Calcium Homeostasis Flashcards
State the roles of calcium in the body
- Signalling
- Skeletal strength
- Blood clotting (Ca2+ is a cofactor in the coagulation cascade - Silverthorn)
- Apoptosis
- Membrane excitability (affected by Ca2+ plasma concentrations - Silverthorn)
Describe the effects of hypercalcaemia and hypocalcaemia on neuronal Na+ permeability
- Hypercalcaemia: Decreases Na+ permeability which will reduce excitability and depress neuromuscular activity
- Hypocalcaemia: Increases Na+ permeability which will increase excitability and promote neuromuscular activity
How is calcium distributed in the body?
- Bones: 99% (1kg stored in the calcified extracellular matrix of bone, mostly in the form of hydroxyapatite)
- Intracellular: 0.9%
- Extracellular fluid: 0.1%
The homeostasis of what element is important in determining calcium balance?
- Phosphate
(since calcium is stored in the form of hydroxyapatite - (Ca10(PO4)6(OH)2)
Why does calcium have a very high affinity for proteins?
- Small positive charge of the calcium is attracted to the large negative charge of the protein
In the body, calcium situated where is physiologically active?
- Calcium that is free in the plasma
In the plasma, what % of proteins is bound to plasma proteins?
40%
(the rest either bind to plasma anions or are free in solution)
How does the binding capacity of calcium for plasma proteins change with pH?
- Binding capacity is increased under alkalotic conditions
In terms of calcium balance, explain what hyperventilation leads to?
- Hypocalcaemic tetany
- This is becuase if you hyperventilate, plasma pH ↑ (alkalosis) (this is becuase as you breathe out more rapidly, you effectively drive out the CO2 which is acidic)
- As plasma pH ↑, then more plasma proteins bind with Ca2+
- Therefore ↓ Ca2+ plasma concentration
- Hypocalcaemia results leading to hypocalcaemic tetany (not in lecture but REMEMBER: ↓ Ca2 increases neuronal Na+ permeability. This leads to depolarisations and action potentials)
(opposite occurs with acidosis)
Look at the following diagram:
Does the Ca2+ which is stored in bone always stay in bone?
- No
- Can be added to or released as requried
(Ca2+ balance is a very important function of bone)
Define/give the function of the following type of cells in bone:
(a) Osteoblasts
(b) Osteoclasts
(c) Osteocytes
(a) Osteoblasts = Bone building cells that control the deposition of Ca2+ in bone
(b) Osteoclasts = Mobilises/absorbs bone tissue by secreting H+ (pH ≈ 4) ions to dissolve the calcium salts
(c) Osteocytes = Differentiated osteoblasts in established bone
State the 2 key hormones which act to increase the concentration Ca2+ in the plasma
- Parathyroid hormone (PTH) (polypeptide hormone produced by the parathyroid hormones)
- Calcitriol (steroid hormone produced from vitamin D by the liver ans kidneys - active form of Vitamin D3)
How many parathyroid glands are there?
4
State the function of parathyroid hormone (PTH) AND explain how it does this
Function: Acts to increase free [Ca2+]plasma due to ↓ in free [Ca2+]plasma
It does this by:
- Stimulating osteoclasts to ↑ resorption (release) of Ca2+ and phosphate in bone
- Inhibiting osteoblasts to ↓ Ca2+ deposition in bone
- Increasing reabsorption of Ca2+ from the kidney tubules, therefore decreasing its excretion in the urine
- Increasing renal excretion of phosphate. This elevates free [Ca2+] by preventing it from being deposited back into bone, a process that requires phosphate
- Stimulates the kidney to synthesise calcitriol from vitamin Dwhich promotes calcium absorption at the gut and kidney
Look at the following diagram:
How does calcitriol increase the absorption of Ca2+ from the gut?
- Calcitriol controls an ACTIVE TRANSPORT SYSTEM which moves Ca2+ from the intestinal lumen to the blood
(REMEMBER: much of Ca2+ is excreted in the faeces)
Look at the following diagram:
How does the hormone prolactin relate to calcitriol?
- Prolactin increases the synthesis of calcitriol
How is plasma [Ca2+] maintained in those people that are vitamin D deficient?
- Because vitamin D is low, parathyroid hormone (PTH) must work hard to maintain plasma [Ca2+]
- To do this, it needs to continually remove Ca2+ from bone resulting in bones which are soft and, if still growing, become bent
(Rickets in children, Osteomalacia in adults)
Explain how calcitonin performs its function
The function of calcitonin is to decrease [Ca2+]plasma
It does this by:
- Binding to osteoclasts and inhibit bone resorption
- Increasing renal excretion so preventing a further increase in Ca2+
