Calcium

Question 1

What are the functions of calcium?

Answer 1

• Bone and tooth structure
• Mineral store
• Action potentials - cardiac muscle
• Membrane excitability
• 2nd messenger
• Co-factor in metabolic pathways
• Blood clotting

Question 2

What are the concentrations of diffusible calcium in the body?

Answer 2

Ionised calcium = 1.2 mmol/l

Bound to citrate = 0.2 mmol/l

Question 3

What is the concentration of non-diffusible calcium in the body?

Answer 3

Bound to protein = 1.2 mmol/l

Question 4

Describe calcium turnover in the body

Answer 4

The kidneys excrete calcium
Calcium is also excreted from G.I. tract through faeces
Sources of calcium: diet, bones (exchangeable bone)

Question 5

What is calcium homeostasis maintained by?

Answer 5

Parathyroid hormone
Calcitonin
Vitamin D

Question 6

When is parathyroid hormone secreted?

Answer 6

Secreted by the parathyroid glands in response to LOW plasma calcium concentration

Question 7

How does parathyroid hormone increase plasma calcium?

Answer 7

• Increased resorption of bone (osteoclasts)
• Increased calcium reabsorption in the kidney (with decreased phosphate reabsorption)
• Increased uptake of calcium from the intestines (assisted by vitamin D)

Question 8

When is calcitonin secreted?

Answer 8

Secreted from thyroid glands in response to HIGH plasma calcium

Question 9

How does calcitonin decrease plasma calcium?

Answer 9

• Increased formation of bone (osteoblasts)

- Decreased Ca2+ reabsorption in the kidney

Question 10

What evidence suggests that calcitonin does not play a major role in calcium homeostasis?

Answer 10

Lack of calcitonin has no pathological effect

Question 11

Describe the steps in the formation of ‘calcitrol’

Answer 11

• Vitamin D3 obtained from diet, and synthesised in the skin from 7-dehydrocholesterol
• Vitamin D3 converted to 25(OH)Cholecalciferol by 25-hydroxylase
• 25(OH)Cholecalciferol converted to 1,25(OH)Cholecalciferol (i.e. calcitrol) by 1alpha-hydroxylase (regulated by PTH)

Question 12

What effects does 1,25(OH)Cholecalciferol (calcitrol) have on plasma calcium concentration?

Answer 12

• Intestine: increased calcium absorption
• Kidney: calcium retention and phosphate retention
• Bone: calcium release and phosphate release
  Overall effect: increased levels of calcium and phosphate in the plasma

Question 13

Which hormones increase bone formation and increase bone mass?

Answer 13

Calcitonin 
Growth hormone 
IGF-1
Insulin 
Oestrogen 
Testosterone

Question 14

Which hormones increase bone resorption and reduce bone mass?

Answer 14

Cortisol
Parathyroid hormone
Thyroid hormones

Question 15

What is the function of osteoblasts?

Answer 15

Synthesis and secrete collagen fibres forming a matrix (osteoid) later mineralised by calcium salts

Question 16

What are osteocytes?

Answer 16

Trapped osteoblasts in the bone matrix

They lie in the bony lacunae and contact other cells via long cytoplasmic processes

Question 17

What are osteoclasts?

Answer 17

Large, multinucleate cells derived from macrophages that resorb bone

Question 18

What effect does parathyroid hormone have on bone?

Answer 18

Induces cells to differentiate into osteoclasts to start resorbing bone
Decreases bone mass

Question 19

What is the name of the matrix that forms bone secreted by osteoblasts?

Answer 19

Osteoid

Question 20

What is hypercalcaemia?

Answer 20

Raised plasma calcium concentration

Question 21

What is hypocalcaemia?

Answer 21

Reduced plasma calcium concentration

Question 22

Describe hypocalcaemic tetany

Answer 22

Decreased plasma calcium concentration
Due to: decreased calcium intake, excessive calcium loss, alkalosis
Low calcium leads to increased nerve excitability: pins and needles, muscle spasms

Question 23

What can hyperventilation cause?

Answer 23

Alkalosis
Caused by ‘blowing off’ CO2
Remedy is to trap CO2 in the patient’s lungs
Achieved by rebreathing expired air from a paper bag

Question 24

What are some disorders of hormones regulating calcium?

Answer 24

Over and under secretion of parathyroid hormone, vitamin D and calcitonin

Question 25

What is hyperparathyroidism?

Answer 25

Parathyroid hormone released in high concentration Promotes resorption of bone May see osteitis fibrosa cystica on radiographs - areas of demineralisation in skull and leg bones May also be lesions in alveolar bone

Question 26

What is hypoparathyroidism?

Answer 26

Not enough parathyroid hormone being released Defective mineralisation of teeth due to low blood calcium levels due to undersecretion of PTH Occurs in-utero

Question 27

When may Vitamin D deficiency occur?

Answer 27

``` Dietary deficiency Failure to synthesise in body -> decreased calcium uptake from G.I. tract Under-mineralised bone Bone lacks rigidity Rickets in children Osteomalacia in adults ```

Question 28

What clinical consequences do increased or decreased levels of calcitonin have?

Answer 28

No obvious clinical consequences => calcitonin is probably not essential for the regulation of plasma [Ca2+]

Question 29

Name 2 types of bone diseases

Answer 29

Osteoporosis: Decreased bone mass and density Osteopetrosis: Increased bone mass and density

Question 30

Describe osteoporosis

Answer 30

Decreased bone mass and density Loss of matrix with secondary mineral loss Fractures common Common in the elderly Affect men and women but is evident earlier in women (menopause) Can be caused by corticosteroids, nutritional deficiency

Question 31

Describe osteopetrosis

Answer 31

Increased bone mass and density Reduced blood supply Prone to fracture and chronic infection Difficult extractions Tooth roots are indistinct on radiographs Effects are greater in the mandible compared to the maxilla