Physiology Of Calcium Regulation

Question 1

Where is the majority of calcium in the body stored?

Answer 1

Within bone ECM

Question 2

Total calcium in the blood consists of

Answer 2

40% bound to plasma proteins
10% in complexes (citrate, phosphate)
50% in ionised (active) form

Question 3

What portion of calcium is monitored by the parathyroid gland?

Answer 3

Ionised calcium

Question 4

What is PTH and where is it produced?

Answer 4

Parathyroid hormone

Produced by Chief cells of the external and internal parathyroid glands

Question 5

Where is calcitonin produced?

Answer 5

Produced by parafollicular (C-cells) of the thyroid

Question 6

What is calcitriol and where is it produced?

Answer 6

Active vitamin D3

Activated in the kidney

Question 7

If an animal is hypocalcaemic, what is the body’s response?

Answer 7

Increased PTH secretion

Production of more activated vitamin D3

Question 8

If an animal is hypercalcaemic, what is the body’s response?

Answer 8

Decreased PTH secretion

If the hypercalcaemiq is severe: calcitonin is produced

Question 9

What are the four actions of PTH within the body?

Answer 9

1: Bone - fast phase - calcium from bone fluid
2: Bone - slow phase
3: Kidney - re absorption in tubules to recover more calcium from urinary filtrate
4: Intestines - indirect effect through the activation of Vitamin D to get calcium from the gut

Question 10

Describe the fast phase of releasing calcium from bone

Answer 10

PTH acts on existing osteocytes and osteoblasts
Action is within minutes and progressively increases for hours
Calcium is taken up from bone fluid

Question 11

Describe the slow phase of releasing calcium from bone

Answer 11

Activation of existing osteoclasts and new osteoclasts are formed
This takes 48hrs-days and results in progressive depletion of bone mineral

Question 12

Describe the action of PTH on the kidney

Answer 12

PTH increases calcium re absorption in the late distal tubules and collecting tubules
This results in retention of Ca and Mg
There is decreased phosphorus reabsorption in the renal proximal tubule —> results in rapid loss of phosphorus

Question 13

Describe the role of PTH in the activation of vitamin D

Answer 13

Final conversion step to active vitamin D (aka calcitriol) occurs in the renal tubules
It is catalysed by the enzyme 1-alpha-hydroxylase which is activated by PTH
Rise in PTH —> more calcitriol

Question 14

What are the effects of calcitriol?

Answer 14

Increases calcium absorption from the intestine
Decreases calcium excretion by the kidneys
Is needed for normal functioning of bone (osteoblasts and osteoclasts)

Question 15

What effect does calcitonin have on the body?

Answer 15

Stimulated by hypercalcaemia - lowers blood calcium
Opposite (and weaker effect) compared to PTH
Fast phase - puts calcium into bone fluid
Slow phase - puts calcium into bone

Question 16

What are the two lines of defence against fluctuations in blood calcium?

Answer 16

1. First line - buffering (exchangeable calcium in bone salts and mitochondria)
2. Second line - hormonal control (PTH, calcitonin, calcitriol)

Question 17

What is the normal range for total calcium?

Answer 17

2.2 - 2.6 mmol/L

Question 18

What form of calcium is free to interact with tissues?

Answer 18

Ionised calcium

Question 19

Normal range for ionised calcium?

Answer 19

1.00-1.25 mmol/L

Question 20

Describe the effects of PTH on phosphorus homeostasis

Answer 20

Decreased renal phosphate reabsorption

Increased intestinal phosphate (and calcium) absorption