after each loose stool, not to exceed 8 mg daily Imodium A-D, various generic Loperamide, a synthetic opioid, acts on intestinal smooth muscle to decrease motility allowing for absorption of water and electrolytes. Poorly penetrates the CNS and has a lower risk of side effects compared with diphenoxylate or opiates. Not considered a controlled substance. 

The functions of the colon are (1) absorption of water and electrolytes from the chyme and (2) storage of fecal matter until it can be expelled. The proximal half of the colon, illustrated in Figure 12.2, is concerned principally with absorption, and the distal half with storage. Because intense movements are not required for these functions, the movements of the colon are normally sluggish. Although sluggish, the movements still have characteristics similar to those of the small intestine and can be divided into mixing movements and propulsive movements. 

Bile is a mixture of electrolytes, bile acids, cholesterol, phospholipids and bilirubin. Adults produce between 400 and 800 ml of bile daily. Hepatocytes secrete bile into canaliculi, then into bile ducts, where it is modified by addition of a bicarbonate-rich secretion from ductal epithelial cells. Further modification occurs in the gall bladder, where it is concentrated up to fivefold, through absorption of water and electrolytes. Gallstones, most of which are composed... 

Wapnir RA, Wingertzahn MA, Teichberg S. Cellulose derivatives and intestinal absorption of water and electrolytes potential role in oral rehydration solutions. Proc Soc Exp Biol Med 1997 215(3) 275-280. 

Enkephalins are pentapeptides that bind to opiate receptors. In the gut, enkephalins promote the absorption of sodium, chloride and water (Dobbins et al 1980). Racecadotril is an oral enkephalinase inhibitor used in France and the Philippines for the treatment of acute diarrhea. It prevents the degradation of endogenous opioids (enkephalins) and thus promotes absorption of water and electrolytes from the intestinal lumen (Matheson Noble 2000). Studies have demonstrated the efficacy of racecadotril in two models of hypersecretory diarrhea infusion of cholera toxin and castor oil induced diarrhea. Moreover, unlike loperamide, racecadotril did not prolong transit time in the small intestine or colon. Further experiments have shown that racecadotril does not promote bacterial overgrowth in the small intestine (Duval-Iflah et al 1999). There are no reports on the use of racecadotril in horses. 

The primary active ingredients of cascara sagrada include cascarosides A, B, C, and D, but barbaloin, chrysaloin, chryophanol, emodin, and aloe-emodin are also present (Tyler, 1994 Anonymous, 1996). The anthrone glucofrangulin is present in the cortex of the European species Rhamni frangula (De Witte, 1993). As with senna and aloe constituents, these anthrones produce an active secretion of water and electrolytes within the lumen of the small intestine. In addition, the anthrones inhibit absorption of water and electrolytes from the large intestine. This causes an increase in the volume of bowel contents, and strengthens the dilatation pressure in the intestine to stimulate peristalsis (Anonymous, 1996). 

Gastrointestinal constipation and nausea are common. Nausea may be treated with antiemetics, and frequently improves with ongoing therapy. Virtually all patients taking opioids become constipated and do not become tolerant to this side effect. Activation of mu receptors in the gastrointestinal tract slows peristalsis, which promotes further absorption of water and electrolytes in the colon. Patients should be treated prophylactically with stool softeners and/ or laxatives. There is an oral oxycodone/naloxone prolonged-release tablet in clinical trials to counteract opioid-induced constipation, which is often debilitating. 

Diarrhea is an imbalance in absorption and secretion of water and electrolytes. Diarrhea may be associated with a specific disease of the GI tract or with a disease outside the GI tract. 

Secretory diarrhea occurs when a stimulating substance (e.g., vasoactive intestinal peptide [VIP], laxatives, or bacterial toxin) increases secretion or decreases absorption of large amounts of water and electrolytes. 

Kendrick ML, Zyromski NJ, Tanaka T, Duenes DA, Libsch K, Sarr MG. Postprandial absorptive augmentation of water and electrolytes in the colon requires intraluminal glucose. J Gas-trointest Surg 2002 6 310-315. 

Two terms are often used /axat/Ves and caihartics. Cathartics produce prompt fluid evacuation, whiie iaxatives produce soft-formed stoois over a protracted period. Mechanisms of action vary, but the net overall effect is fluid accumuiation within the bowei iumen by a hydrophiiic action, an osmotic action and/or a direct action on mucosal cells to decrease absorption or to enhance secretion of water and electrolytes. Changes in Na-f-/K-i-ATPase, adenylyl cyclase and prostaglandins may be invoived in these actions. 

