Phosphorus Homeostasis

Question 1

• What is the relation between phosphorus and calcium?

Answer 1

85-90% of phosphorus is combined with calcium in bone

Question 2

• What are the two routes of phosphorus absorption from the intestine?

Answer 2

Passive diffusion & vitamin-D mediated active transport

Question 3

• What happens in response to low blood phosphorus levels?

Answer 3

Calcitriol increases phosphorus absorption from intestine and PTH mobilizes phosphorus from bone

Question 4

• What happens in response to high blood phosphorus levels?

Answer 4

PTH increases and more phosphorus is excreted in the urine and saliva

Question 5

• How is vitamin D3 produced?

Answer 5

7dehydrocholesterol is converted into cholecalciferol (vitamin D3) in the skin, converted to 25-hydroxycholecalciferol by the liver

Question 6

• Which enzymes regulated vitamin D3?

Answer 6

1α-hydroxylase – active vitamin D3, 24-hydroxylase – inactive vitamin D3

Question 7

• What are the four actions of calcitriol?

Answer 7

Increases calcium absorption from the intestine, increases phosphorus absorption from the intestine, decreases calcium and phosphorus excretion by the kidneys, allows normal functioning of bone

Question 8

• How does calcitriol affect phosphorus absorption?

Answer 8

Increases active transport

Question 9

• What are the possible causes of hypophosphataemia?

Answer 9

Rickets, phosphorus deficiency, refeeding syndrome, diabetes mellitus, primary hyperparathyroidism

Question 10

• What are the results of rickets?

Answer 10

Thickening of metaphyseal region of bones, bending of long bones

Question 11

• What are the effects of phosphorus deficiency in cattle?

Answer 11

Poor health, role in parturient paresis, postparturient hemoglobinuria

Question 12

• What are the effects of hypocalcaemia on hypophosphataemia?

Answer 12

PTH released in response to low calcium, increases urinary excretion of phosphorus

Question 13

• How does hypophosphataemia affect erythrocytes?

Answer 13

Effects their ability to produce ATP, inadequate ATP inhibits 3Na-2K ATPase pump, cells swell and rupture, haemoglobin passed in urine

Question 14

• What is refeeding syndrome?

Answer 14

Concentrated calories fed too quickly, phosphorus moves into tissue for increased ATP production, erythrocytes become deficient in ATP, oxygen carrying capacity is decreased

Question 15

• How can diabetes mellitus cause hypophosphataemia?

Answer 15

When insulin is administered glucose moves into the tissues, rapid production of ATP lowers plasma phosphorus levels resulting in haemolysis

Question 16

• How does primary hyperparathyroidism result in hypophosphataemia?

Answer 16

High PTH levels, loss of phosphorus in the urine