59. Electrolyte cycle of intestinal canal, water transport, iron absorption

Question 1

Electrolyte and water transport

Answer 1

• One of the most important critical functions of the GI-tract is the reabsorption of digestive secretion delivered into the proximal part of the intestines.
• The sum of GI secretion forms a substantial fraction of ECF from which they are derived.
• These compounds must return to ECF in order to maintain homeostasis
• Reabsorption starts in the distal section of the small intestines, and ends in the first part of the large intestines.
• Water will follow the transport of the osmotically bound active substances.
• 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 2

Water

Answer 2

• Enters with the food and GI secretion
• The small intestine water transport is mainly regulated by osmosis.
• By absorption of digested luminal contents, contents in the lumen become hyperosmotic, and water reabsorption rapidly occurs.

Question 3

Sodium

Answer 3

• Enters the lumen by secretion, and is almost entirely reabsorbed.
• Three fundamental Na+ uptake mechanisms on the luminal side of enterocytes:
• Na+/Cl- co-transport
• Na+/organic substance co-transport
• Na+ being absorbed by its own
• Aldosterone facilitate all these processes.

Question 4

Potassium

Answer 4

• Moves into mucosa cells, out towards the blood, or into the interstitial lumen based on its electrochemical gradient.

Question 5

Calcium

Answer 5

• Absorption occurs in the upper region of the small intestines.
• It is influenced by proteins, amino acids (Lys, Arg), citrate and 1,25-OH-D-hormone.
• Its insoluble compound is not absorbed, and can be bound to oxalate or free fatty acids.

Question 6

Magnesium

Answer 6

• Absorption occurs in the upper region of the small intestines
• Protein facilitates its transport
• High concentration of Mg2+ obstruct water reabsorption, thus water containing MgSO4 (“bitter water”) cause diarrhea and hyperosmosis.

Question 7

Chloride

Answer 7

• Absorbed mainly by rapid passive diffusion following Na+ in small intestines.

Question 8

Bicarbonate

Answer 8

• Comes from pancreatic juice, intestinal secretion and (in Ru) saliva.
• Has an important buffer role in the intestinal fluid.
• Absorption is an indirect process:
• Mucosa cells exchange hydrogen ions with sodium ion absorption
• H+ binds to bicarbonate in the lumen forming carbonic acid. Carbonic acid decays to water and CO2 which can freely diffuse into the blood.

Question 9

Absorption of iron

Answer 9

• Iron is absorbed according to the need of the animal, therefore the absorption of iron in e.g. immature and pregnant animals is significantly increased.
• Iron transported to the animal by food is generally in water-nonsoluble complexes, which abstract absorption.
• These complexes dissociate at low pH, with the help of gastric HCl secretion.
• The destruction of the complexes reduces Fe3+ to Fe2+.
• Cellular level: Enterocytes in the duodenum and jejunum secrete an iron-binding protein into the lumen → Transferrin (Tf)
• Transferrin binds two iron ions and forms a Tf-Fe complex.
• Receptors at the surface of the brush border bind the complex, and by endocytosis the cell can take them up.• In the cytoplasm, the receptor is released, and the complex moves to the basolateral side where iron s released.
• The free transferring molecules are secreted back to the intestinal lumen.
• Ferritrin will bind to excess Fe molecules!