calcium homeostasis

Question 1

role of calcium in the body

Answer 1

• signalling (Ca2+ important signalling molecule)
• blood clotting
• apoptosis
• skeletal strength
• membrane excitability (Ca2+ decreases Na+ permeability)

Question 2

Ca2+ and membrane excitability - hypocalaemia

Answer 2

increases neuronal Na+ permeability leading to hyperexcitation of neurones
in extreme cases causes tetany, if spreads to larynx and resp muscles - asphyxiation

Question 3

Ca2+ and membrane excitability - hypercalaemia

Answer 3

decrease in neuronal Na+ permeability which will reduce excitability and depress neuromuscular activity and in extreme cases, trigger cardiac arrythmias

Question 4

calcium distribution in body

Answer 4

bones 99%
Intracellular 0.9%
extracellular 0.1%

Question 5

calcium in extracellular fluid

Answer 5

nearly half bound to protein so only 0.05% of calcium in body is free in solution and physiologically active

Question 6

physiologically active calcium

Answer 6

0.05% in ECF

Question 7

why is phosphate homeostasis also important in determining calcium balance

Answer 7

calcium stored in calcified extracellular matrix of bone is mostly in the form of hydroxyapatite
(Ca10(PO4)6(OH)2)

Question 8

how much calcium in plasma

Answer 8

2.2-2.5mM

Question 9

extracellular calcium stores

Answer 9

calcium has high affinity for proteins an in plasma around 40% is bound to plasma proteins

10% are neither bound to plasma protein or free in solution, they bind to plasma anions

Question 10

how much calcium is physiologically active

Answer 10

about 1.2mM (~50% plasma calcium)

Question 11

calcium distribution in the blood

Answer 11

40% protein bound (albumin 80%, globulin 20%)
50% free
10% complexed (anions)

Question 12

how does pH affect binding capacity of plasma protiens

Answer 12

binding capacity is increased under alkalotic conditions

Question 13

protein binding when alkalosis

Answer 13

plasma proteins bind more Ca2+ causing plasma concentration to fall
may precipitate hypocalcaemia

Question 14

protien binding when acidosis

Answer 14

binding capacity reduces and free plasma Ca2+ rises

Question 15

why does alkalosis cause more protein binding

Answer 15

hydrogen and calcium ions are postively charges and so compete for binding sites on the negative proteins

less H+ means less competition for binding sites

Question 16

total body calcium =

Answer 16

calcium in - calcium out

Ca2+ from diet - Ca2+ excretion kidney and faeces

Question 17

Ca2+ in bone

Answer 17

99% total body Ca2_ in bone, this can be added to or released as required

bone functions to provide mechanical support for body but its role in maintaining ca balance takes precendence over this

Question 18

osteoblasts

Answer 18

bone building cells

highly active cells which lay down a collagen extracellular matrix which they then calcify

Question 19

osteocytes

Answer 19

differentiates osteoblasts in established bone

regulate activity of osteoblasts and osteoclasts

Question 20

osteoclasts

Answer 20

cells that are responsible for mobilising bone

they secrete H+ ions to dissolve calcium salts and provide proteolytic enzymes to digest the extracellular matrix

Question 21

which two hormones act to increase calcium in plasma

Answer 21

parathyroid hormone

calcitriol

Question 22

calcitriol

Answer 22

active form of viD3

steroid hormone produced from vitamin D by the liver and kidneys

Question 23

parathyroid glands

Answer 23

4 on posterior surface of thyroid gland

essential for life

Question 24

when it PTH released

Answer 24

in response to drop in plasma calcium

Question 25

how does PTH act to increase Ca2_

Answer 25

1. stimulate osteoclasts to release Ca2+ and phosphate in bone
2. inhibit osteoblast to reduce Ca2+ deposition
3. inc reabsorption of Ca2+ from kidney tubules
4. inc renal excretion of phosphate - less phosphate = less calcium deposited back in bone
5. stimulates kidney to synthesise calcitriol from vitamin D which promotes calcium absorption at gut and kidney

Question 26

calcitriol production

Answer 26

steroid hormone produced in 2 steps (.1/ liver 2 kidneys) from dietary vitD or from precursors activated by sunlight on skin

Question 27

actions of calcitriol

Answer 27

• inc absorption of Ca2+ from gut
• facilitates renal absorption of Ca2+
• mobilise calcium stores in bone by stimulating osteoclast activity

Question 28

vitamin D deficiency

Answer 28

rickets in children
osteomalacia in adults

calcium and phosphate deficiency

Question 29

how does VitD3 deficiency cause Ca2+ and phosphate deficiency

Answer 29

intenstinal malabsorption of Ca2+ so low plasma calcium which increases PTH which promotes phosphate deficiency (aggravates Ca2+ loss from bone)

Question 30

which hormone acts to decrease plasma calcium

Answer 30

calcitonin

Question 31

calcitonin

Answer 31

peptide hormone produced by thyroid gland

secretion stimulated by rise in plasma calcium

Question 32

calcitonin actions

Answer 32

bind to osteoclats and inhibit bone resorption
also increase renal excretion

however, little evidence it is important in humans

Question 33

other endocrine hormones altering calcium balance

Answer 33

cortisol
insulin
oestrogen
prolactin

Question 34

how does cortisol affect calcium balance

Answer 34

inhibits osteoblasts, increase renal excretion and reduces intestinal absorption
so decreases plasma calcium
which increases PTH
which causes increased bone resorption

Question 35

how does insulin affect calcium balance

Answer 35

increases bone formation and antagonises the action of cortisol
diabetics ma have significant bone loss

Question 36

how does oestrogen affect calcium balance

Answer 36

constant stimulus for bone formation

Question 37

how does prolactin affect calcium balance

Answer 37

promotes calcium absorption from gut by stimulating syntehsis of calcitriol