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Endocrine & Metabolism
MAKEUP: Calcium absorption & metabolism
General: Calcium is a vital ion
Required for:
- Membrane excitation: Most membrane depolarisation requires rapid
influx of Ca2+
- Homeostasis: Clotting factors require Ca2+ for activation
o Coagulopathy doesn’t occur with hypocalcaemia → to get that low
would be incompatible with life
- Muscle contraction: excitation-contraction coupling
- Excitation-Secretion: Secretion for endocrine & exocrine organs require
Ca2+ influx. Neurotransmitter release as well.
- Structural support: Ca2+ bound to outer surface cell membranes for
structural stability and intercellular adhesion
- Normal range total calcium 2.45-2.55mmol/L
- Readily exchangeable pool: 1%
- Non-readily exachangeable pool: 99%
- Plasma Calcium
o Free / ionised calcium (diffusible): 45%
o Complexed with HCO3- / citrate (diffusible): 10%
o Bound to globulins / albumin (non-diffusible): 45%
Calcium is under tight hormonal control → Release of hormones is based on
plasma [Ca2+]
Parathyroid Hormone (PTH)
- Produced in chief cells of parathyroid glands
o Pre-prohormone → cleaved to prohormone → hormone (ER &
Golgi apparatus)
- ↓[Ca ] → ↑synthesis / release PTH
- Actions: Overall effect → ↑[Ca2+] plasma
o Kidney:
 ↑DCT / CD Ca2+ reabsorption,
 ↓phosphate reabsorption in PCT
 ↑1,25-dihydroxycholecalciferol (calcitriol) production
o Bone: ↑bone resorption / ↓bone formation → ↑Ca2+ / phosphate
 Initial rapid phase → osteolysis (osteoblast, osteocyte
 Slow phase → ↑osteoclast activity
Vitamin D
- Cholecalciferol (Vit D3) produced in skin exposed to UV light
- Hydroxylated in liver → 25-hydroxycholecalciferol
- Hydroxylated in proximal nephron → 1,25-dihydroxycholecalciferol
o Hydroxylase activity dependent on PTH
By Amanda Diaz
Endocrine & Metabolism
o GIT: Calcitriol → ↑Ca2+ absorption in SI by ↑Calcium binding protein
in mucosa
o Bone: ↑bone reabsorption
o Kidney: ↑Ca2+ and phosphate reabsorption from PCT
- Secreted by parafollicular cells of thyroid
o Stimulated by ↑[Ca2+] > 2.4mmol/L, stimulated by gastrin
- Actions:
o Bone: Direct inhibition of osteoblast activity (↓bone reabsorption)
o Kidney:
 ↑Ca2+ / phosphate excretion
 ↓hydroxylase activity → ↓calcitriol synthesis
o GIT: ↓jejunal absorption of dietary calcium
By Amanda Diaz