Calcium Flashcards
What are the functions of calcium?
bone and tooth structure, mineral store, action potentials, membrane excitability, second messenger, co-factor in metabolic pathways, blood clotting factor
How can we be certain that cause of tooth weakness after development is not due to Ca deficiency?
detine/enamel cannot be broken down to recover Ca because odontoclasts/ameloclasts do not exist
Examples of Ca acting as a second messenger
muscle (excitation-contraction coupling), gland secretion, non-steroid hormone action (non-steroid therefore cannot penetrate membrane)
What is meant by diffusible calcium?
calcium that is available - it is either ionised or bound to citrate
What is the concentration of ionised calcium?
1.2 mmol/l
What is the concentration of diffusible calcium bound to citrate?
0.2mmol/l
What is non-diffusible calcium?
calcium that’s bound to proteins
What is the concentration of calcium bound to protein (non-diffusible)?
1.2 mmol/l
What is the total concentration of calcium in the body?
2.6 mmol/l (diffusible 1.4 + non-diffusible 1.2)
Where do we get calcium from?
our diet
Where is calcium absorbed from our diet?
GI tract
What happens to the calcium that is not absorbed in the GIT?
the calcium is excreted in faeces
What is the name of the type of bone that is remodelled to achieve Ca balance?
exchangeable bone
What is the name of the type of bone that is not demineralised/remodelled?
stable bone
Where does the Ca that is absorbed from the GIT go?
Ca enters the plasma and interstitial fluid
How is excess Ca excreted from the plasma?
blood is filtered by the kidneys and excess Ca is excreted in urine
How does Ca become incorporated into bone?
Ca from the plasma/interstitial fluid is used to mineralise osteoid of exchangeable bone which then becomes stable bone
Which substances are required for calcium homeostasis?
parathyroid hormone, calcitonin, vitamin D
Where is PTH secreted from?
parathyroid glands
When in PTH secreted?
when plasma [Ca2+] is low
Function of PTH
increases plasma calcium
How does PTH increase plasma calcium?
increased bone resorption, increased Ca2+ resorption in kidney (in exchange with decreased PO4 resorption), increased Ca2+ uptake from intestines (assisted by vit D)
Which cells carry out bone resorption?
osteoclasts
What is the effect of PTH on the kidney?
increases Ca2+ resorption, increases PO4 excretion, increases 1,25 (OH)2 vit D (calcitriol) formation
What is the effect of PTH on bone?
bone resorption - releases Ca2+ and PO4
What is the overall effect of PTH on PO4 concentration?
no effect - although there is increased PO4 excretion from kidney, bone resorption releases PO4
How is absorption of Ca from the intestines increased?
PTH increases the formation of 1,25 (OH)2 vit D (calcitriol) in the kidneys which aids the absorption of Ca from the GIT
Where is calcitonin secreted from?
C cells (parafollicular) in the thyroid gland
When is calcitonin released?
When plasma [Ca2+] is high
What is the function of calcitonin?
reduces plasma [Ca2+]
How does calcitonin lower plasma calcium?
increases bone formation and decreases Ca2+ resorption in the kidneys
Which cells are involved in bone formation?
osteoblasts
Why does it seem that calcitonin does not play a major role in calcium homeostasis?
a lack of calcitonin has no pathological effects
What is the effect of calcitonin on kidneys?
increases Ca2+ excretion
What is the effect of calcitonin on bone?
increases bone deposition
Where is vitamin D derived from?
dietary vitamin D and 7-dehydrocholesterol synthesised in the skin
What is the name of the precursor to vitamin D synthesised in the skin in sunlight?
7-dehydrocholesterol
What is the first product from dietary vitamin D and 7-dehydrocholesterol?
vitamin D3 (cholecalciferol)
What is vitamin D3 (cholecalciferol) converted into?
25 (OH) cholecalciferol
How is vitamin D3 (cholecalciferol) converted into 25 (OH) cholecalciferol?
vitamin D3 (cholecalciferol) is hydroxylated by 25-hydroxylase in the liver
Where is vitamin D3 (cholecalciferol) hydroxylated into 25 (OH) cholecalciferol?
in the liver by 25-hydroxylase
What is 25 (OH) cholecalciferol converted into?
1,25 (OH)2 cholecalciferol (calcitriol)
How is 25 (OH) cholecalciferol converted into 1,25 (OH)2 cholecalciferol?
hydroxylated by 1a (alpha) hydroxylase in the kidney
Which enzyme converts 25 (OH) cholecalciferol into 1,25 (OH)2 cholecalciferol (calcitriol)?
1 alpha hydroxylase (in the kidney)
Which hormone regulates the hydroxylation of 25 (OH) cholecalciferol into 1,25 (OH)2 cholecalciferol in the kidney?
PTH
What is the active form of vitamin D?
1,25 (OH)2 cholecalciferol (calcitriol)
What is the overall function of 1,25 (OH)2 cholecalciferol (calcitriol)?
increase plasma Ca and PO4
How does 1,25 (OH)2 cholecalciferol (calcitriol) increase plasma Ca and PO4?
increases Ca absorption from intestines, increase Ca and PO4 retention in kidneys, and increases Ca and PO4 release from bone
What is the effect of 1,25 (OH)2 cholecalciferol (calcitriol) on the intestine?
increases Ca absorption in intestine
What is the effect of 1,25 (OH)2 cholecalciferol (calcitriol) on the kidneys?
increases Ca and PO4 retention
What is the effect of 1,25 (OH)2 cholecalciferol (calcitriol) on bone?
increases Ca and PO4 release
What is a similarity in the effects of vitamin D (calcitriol) and PTH?
both increase plasma [Ca]
What is a difference between the effects of PTH and vitamin D (calcitriol)?
