Calcium Homeostasis

Question 1

• What percentage of calcium is stored within bone?

Answer 1

99%

Question 2

• What three forms of calcium can be found in the blood?

Answer 2

40% bound to plasma proteins, 10% in complexes (chelated), 50% in an ionized (active) form

Question 3

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

Answer 3

Buffering & hormonal control

Question 4

• What are the three hormones involved in the control of calcium?

Answer 4

PTH, calcitonin & calcitriol (active vitamin D3)

Question 5

• If hypocalcaemic and need more calcium, which hormones are released?

Answer 5

Increased PTH & calcitriol

Question 6

• When is calcitonin secreted?

Answer 6

Severe hypercalcaemia (first response is to decrease PTH)

Question 7

• What is the source of PTH?

Answer 7

Secreted by the principal (chief) cells of the parathyroid glands

Question 8

• What is contained between the osteocytic membrane (layer of osteocytes and osteoblasts) and the bone?

Answer 8

Bone fluid

Question 9

• How is PTH secretion controlled?

Answer 9

Secreted continuously but increases as ECF Ca level decreases, membrane receptors coupled to G proteins which control exocytosis of PTH containing vesicles

Question 10

• What are the four actions of PTH within the body?

Answer 10

Bone: fast phase – increases calcium uptake from bone fluid, PTH increases membrane permeability to calcium
Bone: slow phase – activation of osteoclasts, bone resorption releases Ca and P – cannot take place without the presence of calcitriol
PTH & the kidney – increases calcium reabsorption in the late DT and CD, decreases phosphorus reabsorption in the PT
PTH & vitamin D – activates 1α-hydoxylase, catalyses the conversion to calcitriol in the kidney

Question 11

• What is osteolysis?

Answer 11

The replacement of calcium in the bone fluid by calcium phosphate crystals

Question 12

• What are the effects of calcitriol?

Answer 12

Increases calcium absorption from the intestine, decreases calcium excretion by the kidneys (weak affect compared to PTH)

Question 13

• How does calcitriol affect intestinal calcium uptake?

Answer 13

Increases the synthesis of calbindin (calcium binding protein), activates calcium ATPase pumps on the basolateral membranes

Question 14

• What two factors activate calcitriol?

Answer 14

PTH & low blood phosphorus levels

Question 15

• What effects does calcitonin have on bone?

Answer 15

Opposite affects to PTH

Question 16

• What cells secrete calcitonin?

Answer 16

Para follicular/C-cells

Question 17

• What factors stimulate secretion of calcitonin?

Answer 17

High levels of iCa2+ in plasma and gastrointestinal hormones such as gastrin & secretin

Question 18

• How is magnesium controlled?

Answer 18

No hormonal control, whole body balance between inflow and outflow, outflow via urine is affected by PTH – excessive outflow is not easily remedied