Physiology · MU Sofia

51. Absorption of electrolytes, water, vitamins and bile acids in the gastrointestinal tract

Covers how sodium, calcium and iron are taken up in the small intestine and colon, with sodium mechanisms set out segment by segment, then water movement that follows solute transport. It describes absorption of fat-soluble and water-soluble vitamins, including the intrinsic factor pathway for vitamin B12, and closes with bile salt reabsorption through enterohepatic circulation. Includes three pictures and a set of recap questions.

Inside: 6 sections · 3 labelled pictures · 16 flashcards · 5 min

Topic 51 of 104 in the MU Sofia syllabus for Physiology (Dental Medicine, Year 2). The free AceMed note covers it in 6 sections, about 647 words (5 min of reading), with 16 flashcards.

In this note

  1. Absorption of electrolytes, water, vitamins and bile acids in the gastrointestinal tract.
  2. Electrolyte Absorption.
  3. Water Absorption
  4. Vitamin Absorption.
  5. Bile Acid Absorption.
  6. Recap Questions

Electrolyte Absorption.

The absorption of electrolytes, primarily sodium (Na+), is essential for maintaining fluid balance and overall physiological function. The small intestine and colon are the primary sites for electrolyte absorption, with specific mechanisms tailored to different segments.

  • Sodium (Na+) Absorption:
  • Nutrient-coupled absorption: The jejunum utilizes Na+-dependent cotransporters to absorb Na+ along with nutrients like glucose and amino acids. This mechanism leverages the electrochemical gradient established by the Na+/K+ ATPase pump on the basolateral membrane. This is similar to the process in the early proximal tubule of the kidney.
  • Electroneutral Na+/H+ exchange: This mechanism involves the exchange of Na+ for H+ across the apical membrane. It occurs predominantly in the jejunum, with minimal activity in the duodenum.
  • Na+/H+ and Cl-/HCO3- exchange: This coupled exchange mechanism operates in the ileum and proximal colon. Sodium absorption is linked to chloride (Cl-) influx and bicarbonate (HCO3-) efflux, ensuring electroneutrality.
  • Calcium (Ca2+) Absorption:
  • Passive paracellular absorption: Ca2+ can move passively between epithelial cells.
  • Active transcellular absorption: This process involves Ca2+ entry through channels on the apical membrane, binding to intracellular buffering proteins like calbindin, and subsequent transport across the basolateral membrane via antiporters (exchanging Ca2+ for H+ or Na+). The synthesis of calbindin is dependent on the active form of vitamin D (1,25-dihydroxycholecalciferol).

Vitamin D deficiency can lead to impaired Ca2+ absorption and conditions like rickets in children and osteomalacia in adults.

  • Iron (Fe2+) Absorption:
  • Iron is absorbed in two forms: free iron (Fe2+) and heme iron (iron bound to hemoglobin or myoglobin). Heme iron is broken down by lysosomal enzymes within intestinal cells, releasing free iron. Free iron binds to apoferritin and is transported across the basolateral membrane into the blood. In the circulation, iron binds to transferrin, a protein that delivers it to the liver for storage and to the bone marrow for hemoglobin synthesis.

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