PTH doesn’t affect [PO4] as PO4 excretion from kidney is increased whereas calcitriol increases PO4 retention in the kidneys resulting in an overall [PO4] increase (bone resorption increases PO4 too)
Which hormones increase bone formation/mass?
calcitonin, growth hormone, IGF-1, insulin, oestrogen, testosterone
Which hormones increase bone resorption and therefore decrease bone mass?
cortisol, parathyroid hormone, thyroid hormones
Function of osteoblasts
synthesis and secrete collagen fibres forming a matrix (osteoid) which is later mineralised by calcium salts
How do osteocytes arise?
when osteoblasts become trapped in the matrix they mature into osteocytes
Where are osteocytes found?
in bony lacunae
How do osteocytes communicate?
via long cytoplasmic processes
Describe the structure of an osteoclast
a large, multinucleate cell
What cells are osteoclasts derived from?
monocyte/macrophage lineage
Function of osteoclasts
resorb bone
Where are osteoclasts found?
in depressions called Howship’s lacunae
What concept underpins orthodontic treatment?
bone remodelling
Function of bone remodelling
allows a constant source of Ca
What is the term for raised [Ca2+]?
hypercalcaemia
What is the term for reduced [Ca2+]?
hypocalcaemia
What condition may result from low calcium levels in the blood?
Hypocalcaemic tetany
What does hypocalcaemic tetany cause?
involuntary muscle contractions
What are the causes of hypocalcaemic tetany?
decreased Ca2+ intake, excessive Ca2+ loss, alkalosis
What is alkalosis?
condition where blood plasma pH is too high
How can alkalosis contribute to hypocalcaemia?
a high blood plasma pH increases protein binding of calcium which decreases ionised calcium (diffusible)
How does low [Ca2+] impact nerve excitability?
low [Ca2+] increases nerve excitability
What are the symptoms of low [Ca2+]?
pins and needles, muscle spasms
What are the 2 clinical indicators of hypocalcaemia?
Trousseau’s sign and Chvostek’s sign
What is Trousseau’s sign?
involuntary contractions of muscles in hand and wrist due to hypocalcaemia
What is Chvostek’s sign?
tapping of the facial nerve resulting in muscle contraction due to hypocalcaemia
What can cause alkalosis?
hyperventilation (increase in O2 which expels more CO2 causing increase alkalinity of blood)
What is the treatment for alkalosis due to hyperventilation?
trapping CO2 in the lungs by rebreathing expired air from a bag
Which hormones can cause calcium regulation disorders if they are over or under secreted?
parathyroid hormone, vitamin D, calcitonin
What is the consequence of hyperparathyroidism?
increased bone resorption
Which condition can result from hyperparathyroidism?
osteitis fibrosa cystica
What is osteitis fibrosa cystica?
areas of demineralisation in skull and leg bones
What is unusual about the name of the disease, osteitis fibrosa cystica?
it names features that do not occur in the condition - there is no inflammation of the bone, no fibrous tissue or cysts
How would osteitis fibrosa cystica be diagnosed?
no infection, no cysts, teeth vital. Panoramic radiograph of the skull shows other areas of radiolucency / lesions. Refer to GP to test [Ca2+]
What consequence does hypoparathyroidism have on teeth?
defective teeth mineralisation due to low blood [Ca2+] from under secretion of PTH (congenital as enamel is not broken down following development)
What is the cause of vitamin D deficiency?
dietary deficiency results in failure to synthesise 1,25 (OH)2 cholecalciferol (calcitriol)
What are the consequences of vitamin D deficiency?
decreased Ca uptake from GIT, undermineralised bone causing a lack of rigidity
What is the condition of a lack of bone rigidity in children?
rickets
What is the name of the condition in which adults lack bone rigidity?
osteomalacia
Cause of rickets / osteomalacia
vitamin D deficiency
What are the symptoms of rickets?
bowlegged, cowboy walk, flexible bones
What are the consequences of increased or decreased calcitonin levels?
no obvious clinical consequences - so calcitonin is likely not essential for plasma [Ca2+] regulation
Which bone disease results in a decreased bone mass and density?
osteoporosis
Which bone disease results in an increased bone mass and density?
osteopetrosis
Which part of the population is more affected by osteoporosis?
elderly
Although osteoporosis affects both men and women, why is it evident earlier in women?
due to menopause (decreased oestrogen)
Causes of osteoporosis
menopause, corticosteroids (cortisol increases bone resorption), nutritional deficiency
How does osteopetrosis affect blood supply?
osteopetrosis reduces blood supply
What are the consequences of osteopetrosis?
bone is prone to fracture and chronic infection, difficult extractions and healing
Why is bone healing reduced in osteopetrosis?
due to reduced blood vessels
Why are tooth roots indistinct from bone on radiographs in patients with osteopetrosis?
osteopetrosis results in more radiopaque bone which is difficult to distinguish from teeth
How do the maxilla and mandible compare in osteopetrosis?
mandible is enlarged and has a greater density than mandible