Q6 Loperamide hydrochloride is an opioid. The starting dose will be 4 mg, which can be reduced to 2 mg, three times a day for five days if necessary. Opioids act on // opiate receptors in the myenteric plexus of the intestine and may modulate acetylcholine release to reduce peristalsis. They trigger mucosal transport of ions and water out of the lumen and cause a reduction in secretion. The absorption of fluid and electrolytes is increased since the stool remains in the colon for a longer period. Loperamide does not produce sedation or other central effects associated with opiates, since it does not cross the blood-brain barrier. 

In 1989 a renal natriuretic factor (RNF) was detected for the first time and termed urodilatin (P. Schulz-Knappe et at). As examinations have hitherto shown, urodilatin is formed in the medial nephron of the kidney and causes a distad inhibition in the absorption of water and sodium. Its half-life is likewise about 3 minutes. Although ANF is most probably of limited significance for the excretion of sodium and its influence on sodium homoeo-stasis in cases of liver cirrhosis (with or without ascites) has still not been fully clarified, urodilatin is deemed to be important for the regulation of the water and electrolyte balance. (3) (s. tab. 16.5)... 

Secretory diarrhea occurs when a stimulating substance either increases secretion or decreases absorption of large amounts of water and electrolytes. Substances that cause excess secretion include vasoactive intestinal peptide (VIP) from a pancreatic tumor, unabsorbed dietary fat in steatorrhea, laxatives, hormones (such as secretin), bacterial toxins, and excessive bile salts. Many of these agents stimulate intracellular cyclic adenosine monophosphate and inhibit Na+/K+-ATPase, leading to increased secretion. Also, many of these mediators inhibit ion absorption simultaneously. Clinically, secretory diarrhea is recognized by large stool volumes (>1 L/ day) with normal ionic contents and osmolality approximately equal to plasma. Fasting does not alter the stool volume in these patients. 

OVERVIEW OF GI WATER AND ELECTROLYTE FLUX Huid content is the principal determinant of stool volume and consistency water normally accounts for 70-85% of total stool weight. Net stool fluid content reflects a balance between luminal input (ingestion and secretion of water and electrolytes) and output (absorption) along the length of the GI tract. The daily challenge... 

In summary, bile salts are catabolic products of cholesterol. Cholesterol is converted to bile salt in the liver. The liver cells excrete the bile salts into the biliary canaliculi from there they are poured into the intestines, where they exert their detergent effect and facilitate the absorption primarily of fats but also of water and electrolytes. The intestinal bacteria convert the primary bile salts into secondary bile salts, and the intestinal mucosa quickly reabsorbs both primary and secondary salts. All circulating biliary salts are reabsorbed by the liver, reconjugated, and resecreted in the bile. The formuls of the principal bile acids are given in Fig. 9-17 and the metabolism of bile acids is outlined in Fig. 9-18. 

Charlie Code reached his concept of the barrier by a different route. In the 1950s Code began to study the absorption of water, sodium, and potassium in the stomach and small intestine. At that time it was well known, chiefly as the result of the work of Maurice Visscher at the University of Minnesota, that there are unidirectional fluxes across the mucosa and that net flow of water and electrolytes is the result of two opposing fluxes. Visscher had said that flux from lumen to blood is out of the gut and that flux from blood to lumen is into the gut (Fig. 7-1 A). Anyone who has studied embryology knows that the lumen of the gut is outside the body and that Visscher s designation of directions is anatomically incorrect. The confusion was compounded by those who studied fluxes across everted sacs of intestine for them, out was from fluid outside the sac, and in was into fluid inside the sac. In 1960 Code proposed unambiguous nomenclature for unidirectional and net fluxes (Fig. 7-1Code s terms were not universally adopted, but I will use them here. 

Water and electrolytes. Each day in an average adult, about 5.51 of food and fluids move from the stomach to the small intestine as chyme. An additional 3.5 1 of pancreatic and intestinal secretions produce a total of 9 1 of material in the lumen. Most of this (>7.5 1) is absorbed from the small intestine. The absorption of nutrient molecules, which takes place primarily in the duodenum and jejunum, creates an osmotic gradient for the passive absorption of water. Sodium may be absorbed passively or actively. Passive absorption occurs when the electrochemical gradient favors the movement of Na+ between the absorptive cells through "leaky" tight junctions. Sodium is actively absorbed by way of transporters in the absorptive cell membrane. One type of transporter carries a Na+ ion and a Cl ion into the cell. Another carries a Na+ ion, a K+ ion, and two Cl ions into the cell. 

Billich, C. O., Levitan, R., Effects of sodium concentration and osmolality on water and electrolyte absorption form the intact human colon, J. Clin. Invest. 1969, 48, 1336-1347. 

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