Fluid and Electrolytes

• How much water is ingested daily into the gastrointestinal tract?

  About 2L of water is ingested daily.
• How much water enters the GI tract from secretions daily?

  About 7L of water enters the GI tract from secretions.
• How much water is lost in feces daily?

  Only about 100-200ml of water is lost in feces.
• How much water does the GI tract absorb daily?

  The GI tract absorbs about 9L of water per day.
• Where is the majority of water absorbed in the GI tract?

  1. 5L is absorbed in the small intestine and about 1.5L in the colon.
• What happens to the osmolarity of chyme as it moves from the stomach to the colon?

  The osmolarity generally decreases due to the absorption of water and solutes.
• What causes the significant drop in osmolarity in the duodenum?

  Pancreatic juices and bile neutralize stomach acid, facilitating nutrient absorption.
• What is the state of chyme by the time it reaches the colon?

  Most nutrients have been absorbed, leaving a more concentrated, slightly hypertonic stool.
• What is the osmolarity of chyme leaving the stomach?

  Chyme leaving the stomach is relatively hypertonic.
• How do pancreatic juices affect chyme in the duodenum?

  They are rich in bicarbonate ions, which neutralize stomach acid and reduce osmolarity.
• What role does bile play in digestion?

  Bile aids in digestion and absorption, contributing to the decrease in osmolarity.
• How does the intestinal lining affect osmolarity as chyme travels through the small intestine?

  The intestinal lining actively transports solutes like sodium and glucose, drawing water via osmosis.
• What is the primary function of the colon regarding water absorption?

  The primary function is to absorb remaining water, resulting in a further decrease in volume.
• How are sodium ions absorbed in the small intestine?

  Sodium ions are primarily absorbed through active transport mechanisms coupled with glucose and amino acids.
• What drives water reabsorption in the colon?

  Sodium absorption is the key driver of water reabsorption, primarily through aldosterone's action.
• How does aldosterone facilitate sodium absorption in the colon?

  Aldosterone facilitates sodium absorption by increasing the movement of sodium ions across epithelial cells.
• How does sodium enter the epithelial cells in the colon?

  Sodium enters through epithelial sodium channels (ENaC) driven by the electrochemical gradient.
• What is the role of aldosterone in potassium secretion?

  Aldosterone indirectly stimulates potassium secretion by maintaining the sodium gradient necessary for potassium channels to function.
• How is bicarbonate transported into the lumen of the colon?

  Bicarbonate is actively transported into the lumen via specific bicarbonate transporters, often coupled with chloride ions.
• How does aldosterone affect bicarbonate transport in the colon?

  Aldosterone increases the activity of apical bicarbonate-chloride exchangers in non-A intercalated cells.
• What are the key processes involved in water absorption in the GI tract?

  • Ingestion of water (2L daily)
  • Secretions contributing water (7L daily)
  • Absorption in the small intestine (7.5L)
  • Absorption in the colon (1.5L)
  • Loss in feces (100-200ml)
• What are the main components affecting osmolarity of chyme as it moves through the GI tract?

  • Initial hypertonicity due to digestive enzymes and stomach acid
  • Neutralization by pancreatic juices and bile in the duodenum
  • Active transport of solutes in the small intestine
  • Water absorption in the colon leading to semi-solid stool
• What are the mechanisms of nutrient absorption in the small intestine?

  • Sodium absorption via active transport
  • Glucose and amino acids coupled with sodium
  • Iron absorption via specific transporter in the duodenum
  • Calcium absorption dependent on vitamin D
• What is the role of aldosterone in the colon?

  • Facilitates sodium absorption
  • Increases water reabsorption
  • Stimulates potassium secretion
  • Enhances bicarbonate transport