Calcium Homeostasis Flashcards
What does removal of calcium require
Active transport
What is the role of calcium in the body
Signalling Blood clotting Apoptosis Skeletal strength Membrane excitability
What is calcium’s role in signalling
Exocytosis of vesicles containing hormones
Contraction of muscles
Alters enzymes function
What does hypocalcaemia do to membrane excitability
Increases Na permeability
Leads to depolarisation
If extreme = tetany (sustained contraction)
Asphyxiation if resp muscles affected
What does hypercalcaemia do to membrane excitability
Decreased Na permeability
Reduces excitability
Can trigger arrhythmia
How is calcium distributed in the body
Bone (99%) Intracellular fluid (0.9%) Extracellular fluid (0.1%)
How is calcium stored in the bone and why is this important
Hydroxyapatite (Ca10(PO4)6OH2)
If phosphate deficient can’t lay down calcium in bone
How is calcium stored in extracellular fluid and why important
This is the value hypo / hyper relates too
Tightly regulated
1/2 bound to protein
What affected binding capacity
pH
What does alkalosis cause
H+ ions leave protein to fix alkalosis so are space for Ca to bind
Plasma conc of Ca fall = hypcalcaemia
What does acidosis cause
Increased H+ ions in plasma displaces bound Ca from protein = hypercalcaemia
What takes precedence over skeletal structure of bone
Calcium levels in ECF so if low Ca released from bone
What do osteoblasts do
Bone building cells
Lay down collagen
What do osteocytes do
Regulate bone metabolism
What do osteoclasts do
Calcium releasing cells
Secrete H+ ions to dissolve calcium salts and digest matrix
What are three key hormones involved in calcium homeostasis
PTH
Calcitriol
Calcitonin
What produced PTH and when
Parathyroid hormones in response to low calcium
How is calcitriol produced
Steroid hormone
Produced from diet vit D in liver and kidney
Inactive vit D goes to liver
Transported to kidney
What stimulates calcitriol
PTH stimulates kidney to produce from vitamin D
Prolactin in lactating women as need Ca for milk
What is full name of calcitriol
1,24 dihydroxycholecalciferol
What is the role of PTH
Stimulate osteoclast to release Ca + phosphate
Inhibit osteoblasts
Increase reabsorption of Ca from kidney
Increase renal excretion of phosphate so can’t deposit in bone
Stimulates kidney to synthesise calcitriol from vitamin D
Overall effect is increased calcium and decreased phosphate
What do you need in renal failure
Active vit D (calcitriol) as unable to make
What does calcitriol do
Increases Ca uptake from small intestine
esp during pregnancy / lactation + growth
Increases renal absorption
Increases osteoclast activity
Where do we get our vit D and where is it stored
Diet
Sunlight activates so clothing / sunscreen = deficient
Stored in fat
Who is at risk of vitae’s D deficiency
Children
Elderly
Asian as pigmented skin less able to make
What happens if deficient
Decreased plasma Ca which leads to increased PTH to increase
Calcium and phosphate removed from bone = osteoporosis
What is vit D deficiency implicated in
MS
Cancer
Arthritis
CVD
What is calcitonin and what does it do
Peptide hormone produced by thyroid
Acts to decrease plasma Ca
Stimulated by hypercalcaemia
How does calcitonin work
Binds to osteoclast and inhibits release of Ca
Increases renal excretion of Ca
What are other hormones involved in Ca homeostasis
Cortisol = lose Ca Insulin = Ca in bone Oestrogen = Ca in bone GH = Ca in bone Prolactin
What does cortisol do
Inhibits osteoblasts
Increases renal excretion of Ca and phosphate
Reduced absorption of Ca
Leads to decreases Ca and increased PTH
What does this lead too
Osteoporosis
What does insulin do
Increased bone formation as glucose used for energy
Antagonises cortisol
DM = bone loss
What does oestrogen do
Simulates bone formation via osteoblasts
If post-menopausal = osteoporosis
Osteoporotic fracture rare if pre-menopausal women
What does GH do
Stimulates bone formation
What does prolactin do
Stimulates synthesis of calcitriol
