16 - CALCIUM AND PHOSPHATE HOMEOSTASIS. THE PARATHYROID HORMONE, CALCITONIN VIT D, PHYSIOLOGICAL EFFECTS AND CONTROL OF SECRETION. IMPAIRED CALCIUM PHOSPHATE HOMEOSTATIS.

section

role of calcium

4 main components of calcium and phosphate homeostasis

PTH

PTH regulation

vitamin D

calcitonin

hypocalcaemia

hypercalcaemia

role of calcium

Calcium used in nerve conduction, muscle contraction, hormone release and cell signalling, blood coagulation. Phosphate in ATP

4 main components of calcium and phosphate homeostasis

1. serum calcium

2. serum phosphate

3. 1, 25-dihydroxyvitamin D-3

4. parathyroid hormone (PTH).

• About 99% of the total body calcium is stored in bones in the form of calcium phosphate and calcium hydroxide salts (hydroxyapatite).

• a very small portion of this calcium is available for exchange in the serum.

PTH

hormone is secreted by the parathyroid glands

The major target organs are the kidneys, skeletal system, and intestine. 2+

It is involved in the homeostasis of blood calcium (Ca ) and phosphate (HPO 2 4) levels:

• Helps activate vitamin D and increases the rate of calcium and phosphate absorption from the GIT.

• Increases the number and activity of osteoclasts, which cause bone tissue to break down and additional calcium and phosphate → blood.

• Increases the rate at which the kidneys remove phosphates loss in urine.

PTH regulation

• When the calcium level of the blood falls, more PTH is released.

• Conversely, when the calcium level of the blood rises, less PTH (and more CT) is secreted

• This is an example of a negative feedback control system.

vitamin d

• formed in skin when a cholesterol is exposed to UV light.

• Its then activated in the liver and in the kidney.

• it increases gut absorption of calcium by stimulating the production of calcium binding protein in the intestinal epithelial cells

calcitonin

secreted by the parafollicular- thyroid gland. Is released in response to hypercalcemia and has an aim of decreasing blood levels of ca2+ ions.

2 main effects:

• t suppresses renal tubule reabsorption of calcium , increases excretion of calcium into urine

• Inhibits bone resorption, minimizing movement calcium from bone to blood.

• Potassium-3.5-5.6mmol/l

• Normal levels of calcium- 2.12-2.62mmol-1

hypocalcaemia

low levels of ca2+ in blood

• Neuromuscular excitability, muscle spasms, urinal dysfunction- caused by low vit D, hypoparathyroidism

hypercalcaemia

too high organ disfunction and damage, precipitaton of ca2 and phosphate in tissue, widespread organ damage and dysfunction