12 - Absorption of Ions and Water

Vocabulary flashcards covering water and ion absorption, transport mechanisms, effects of enterotoxins, calcium and iron absorption, and regulatory factors like hepcidin and related diseases.

Water absorption depends on ions

Water uptake in the intestine is driven by the absorption of ions (principally Na+ and Cl−); creates a hypertonic NaCl region near the apical intercellular space, pulling water through tight junctions into the intercellular spaces.

Transcellular transport

Solutes cross two cell membranes, typically via active transport (ATP), moving from the intestinal lumen into the blood.

Paracellular transport

Solutes move passively between adjacent cells through tight junctions, without crossing cell membranes.

SGLT1 (Na+/Glucose transporter)

Luminal Na+-dependent transporter in the jejunum that brings Na+ in with glucose/galactose and neutral amino acids, helping Na+ absorption.

Na+/H+ exchanger

Transporter exchanging Na+ for H+; contributes to Na+ absorption and is modulated by luminal pH.

Na+-H and Cl−-HCO3− exchange (electroneutral)

Coupled Na+ and Cl− absorption with H+ and HCO3− exchange; electroneutral and regulated by intracellular signals (cAMP/cGMP and Ca2+).

E. coli enterotoxin mechanism

Enterotoxin binds enterocytes, is internalized, increases cAMP via Gs, enhancing Cl− secretion and reducing Na+/Cl− uptake, causing diarrhea.

TRPV6

Apical calcium channel in enterocytes that mediates Ca2+ uptake in the duodenum (active Ca2+ absorption).

Calbindin-D9K

Calcium-binding protein that buffers intracellular Ca2+ and aids transfer to the basolateral side.

PMCA

Plasma membrane Ca2+ ATPase; pumps Ca2+ out of enterocytes to maintain low intracellular Ca2+.

NCX

Sodium–calcium exchanger; exports Ca2+ from enterocytes in exchange for Na+.

Vitamin D receptor (VDR)

Nuclear receptor; upon activation by vitamin D, increases transcription of Ca2+ transport proteins to promote Ca2+ uptake.

Hypocalcaemia

Low levels of calcium in the blood; can result from vitamin D deficiency and impaired Ca2+ absorption.

DMT1 (divalent metal transporter 1)

Transporter that mediates import of Fe2+ (ferrous iron) into enterocytes.

Ferric reductase

Enzyme that reduces Fe3+ to Fe2+ in the intestinal lumen to enable DMT1 uptake.

Hephaestin

Cu-containing oxidase that oxidizes Fe2+ to Fe3+ inside enterocytes for binding to transferrin in blood.

Ferroportin

Cellular iron exporter on enterocytes and macrophages; releases Fe2+ into circulation; inhibited by hepcidin.

Hepcidin

Master regulator of iron homeostasis; inhibits ferroportin, lowering plasma iron and intestinal iron absorption.

IL-6

Inflammatory cytokine that stimulates hepcidin production, linking inflammation to iron sequestration.

Haemochromatosis (HFE mutation)

Mutation in HFE gene associated with iron overload; leads to reduced or dysregulated hepcidin and increased tissue iron.

Transferrin

Plasma protein that binds ferric iron (Fe3+) for transport to tissues.

Ferritin

Intracellular iron storage protein; levels reflect iron stores in the body.

Anemia of Chronic Disease (ACD)

Anemia associated with chronic inflammation; high hepcidin, reduced erythropoiesis, often high ferritin.

Iron Deficiency Anemia (IDA)

Anemia caused by insufficient iron intake/absorption; typically low ferritin and low available iron.

Ferritin as diagnostic discriminator

Low ferritin suggests IDA, while high ferritin suggests ACD or iron overload conditions.

Duodenum as site of iron absorption

Iron absorption primarily occurs in the duodenum via Fe2+ uptake (DMT1) and export (ferroportin) into blood.