Renal regulation of minerals

Question 1

How is calcium moved in the blood?

Answer 1

50% ionised (biologically active, filtered)
40% protein-bound
10% chelated to anions (e.g. Phosphate)

Question 2

The bone acts as a reservoir for what minerals?

Answer 2

calcium, magnesium and phosphate

Question 3

What can cause a loss of calcium?

Answer 3

Excess P intake or diets high in cation binders such as cereals may weaken bones and oxalates may also bind calcium and this can lead to nutritional secondary hyperparathyroidism

Question 4

What increases the gut absorption of calcium?

Answer 4

Vit D increases efficiency of gut absorption

Question 5

How is calcium reabsorbed?

Answer 5

Via Epithelial apical Ca channels
Intracellular transport by Ca-binding proteins (up-regulated by Vit-D)
Basolateral Ca ATPase (stimulated by PTH and Vit D)

Question 6

Hypocalcaemia will generally lead to what type of thyroid disease?

Answer 6

secondary hyperparathyroidism

Question 7

Hypercalcaemia will generally lead to what type of thyroid disease?

Answer 7

secondary hypoparathyroidism

Question 8

How is phosphate usually stored in the body?

Answer 8

80-90% inorganic in bone (hydroxyapatite)

15% organic in soft-tissues (major intracell anion)

Question 9

What causes an increase in dietary absorption of P?

Answer 9

Vit D

Question 10

Free phospahate (not protein bound) is filtered by the kidney so it must be reabsorbed, where does this take place?

Answer 10

80% uptake in PT transcellularly with Na (inhibited by PTH)
No significant uptake in the loop
Some uptake in the CD and DT

Question 11

How does CKD cause hypophosphataemia and hypocalacamia?

Answer 11

CKD increases blood parathyroid hormone (PTH) levels which stimulates renal excretion of phosphate
PTH remains high = Ca and P release from bone (worsens situation)
Hyperphosphatemia also inhibits production of calcitriol and therefore reduces intestinal calcium absorption

Question 12

How is magnesium stored in the body?

Answer 12

54% bone, 45% soft-tissues, 1% ECF (active, filtered)

Question 13

Non protein-bound magnesium is freely filtered so it must be reabsorbed, where does this take place?

Answer 13

30% reabsorbed in PT, Passive driven by trans-epithelial gradient
65% reabsorbed in Thick Ascending Limb (unusual).
5% in DT (active uptake)
Apical Mg channel and basolateral Mg ATPase

Question 14

What effect does PTH and calcitonin have on Mg absorption/excretion?

Answer 14

PTH and calcitonin increase passive Mg reabsorption

Question 15

What effect do loop diuretics have on Mg absorption/excretion?

Answer 15

Loop diuretics and thiazides increase Mg excretion

Question 16

What is the effect of secondary hyperparathyroidism (excessive PTH) on phosphate and calcium?

Answer 16

Renal failure causes a decrease in P excretion so serum P rises.
This increase in serum P binds to Ca (which is just like decreasing free calcium) and this will increase PTH production more