Calcium Flashcards
What are the roles of calcium in the body
Signalling Blood clotting Apoptosis Skeletal strength Membrane excitability
How does hypercalcaemia affect membrane excitability
o Hypercalcaemia decreases neuronal Na+ permeability which will reduce excitability and depress neuromuscular activity and in extreme cases, trigger cardiac arrhythmias.
How does hypocalcaemia affect membrane excitability
o Hypocalcaemia increases neuronal Na+ permeability leading to hyperexcitation of neurons. In extreme cases causes tetany, if spreads to larynx and respiratory muscles – asphyxiation.
How much total body calcium is stored in bones
99%
How much total body calcium is stored intracellularly
0.9% (mitochondria and sarcoplasmic reticulum)
How much total body calcium is stored extrecellularly
0.1%
How much of the extracellular calcium is bound to plasma proteins
40%ish
What proteins does calcium bind to
Globin 20%
Albumin 80%
What is normal plasma calcium range
2.2-2.6mM (0.1%) (not all of this is free)
What effect will high albumin have on plasma Ca levels
Decrease amount of free plasma calcium (as more is bound to albumin)
What condition features high plasma albumin
Multiple myeloma
What condition features low plasma albumin
Malnutrition
Nephropathy
What effect will low albumin have on plasma Ca levels
Low albumin levels increase free or available calcium.
What % of filtered calcium is reabsorbed in the kidney
97.99%
What affects the total amount of calcium in the plasma
pH
Bones
How does pH affect plasma calcium levels
Low pH increases free plasma calcium because the hydrogen ions compete with the Calcium ions for a spot on the plasma proteins
How do bones affect plasma calcium levels
99% of total body Ca2+ is stored in bone and this can be added to or released as required and there is a continuous turnover of bone, it is continually being deposited and then resorbed.
What bone cell is responsible for breaking down bone and reelaseing calcium
Osteoclast
What bone cell is responsible for building bone from calcium and phosphate
Osteoblasts
What is an osteocyte
A matured osteoblast that is less active
How do osteoclasts mobilise bone
They secrete H+ ions (pH 4) to dissolve the calcium salts and also provide proteolytic enzymes to digest the extracellular matrix.
What hormones are involved in calcium homeostasis
Parathyroid hormone
Calcitriol
Calcitonin
What kind of hormone is PTH
polypeptide
What kind of hormone is calcitriol
Steroid
What kind of hormone is calcitonin
Peptide
Where is calcitonin released from
The parafollicular (clear) cells of the thyroid gland
Where are parathyroid glands located and how many are there
There are usually 4 lying on the posterior surface of the thyroid gland, although variations in number and location of are common, 1 in 10 people have aberrant distribution.
How does parathyroid hormone affect Calcium levels (x5)
Stimulating osteoclasts to increase resorption (release) of Ca2+ and phosphate in bone
Inhibiting osteoblasts thus reducing 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 (into hydroxyappetite), a process that requires phosphate.
Stimulates the kidney to synthesise calcitriol from vitamin D which promotes calcium absorption at the gut and kidney.
How long does it take for osteoclast stimulating effects of PTH to be seen after release
12-24hrs
What is calcitriol
the active form of Vitamin D3
What is calcitriols effect on calcium
It complements the actions of Parathyroid hormone and increases plasma calcium.
How is calcitriol produced
From a two step process from inactive Vit D(cholecalciferol)
Where does stage one of calcitriol formation occur
Liver
Where does stage two of calcitriol formation occur
Kidneys
What stage of calcitriol production is PTH needed
Stage 2 (in the kidneys)
What hormones enhance the effect of calctriol
PTH
Prolactin
How does calcitrol increase plasma calcium
Increase absorption of Ca2+ from the gut.
Facilitates renal absorption of Ca2+ and Phophate
Mobilises calcium stores in bone by stimulating osteoclast activity.
If PTH increases phosphate excretion then why does calcitriol stimulate phosphate resabsorpion
To ensure any excess calcium to be laid down as bone.
When does calcitriol Mobilises calcium stores in bone by stimulating osteoclast activity.
Only when calcium levels are low
