Calcium Homeostasis Flashcards
What are the roles of calcium within the body?
Signalling - Ca2+ important signalling molecules e.g. NT’s, muscle fibres
Blood Clotting - Essential component of clotting cascade
Apoptosis
Skeletal Strength - 99% of calcium in body is wrapped up in bone where it gives strength to skeleton
Membrane Excitability - Ca2+ decreases Na+ permeability
What is the effect of hypocalcaemia?
Increased neuronal Na+ permeability leading to hyperexcitation of neurons. Can cause tetany and if it spreads to larynx/resp muscles = asphyxiation
What is the effect of Hypercalcaemia?
Decreases neuronal Na+ permeability which reduces excitability and depresses the neuromuscular activity. In extreme cases, cardiac arrhythmias
How is calcium distributed within the body?
Bones - 99%
Intracellular - 0.9%
ECF- 0.1%
In what form is calcium stored in the bones?
Hydroxyapatite
How much calcium is found within the plasma
2.2-2.6mM
What does calcium have a high affinity for?
proteins
How much calcium is bound to plasma within plasma?
~40%
How much calcium can be classed as “free calcium”?
1.2mM
What form of calcium is physiologically active?
Free calcium
When is binding capacity increased?
Under alkalotic conditions e.g. hyperventilation
When is binding capacity of calcium reduced?
When acidosis occurs
Define how Total Body Calcium is determined?
Ca2+ from diet - Ca2+ excreted in Kidneys and Faeces
What are osteoblasts?
Bone Building cells
What is the function of osteoblasts?
The lay down a collagen extracellular matrix which they then calcify
What do osteocytes do?
They control the activity of osteoblasts and osteoclasts
What are osteoclasts?
Cells which are responsible for mobilising bone
What are the functions of osteoclasts?
To secrete H+ ions to dissolve calcium salts and provide proteolytic enzymes to digest the extracellular matrix
What are the two hormones which act to increase plasma calcium concentration?
Parathyroid Hormone - released by parathyroid gland
Calcitriol (Active form of Vit D3) steroid hormone produced from Vitamin D by the liver and kidneys
What are the parathyroid glands?
4 glands which lie on the surface of the thyroid
What is the action of PTH?
Stimulates osetoclasts to increase resorption of Ca2+ and phosphate in bone (Effects seen within 12-24hours)
Inhibiting osteoblasts to reduce Ca2+ deposition in bone
Increasing ReAbsorption of Ca2+
Increasing renal excretion of phosphate. This elevates free calcium concentration by preventing it from being deposited back into bone, a process that requires phosphate
Stimulates the kidney to synthesise calcitrol from Vit D which promotes calcium absorption at the gut and kidney
What does Calcitriol do?
Complements action of PTH which increases calcium plasma concentration
How is calcitriol produced?
- Liver 2. Kidneys
From vit d or precursors from sunlight in skin. Formation is stimulated by Prolactin in lactating women
What hormone acts to decrease [Ca2+] plasma?
Calcitonin
Where is calcitonin produced?
The thyroid gland
What is the action of calcitonin?
To bind osteoclasts and inhibit bone resorption as well as increase renal excretion so preventing a further increase in Ca2+
How does cortisol alter Ca2+ balance?
Inhibits osteoblasts by increasing renal excretion of Ca2+ and phosphate. Reduces intestinal absorption of Ca2+ which decreases plasma calcium which increase PTH which increases bone reorption. Produces osteoporosis
How does insulin alter calcium balance?
increases bone formation and antagonises the action of cortisol
How does oestrogen affect calcium balance?
Promotes bone formation via oestrogen receptors on osteoblasts
How does GH affect calcium balance?
Constant stimulus for bone formation
How does prolactin affect calcium balance?
Promotes calcium absorption by the gut by stimulating synthesis of calcitriol
