GIT - Water addition & Absorption

The gastrointestinal (GI) system plays a crucial role in the body's water balance, both through the addition (secretion) of fluids into the lumen and the absorption of water and electrolytes back into the body. This intricate process ensures efficient digestion, nutrient absorption, and the maintenance of overall fluid homeostasis.

Water Addition (Secretion) in the GI Tract

Across a 24-hour period, a significant volume of fluid, approximately 7 to 10 liters, enters the small intestine from various sources, including ingested food and drink (about 2 L) and GI secretions (about 7 L).

The primary sources of fluid added to the GI tract are:

• Saliva: Approximately 1 liter of saliva is secreted daily. Initial saliva from the acinus has an ionic composition similar to plasma, being isotonic with the same concentrations of Na+, K+, Cl-, and HCO3-. As saliva passes through the ducts, Na+ and Cl- are reabsorbed, and K+ and HCO3- are secreted, resulting in hypotonic saliva that is more dilute at lower flow rates because the ducts are relatively impermeable to water. Myoepithelial cell contractions can propel secretion into the main duct. Saliva lubricates food and protects the mouth and esophagus.

• Gastric Juice: The stomach secretes about 2 liters of gastric juice per day. Water in gastric juice acts as a medium for the action of acid and enzymes and helps solubilize food constituents. The secretion of acid by parietal cells involves the conversion of CO2 and H2O to H+ and HCO3-, with H+ and Cl- then secreted into the lumen to form HCl.

• Pancreatic Juice: Pancreatic secretion contributes approximately 2 liters of fluid daily. This secretion contains a high concentration of bicarbonate (HCO3-), which is essential for neutralizing the acidic chyme entering the duodenum. The pancreatic ducts are permeable to water, allowing water to move into the lumen and make the secretion isotonic.

• Bile: The liver continuously produces bile, contributing about 1 liter per day to the GI tract. The secretion of bile acids by hepatocytes drives water and electrolyte movement into the bile via osmotic filtration. Duct cells further add water and sodium bicarbonate (NaHCO3), a process stimulated by secretin. Most secreted bile is stored and concentrated in the gallbladder during fasting through the absorption of water and electrolytes, often increasing the concentration of bile salts and other solutes by 5 to 20 times. Water absorption in the gallbladder is a passive process, dependent on the active absorption of NaCl and NaHCO3.

• Small Intestinal Secretions: About 1 liter of fluid is secreted by the small intestine daily. Secretory mechanisms primarily reside in the crypt cells. Chloride (Cl-) is the main ion secreted into the intestinal lumen through channels in the apical membranes, often activated by increases in cAMP. Sodium (Na+) and water then passively follow Cl- to maintain isotonic conditions. This process is responsible for maintaining a liquid chyme.

Water Absorption in the GI Tract

Water absorption throughout the GI tract is a passive process, always occurring by diffusion down an osmotic gradient created by the active transport and absorption of solutes, including Na+, Cl-, other electrolytes, and nutrients like sugars and amino acids. Epithelial cells do not actively transport water.

The majority of water absorption occurs in the small intestine:

• Small Intestine: Of the 7 to 10 liters of fluid entering the small intestine, approximately 8.5 liters are absorbed here.

    ◦ The duodenum rapidly achieves osmotic equilibrium with plasma.

    ◦ The small intestine is considered a "leaky" epithelium, meaning its tight junctions are relatively permeable to water, facilitating rapid water movement.

    ◦ Sodium (Na+) absorption is a key driver of water absorption. Na+ is absorbed across the apical membrane of epithelial cells via several mechanisms, including restricted diffusion, cotransport with organic solutes (like glucose and amino acids), cotransport with Cl-, and countertransport in exchange for H+. For instance, glucose and galactose are transported into intestinal cells via a Na+-dependent cotransport (SGLT1) system in the luminal membrane, and water follows by osmosis.

    ◦ This solute-driven water absorption is crucial for food assimilation, which takes place primarily in the small intestine, aided by anatomical modifications like Kerckring’s folds, villi, and microvilli that greatly increase the luminal surface area.

• Large Intestine (Colon): About 0.4 to 0.5 liters of fluid is absorbed daily in the colon.

    ◦ This absorption of water and electrolytes transforms liquid chyme into a solid or semi-solid mass known as feces.

    ◦ In the colon, Na+ absorption primarily occurs through restricted diffusion, an electrogenic process that increases from oral to aboral regions and is regulated by mineralocorticoids like aldosterone. An electroneutral process involving Na+-H+ countertransport coupled with Cl--HCO3- countertransport also contributes.

    ◦ The colon's water permeability is considerably lower than that of the small intestine, which can lead to stool water being hypertonic to plasma.

Overall, of the 9 to 10 liters of water entering the GI tract daily, approximately 100 mL is excreted in the stool.

Disorders Affecting Water Balance in the GI Tract

Alterations in normal water absorption and secretion can lead to conditions like diarrhea:

• Osmotic Diarrhea: Occurs when nonabsorbable solutes accumulate in the small intestine, creating an osmotic difference that draws fluid into the lumen. Examples include disaccharidase deficiencies (e.g., lactase deficiency) or fat malabsorption.

• Secretory Diarrhea: Results from factors that stimulate excessive fluid secretion into the lumen, such as toxins (e.g., cholera toxin, Escherichia coli enterotoxins) or GI peptides (e.g., vasoactive intestinal peptide - VIP, secretin, prostaglandin), which increase Cl- secretion into the lumen, with Na+ and water following passively.

• Oral Rehydration Therapy (ORT) leverages the Na+-glucose cotransport mechanism to facilitate the absorption of water and salts, significantly reducing fluid loss in secretory diarrheas.

• Hypokalemia (low potassium) can result from excessive K+ loss in diarrheal fluid, as K+ is actively secreted in the colon, a process stimulated by aldosterone and increased flow rate.