Calcium Flashcards
What are the functions of calcium?
- Bone and tooth structure
- Mineral store
- Action potentials - cardiac muscle
- Membrane excitability
- 2nd messenger
- Co-factor in metabolic pathways
- Blood clotting
What are the concentrations of diffusible calcium in the body?
Ionised calcium = 1.2 mmol/l
Bound to citrate = 0.2 mmol/l
What is the concentration of non-diffusible calcium in the body?
Bound to protein = 1.2 mmol/l
Describe calcium turnover in the body
The kidneys excrete calcium
Calcium is also excreted from G.I. tract through faeces
Sources of calcium: diet, bones (exchangeable bone)
What is calcium homeostasis maintained by?
Parathyroid hormone
Calcitonin
Vitamin D
When is parathyroid hormone secreted?
Secreted by the parathyroid glands in response to LOW plasma calcium concentration
How does parathyroid hormone increase plasma calcium?
- 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)
When is calcitonin secreted?
Secreted from thyroid glands in response to HIGH plasma calcium
How does calcitonin decrease plasma calcium?
- Increased formation of bone (osteoblasts)
- Decreased Ca2+ reabsorption in the kidney
What evidence suggests that calcitonin does not play a major role in calcium homeostasis?
Lack of calcitonin has no pathological effect
Describe the steps in the formation of ‘calcitrol’
- 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)
What effects does 1,25(OH)Cholecalciferol (calcitrol) have on plasma calcium concentration?
- 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
Which hormones increase bone formation and increase bone mass?
Calcitonin Growth hormone IGF-1 Insulin Oestrogen Testosterone
Which hormones increase bone resorption and reduce bone mass?
Cortisol
Parathyroid hormone
Thyroid hormones
What is the function of osteoblasts?
Synthesis and secrete collagen fibres forming a matrix (osteoid) later mineralised by calcium salts
What are osteocytes?
Trapped osteoblasts in the bone matrix
They lie in the bony lacunae and contact other cells via long cytoplasmic processes
What are osteoclasts?
Large, multinucleate cells derived from macrophages that resorb bone
What effect does parathyroid hormone have on bone?
Induces cells to differentiate into osteoclasts to start resorbing bone
Decreases bone mass
What is the name of the matrix that forms bone secreted by osteoblasts?
Osteoid
What is hypercalcaemia?
Raised plasma calcium concentration
What is hypocalcaemia?
Reduced plasma calcium concentration
Describe hypocalcaemic tetany
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
What can hyperventilation cause?
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
What are some disorders of hormones regulating calcium?
Over and under secretion of parathyroid hormone, vitamin D and calcitonin
What is hyperparathyroidism?
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
What is hypoparathyroidism?
Not enough parathyroid hormone being released
Defective mineralisation of teeth due to low blood calcium levels due to undersecretion of PTH
Occurs in-utero
When may Vitamin D deficiency occur?
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
What clinical consequences do increased or decreased levels of calcitonin have?
No obvious clinical consequences => calcitonin is probably not essential for the regulation of plasma [Ca2+]
Name 2 types of bone diseases
Osteoporosis: Decreased bone mass and density
Osteopetrosis: Increased bone mass and density
Describe osteoporosis
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
Describe osteopetrosis
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
