Endocrine Physiology - Calcium

Question 1

How does the body control calcium?

Answer 1

3 Mechanisms:
Absorption via GI tract
Excretion via Kidney
Mobilisation to/from bone

Components of the process:

Vitamin D (Cholecalciferol)
Formed in the skin by UV light, hydroxylated in liver to form 25-hydroxycholecalciferol,

Hydroxylated again by 1-alpha-hydroxylase in kidney to form 1,25 dihydroxycholecalciferol.

Increases calcium/phosphate mobilisation from bone, reabsorption by kidney, and absorption by GI tract

Parathyroid Hormone (PTH)
Peptide hormone produced by chief cells in parathyroid, in response to hypocalcaemia or hypomagnesaemia
Increases calcium mobilisation from bone (acts on osteoclasts & blasts)
Increases calcium reabsorption by kidney in loop of henle, DCT and collecting duct
Increases phosphate excretion by kidney in PCT
Increases renal 1,25-dihydroxycholecalciferol production by upregulating 1-alpha-hydroxylase, influencing GI tract absorption via this process.

Calcitonin
Produced by parafollicular cells in thyroid in response to hypercalcaemia, gastrin, catacholamines

Reduces calcium mobilisation from bone, increaess calcium/phosphate excretion by kidney, decreases calcium absorption by the GI tract

Question 2

Where is calcium stored in the body

IMAGES

Answer 2

Total of ~1.2kg of calcium in the body
98% as hydroxyapatite in skeleton (calcium, phosphate and hydroxide)
1% in tissues
1% in extracellular fluid (50% ionised, 10% complexed (ion couplets with bicarbonate/citrate etc), 40% protein bound (90% albumin, 10% globulins)

Albumin has 12 calcium binding sites, 10-15% taken up by calcium usually.

Question 3

What are normal calcium levels, and what are the causes of hypercalcaemia?

Answer 3

Total calcium 2.2-2.7 mmol/L
Technically all calcium is ionised, but it is used conventionally to describe free ionic portion which is physiologically active.

Causes:
Hyper Calcaemia Suggests Too Many Frappuccinos

Hyperparathyroidism
Cancer
Sarcoid
Thyrotoxicosis
Milk Alkali syndrome
Familiam hypocalciuric hypercalcaemia