Physiology

Question 1

describe the different functions and forms of calcium in the body

Answer 1

functions
- bone and tooth structure
- mineral store
- action potentials
- membrane excitability
- second messenger for excitation contraction coupling, glands, and non steroid hormones
- cofactor in metabolic pathways
- blood clotting

forms
- diffusable calcium
- non diffusable calcium
- ionised calcium is diffusible and at a concentration of 1.2mmol/litre
- bound to citrate is diffusible at 0.2mmol/litre
- bound to protein is non diffusible, 1.2mmol/litre
- total calcium concentration is 2.6 mmol/litre

Question 2

describe calcium turnover

Answer 2

enters through diet in gi tract, where it can enter plasma and interstitial fluid, where it can go to the kidneys or to exchangeable bone to go to stable bone

excreted through urine and faeces

Question 3

describe how calcium homeostasis is maintained

Answer 3

parathyroid hormone
- PT glands
- low calcium levels
- increases resorption of bone through osteoclastogenesis
- increases calcium reabsorbtion in the kidney with decreased phosphate resorption
- increase formation of 1.25 hydroxy vitamin D in the kidney to increase absorption of calcium
- increased uptake of calcium from the intestines is assisted by vitamin D

calcitonin
- thyroid glands from parafollicular cells
- in response to high plasma calcium
- increases bone formation through osteoblastogenesis and decreases calcium resorption in the kidney
- not a major role
- no pathological effects

vitamin D
- calcium absorption in GIT
- calcium retention in the kidney
- phosphate retention in the kidney
- calcium release from bone
- phosphate release from bone

Question 4

describe how vitamin D is formed and what it does

Answer 4

• dietary vitamin D and 7-dehydrocholesterol from the skin
• forms vitamin D3 (cholecalciferol)
• acted on in the liver by 25-hydroxylase to form 25 hydroxycholecalfiferol
• acted on in the kidney by 1alphahydroxylase
• to form 1,25 (OH)2 cholecalciferol
• otherwise known as calcitriol
• calcitriol acts on intestines, kidney and bone to increase plasma levels of calcium and phosphate

Question 5

explain how bone can be affected during maintenance of calcium homeostasis

Answer 5

hormones can affect bone

those that increase bone formation and mass include
- calcitonin
- growth hormone
- IGF1
- insulin
- oestrogen
- testosterone

those that decrease bone mass and increase its resorption include
- cortisol
- parathyroid hormone
- thyroid hormone

osteoblasts are the cells that synthesise and secrete collagen fibres which form a matrix that is mineralised by calcium salts to form bone

osteocytes are trapped osteoblasts in the bone matrix that lie within bone lacunae and contact other cells via long cytoplasmic processes called canaliculi

osteoclasts are large, multinucleated cells that are derived from macrophages that function in bone resorption. they lie in depressions called howships lacunae

Question 6

describe disorders associated with calcium homeostasis

Answer 6

hypocalcaemic tetany
- decreased calcium concentration
- due to decreased intake and increases loss, and this causes alkalosis
- alkalosis is low calcium concentration
- leads to increased nerve excitability
- symptoms are pins and needles, muscle spasms, and trousseaus sign (italian hand)

hypocalcaemia
- hyperventilation can cause alkalosis by blowing off carbon dioxide
- treated by rebreathing expired air

over and under secretion of PTH, vitamin D and calcitonin can have an impact

hyperparathyroidism
- osteitis fibrosa cystica is areas of demineralisation in skull and leg bones caused by primary hyperparathyroidism

hypoparathyroidism
- defective mineralisationo f the teeth due to low calcium levels and undersecretion of PTH

vitamin D deficiency
- dietary deficient or failure to synthesise it
- leads to undermineralised bones lacking rigidity
- rickets in children and osteomalacia in adults

bone diseases are osteoporosis and osteopetrosis

osteoporosis
- decreased bone mass and density
- fractures are common
- happens in the elderly, evident earlier in women due to menopause
- can be caused by corticosteroids and nutritional deficiency
- reduced thickness of corticol bone

osteopetrosis
- increased bone density
- reduced blood supply
- extractions are difficult and tooth roots are indistinct on radiographs
- affects the mandible more than the maxilla