5. Calcium Homeostasis Flashcards
(44 cards)
What are the 5 main roles of calcium in the body
- signalling
- blood clotting
- apoptosis
- skeletal strength - 99% Ca in bone
- membrane excitability
why do people not die directly of clotting in response to low Ca
Ca levels never fall low enough in living people to allow clotting - would die of something else before Ca gets that low
what role of Ca is the most critical in short term homeostasis
membrane excitability - Ca2+ decreases Na+ permeability - can be a medical emergency if not working
what is hypocalcaemia
low calcium in the blood - increases neuronal Na+ permeability leading to hyper excitation of neurons
what can happen in extreme cases of hypocalcaemia
tetany - if spreads to larynx and respiratory muscles causes asphyxiation
what is hypercalcaemia
high calcium in blood - decreases neuronal Na+ permeability which reduces excitability and depresses neuromuscular activity
what can happen in extreme cases of hypercalcaemia
can trigger cardiac arrhythmias
what is the calcium distribution in the body
99% in bone - acts as a store, can sacrifice Ca to maintain plasma levels
1% in blood - (TIGHTLY REGULATED)
- Intracellular = 0.9%
- Extracellular fluid = 0.1%
how is calcium stored intracellularly (inside cells)
mostly stored inside mitochondria and sarcoplasmic reticulum
free [Ca2+]ic very low
how is calcium stored in the extracellular fluid (in plasma)
nearly half bound to protein
= only 0.05% Ca in the body is free in solution and physiologically active
how is calcium stored in bone
99% calcium (~1kg) stored calcified extracellular matrix of bone - in the form of HYDROXYAPATITE
(Ca10(*PO4)6(OH)2)
*phosphate homeostasis therefore very important for Ca balance
what three forms does Ca exist in in the blood
- protein bound - 40% (80%to albumin, 20% to globulin)
- free (ionised) - 50%
- complexed - 10% (eg Ca carbonate, Ca phosphate, etc)
what form of calcium in the blood is physiologically active
free(ionised) Ca
what can change the binding capacity of Ca2+ to plasma proteins
pH Increased pH (alkaline) = increased binding capacity of Ca2+
Decreased pH (acidic) = decreased binding capacity of Ca2+
how does increasing pH lead to increased binding capacity of Ca2+
in alkalosis when pH increases (decreased [H+] in plasma) proteins are deprotonated = overall -ve charge allows Ca to bind to protein
reduces overall amount of ionised Ca BUT total Ca remains constant
how does decreasing pH lead to decreasing binding capacity of Ca2+
in acidosis when pH decreases (increased [H+] in plasma) - increase in protons = displaces Ca from protein
increases overall amount of ionised Ca BUT total Ca remains constant
what principle determines total body Ca2+
total body calcium = calcium in from diet - calcium out excreted at kidney and faeces
how does the Ca stored in bone help maintain Ca balance
at low Ca - bones release Ca (by osteoclasts) into plasma at expense of strength - leads to weak/brittle bones but allows plasma Ca to be maintained (which is more important)
what are osteoBLASTS
bone BUILDING cells - highly active cells - lay down a collagen extracellular matrix which they then calcify
what are oseoCLASTS
bone “CLEARING” cells - responsible for mobilising bone - secrete H+ ions (pH~4) to dissolve Ca salts - provide proteolytic enzymes to digest extracellular matrix - allow for Ca to be released into plasma
what are osteocytes
cells in established bone - much less active than osteoblasts - regulate activity of osteoblasts and osteoclasts
what are the two key hormones that increase [Ca2+] in plasma
- parathyroid hormone (PTH)
2. calcitriol (active form of VitD3)
what is parathyroid hormone
a polypeptide hormone produced by the parathyroid glands
what are the parathyroid glands and where are they found
usually 4 lying on the posterior surface of the thyroid gland - produce parathyroid hormone
variations in number and location are common though - essential for life, be careful to not remove them when removing overactive thyroid gland
