Topic 59 - Electrolyte cycle of intestinal canal, water transport, iron absorption

Question 1

Words to include in electrolyte cycle of intestinal canal

Answer 1

• Reabsorption
• Digestive secretum
• ECF
• Homeostasis
• Small intestines
• Large intestines
• Osmotically
• Glucocorticoids
• Aldosterone
• Na⁺/K⁺ pump
• Na^+​​
  
  • Lumen
  • Enterocyte
  • Na⁺/K⁺​ pump
  • Na⁺-Cl^- co-transport
  • Aldosterone
• K⁺
  
  • ^​Mucosa cells
  • Interstitial lumen
  • Electromechanical gradient
• Ca²⁺
  
  • Duodenum
  • Proteins
  • Amino acids (lysine / arginine)
  • Citrate
  • 1,25-OH-D-hormone
  • Fatty acids
  • Absorption
• Mg²⁺
  
  • Reabsorption
  • MgSO₄
    
    • _​Hyperosmosis
    • Diarrhea
  • Proteins
    
    • Calcium complex
• Cl^-
  
  • Passive diffusion
  • Absoprtion
  • Na⁺/Cl^- cotransporter
• HCO₃^-
  
  • Pancreatic juice
  • Intestinal secretion
  • Saliva (ru)
  • Buffer
  • Absorption
  • H⁺
  • Carbonic acid
  • Water
  • CO₂
  • Lipid soluble

Question 2

Words to include in water transport

Answer 2

• GI secretion
• Small intestines
• Osmosis
• Absorption
• Reabsorption
• Lumen
• Hyperosmotic

Question 3

Words to include in iron absorption

Answer 3

• water-nonsoluble complex
• pH
• HCl secretion
• Fe³⁺
• Fe²⁺
• Absorbed
• Transferrin
  
  • Enterocyte
  • Duodenum
  • Jejunum
  • Fe transport
  • Transferrin-iron complex
  • Tf-Fe receptor
  • Endocytosis
• Ferritin
  
  • Absorption
  • Enterocytes
  • Irreversible
  • Apoferritin

Question 4

Topics to include in the essay

Answer 4

1. Electrolyte cycle of the intestinal canal
   
   • General
   • Na⁺
   • K⁺
   • Ca²⁺
   • Mg²⁺
   • Cl^-
   • HCO₃^-
2. ​Water transport
3. Iron absorption
   
   • Transferrin
   • Ferritin

Question 5

Electrolyte cycle of the intestinal canal

Reabsorption

Answer 5

• Reabsorption of digestive secretion delivered into the proximal part of the intestines is a critical function of the GI-tract
• Reabsorption:
  
  • Starts in the distal section of the small intestines
  • Ends in the first part of the large intestines
• Glucocorticoids (small and large intestines) and aldosterone (only colon) regulates the Na⁺ level by increasing its reabsorption
  
  • When Na⁺ is absorbed, K⁺ secretion increases

Question 6

Electrolyte cycle of the intestinal canal

Na⁺

Answer 6

• Na⁺ in lumen almost entirely reabsorbed
• Low IC Na⁺ concentration maintained by Na⁺/K⁺ pumps on the basolateral side of the enterocytes
• Na⁺ uptake on the luminal side of the enterocytes, facilitated by aldosterone:
  
  • Na⁺-Cl^- co-transport
  • Co-transport of Na⁺ with organic substances
  • Na⁺ absorbed on its own

Question 7

Electrolyte cycle of the intestinal canal

K⁺

Answer 7

• Moves either:
  
  • Into mucosa cells
  • Out towards the blood
  • Into the interstitial lumen
• Based on its electrochemical gradient

Question 8

Electrolyte cycle of the intestinal canal

Ca²⁺

Answer 8

• Absorption in duodenum
• Influenced by:
  
  • Proteins
  • Amino acids (lysine/arginine)
  • Citrate
  • 1,25-OH-D-hormone
• Fatty acids inhibit Ca²⁺ absorption → form calcium complexes

Question 9

Electrolyte cycle of the intestinal canal

Mg²⁺

Answer 9

• Low Mg²⁺ reabsorption in the upper region of the small intestines
  
  • But absorption occurs here
• Proteins facilitates its transport

Question 10

Electrolyte cycle of the intestinal canal

Cl^-

Answer 10

• Absorbed by passive diffusion in the initial region of the small intestines
• Absorption of Cl^- follows Na⁺ movement (Na⁺/Cl^- cotransporter)

Question 11

Electrolyte cycle of the intestinal canal

HCO₃^-

Answer 11

• Source of HCO₃^-:
  
  • Pancreatic juices
  • Intestinal secretion
  • Saliva (Ru)
• Role: buffers intestinal fluid
• Absorption of HCO₃^- is an indirect process:
  
  • H⁺ binds to HCO₃^- in lumen
  • Forming carbonic acid
  • Carbonic acid decays into water and CO₂
    
    • CO₂ is lipid-soluble and absorbed by enterocytes and into blood

Question 12

Water transport

Answer 12

• Water will follow the transport of the osmotically bound active substances
• Enters with the food and GI secretion
• The small intestine water transport is mainly regulated by osmosis
• By absorption of digested luminal contents (chyme), contents in the lumen become hyperosmotic
• Water reabsorption occurs

​

• ​This diffusion is so rapid, that practically there is no difference in the osmotic conditions in the lumen, it is usually isosmotic

Question 13

Iron absorption

General

Answer 13

• Iron transported to the animal by food is generally in water-nonsoluble complexes
  
  • These complexes dissociate at low pH, with the help of gastric HCl secretion
• Fe³⁺ reduced to Fe^{<strong>2+</strong>}
  
  • More easily absorbed
• ​ Fe transporters:
  
  • Transferrin
  • Ferritin

Question 14

Iron absorption

Transferrin

Answer 14

• Produced by enterocytes of duodenum and jejunum
• Role: Fe transport
  
  • Binds 2 Fe atoms to form transferrin-iron complex
  • Receptors on the surface of the brush border (Tf-Fe receptors) bind the transferrin-iron complex
  • Cell can take them up by endocytosis

Question 15

Iron absorption

Ferritin

Answer 15

• Role: protects against excessive Fe absorption
• Produced in enterocytes
• Irreversible binds the iron transported into the cell
  
  • If iron can not get out of the cell it will leave the body in the feces, since the enterocytes will detach from the mucosa
• Apoferrition: ferritin without iron
  
  • Stimulated by iron
  • Increased iron → increased apoferritin