Lecture 11 : Absorption of Ions and Water

Question 1

Transcellular and paracellular definitions

Answer 1

Transcellular - Through cells via transport channels, passive or active, controlled and selective

Paracellular - Between cell junctions, passive only, less selective

Question 2

How does water absorption work

Answer 2

Water absorption depends mainly on ion absorption, Na⁺ and Cl⁻.

NaCl transport into intercellular space → makes space hypertonic → water flows osmotically from lumen via tight junctions.

Question 3

How is Na+ absorbed along intestine

Answer 3

4 main routes

1. Na⁺/Glucose or Na⁺/Amino acid Transport (SGLT1)
   - In jejunum
   - Uses steeper electrochemical Na+ gradient to cotransport Na+ into cell from outside, with glucose or neutral amino acid attached
   - However Na+ / K+ ATPase pump needed to maintain steep Na+ electrochemical fradient
2. Na⁺/H⁺ Exchanger
   - In jejunum
   - Na+ into cell for H+ out
   - in neutral / alkaline pH, due to lots of HCO3-
   - Good for absorbing Na+ when no glucose available
3. Parallel Na⁺/H⁺ and Cl⁻/HCO₃⁻ Exchanger
   - Electrically neutral
   - Dominant during fasting
   - Na+ diffuses in, H+ diffuses out, Cl- diffuses in, HCO3- diffuses out, so no net charge change across membrane
   - Regulated by cAMP, cGMP, Ca²⁺.
   - No glucose required
   - Good for NaCl absorption, maintaining electrolyte balance
4. Na⁺/K⁺ ATPase (basolateral)
   - Maintains low intracellular Na⁺, so high diffusion gradient from Na+ outside cell to inside
   - 3 Na+ pumped out, 2K+ pumped in, using ATP

Question 4

Cholera / E. coli Enterotoxin Effect

Answer 4

Binds enterocytes → ↑ cAMP → ↑ Cl⁻ secretion, ↓ Na⁺/Cl⁻ uptake → water loss → diarrhoea.

Question 5

Calcium (Ca²⁺) Absorption

Answer 5

Can be done passively, paracellularly, throughout small intestine

Can be done actively, transcellularly, mainly in duodenum, regulated by Vitamin D.

VDR, a vitamin D receptor, upregulates Ca²⁺ transport proteins.

Deficiency of this vitamin D receptor = Hypocalcaemia → bone softening

Question 6

Iron in humans

Answer 6

Roles :
- Oxygen transport in haemoglobin and myoglobin
- Energy production in electron transport chain
- Comprises enzymes such as cytochrome P450 or DNA Helicase
.
Sources :
- Haem iron, animals
- Non-haem iron, conversion from other molecules
.
Absorption :
- Fe³⁺ → Fe²⁺
- Via ferric reductase, into a DMT1 transporter

• Fe²⁺ → Fe³⁺ in enterocytes, by hephaestin enzyme
• Exported via ferroportin, binds to transferrin in plasma
• Stored in liver, released by ferroportin
• Released in spleen, where RBCs are recycled
  .
  Regulation of iron in blood
• Hepcidin produced by liver inhibits ferroportin, decreases blood Iron conc

Question 7

Heme Metabolism

Answer 7

Heme is from haemoglobin and myoglobin.

Heme oxidase extracts Fe from heme → leaves porphyrin.

Porphyrin → Biliverdin → Bilirubin
(via biliverdin reductase)

Question 8

Types of Anaemia

Answer 8

Iron Deficiency Anaemia
- Low iron intake
- ↓ Ferritin

Anaemia of Chronic Disease
- Due to inflammation
- ↑ Hepcidin → ↓ Fe absorption + ↑ RBC destruction
- Ferritin levels ↑