Calcium, Phosphate & Magnesium Homeostasis

Question 1

what is the physiological importance of calcium?

Answer 1

important for

• blood clotting
• muscle contraction
• neuronal excitation
• enzyme activity (NA/K ATPase, hexokinase)

Question 2

what is the structural importance of calcium

Answer 2

• Hydroxyapatite Ca₁₀(PO₄)₆(OH)₂ is the predominant mineral in bone

Question 3

what does the total Ca equal

Answer 3

Total Ca = Ionised Ca + Bound Ca + Complexed Ca

Question 4

what is the total body calcium split into?

Answer 4

Bone - 99%
intracellular - 1%
Extracellular - 0.1%

Question 5

At what values does lack or excess of calcium considered a medical emergency

Answer 5

calcium < 1.6 or >3.5 mmol/l

Question 6

What is the adjusted Calcium EQUAL

Answer 6

Adjusted Ca = Total Ca + [ (40 - Alb) x 0.025 ]

Question 7

what is the reference range for adjusted calcium

Answer 7

2.2 -2.6 mmol/l

Question 8

what is the physiological importance of phosphate

Answer 8

• The P in ATP - our fuel
• intracellular signalling
• cellular metabolic processes e.g. glycolysis

Question 9

what is the structural importance of phosphate

Answer 9

• backbone of DNA
• component of hydroxyapatite Ca₁₀(PO₄)₆(OH)₂
• membrane phospholipids

Question 10

what is the distribution of total body phosphorus - 700g?

Answer 10

bone - 85%
intracellular - 14%
extracellular -1%

Question 11

what is the distribution phosphorous in the blood?

Answer 11

Organic form (covalently bound) - 70%

inorganic form as phosphate - 30%

Question 12

what is the normal range for phosphate in adults

Answer 12

0.8 -1.5mmol/l

Question 13

what are the two key controlling factors of homeostasis of calcium?

Answer 13

1. parathyroid hormone (PTH)

2. Vitamin D and metabolites

Question 14

calcium homeostasis is the result of a balance of which processes?

Answer 14

• GI uptake
• Bone Storage
• Renal Clearance

Question 15

what is the function of PTH?

Answer 15

• PTH acts on bone to drive resorption of Ca and PO4
• PTH acts on kidneys to increase reabsorption of Ca from the filtrate but increase excretion of PO4
• PTH also acts on kidneys to increase conversion of vitamin D to its active form which increases Ca and PO4 absorption from the gut

Question 16

what is the net effect of PTH?

Answer 16

• Increase Serum Calcium

- Decrease Serum Phosphate

Question 17

what effect does calcium levels have on PTH?

Answer 17

High calcium inhibits release by negative feedback

low calcium stimulates PTH release

Question 18

what factors affect your vitamin D level?

Answer 18

• the season
• latitude/climate
• clothing
• use of sunscreen
• time spent indoors/outdoors
• skin tone
• Age
• Diet
• Body fat and BMI
• Malabsorption

Question 19

what are the other regulators of calcium and phosphate homeostasis

Answer 19

• PTH
• Vitamin D
• FGF23
• Calcitonin
• Oestrogen

Question 20

what is the role of FGS23 in the regulation of calcium and phosphate homeostasis?

Answer 20

Increases renal phosphate excretion

Question 21

what is the role of calcitonin in regulating calcium and phosphate homeostasis?

Answer 21

• Opposes the effect of PTH by acting on osteoclasts to inhibit bone resorption
• function is usually insignificant in the regulation of normal calcium homeostasis

Question 22

what are the common causes of hypocalcaemia?

Answer 22

• Vitamin D deficiency
• Inadequate dietary calcium intake
• Hypoalbuminaemia
• High phosphate
• Hypoparathyroidism
• EDTA contamination
• Citrate Contamination

Question 23

what are the signs and symptoms of hypocalcaemia?

Answer 23

• Tetany
• Paraesthesia in the extremities
• Cramps
• Convulsions
• Psychosis

Question 24

what are the signs and symptoms of hypercalcaemia?

Answer 24

Stones
- Renal stones due to hypercalciuria causing renal colic
Bone 
pain and osteoporosis 
Moans
- lethargy, fatigue, depression
Groans 
- Abdominal pain, constipation, nausea, vomitting

Question 25

what are the causes of phosphate deficiency

Answer 25

Low intake

• malnutrition
• Malabsorption
• Alcoholism

Excess losses

• Hyperparathyroidism
• Renal tubular damage
• Diarrhoea

ECF/ICF Redistribution

• Refeeding syndrome
• Alkalosis

Question 26

what are the signs/symptoms of phosphate deficiency

Answer 26

• Severe muscle weakness, respiratory muscle failure and rhabdomyolysis
• Haemolysis, thrombocytopenia and poor granulocyte function
• convulsions, coma, death
• rickets/osteomalacia

Question 27

what are the common causes of high phosphate?

Answer 27

Renal failure: acute kidney injury or chronic kidney disease

Hypothyroidism

Spurious

• Haemolysis
• Delayed separation of sample
• Assay interference

Question 28

what is the physiological importance of magnesium?

Answer 28

• Cofactor for ATP - our fuel
• Neuromuscular excitability
• Enzymatic function
• regulates ion channels

Question 29

what is the structural importance of magnesium?

Answer 29

comprises 0.5 - 1% of bone matrix

Question 30

what is the total body distribution of magnesium (-1.1 mol)

Answer 30

Bone - 54%
Intracellular (45%)
Extracellular (1%)

Question 31

what is the distribution of magnesium in the blood?

Answer 31

ionised free Mg2+ (60%)

bound to plasma proteins (25%)

complexed to anions (15%)

Question 32

what predominantly controls homeostasis of magnesium

Answer 32

by the kidneys

Question 33

what are the causes of magnesium depletion?

Answer 33

• Inadequate intake
  
  • malnutrition
  • malabsorption
  • total parenteral nutrition
• Renal loss
  
  • Drugs
  • antibiotics
  • chemotherapy
  • diuretics
  • FK506
• GI Loss
  
  • Diarrhoea
  • PPIs
• Redistribution into cells

Question 34

signs and symptoms of magnesium depletion?

Answer 34

• Neuromuscular hyperexcitability, muscle weakness
• CNS - depression, psychosis
• Cardiovascular - ECG changes
• GI - nausea and anorexia
• Biochemical consequences

Question 35

what are the causes of hypermagnesaemia?

Answer 35

usually Iatrogenic

• cardiac surgery
• Pre-eclampsia