endocrine control of plasma calcium Flashcards
roles of calcium
- Structural component of bones and teeth (99% total calcium)
- Maintains normal excitability of nerve and muscle cells
- Involved in neurotransmitter and hormone release
- Muscle contraction (skeletal and cardiac)
- Activation of many enzymes
- Coagulation of blood
- Milk production… and more!
therefore must be in narrow physiological range
where is calcium found in the body
0.1 % in the extra cellular fluid (ECF)
1 % in the cells
The rest (app. 99 %) in the skeleton: hydroxyapatite
what forms is calcium found in within the body?
- Protein-bound calcium
- cannot diffuse through membranes and thus is not usable by tissues
- Ionized or free calcium
- is the physiologically active form
- Complexed or chelated calcium
- is bound to phosphate, bicarbonate, sulfate, citrate, and lactate
why is plasma calcium regulation so important
what occurs when too low / too high
- Free (ionized) calcium is tightly regulated (±5%)
- Too low = neuronal hyper-excitability (tetany)
- Low ionized calcium levels in the extracellular fluid increase the permeability of neuronal membranes to sodium ion,
- causing a progressive depolarization,
- which increases the possibility of action potentials
- ‘claw hand feature’ - muscles are constantly contracting
- Low ionized calcium levels in the extracellular fluid increase the permeability of neuronal membranes to sodium ion,
- Too high = depression & kidney damage/stones
- Deposition within kidneys
control points for calcium
Absorption – Via intestines
Excretion – Via Kidney/urine
Temporary storage – Via bones
how is normal calcium levels balanced
- Constant exchange between cells and ECF
- When there is not enough calcium in them
- Calcium drawn from bones
- Osteoclast activity to release calcium ions
- Has to be replaced
- Excretion of some amount
- Calcium must be in a soluble and ionized form before it can be absorbed
- Regulation of calcium is closely associated with phosphate.
why do pregnant women who are 18 years or younger require more calcium than those who are 19 years old or older and pregnant
- 18 years or younger
- Growth plates are still open
- Calcium deposition is still required here
- Therefore require more calcium than those 19 years older and pregnant
- Also with breastfeeding
acute control of calcium levels
Must maintain constant free Ca2+ concentration in the plasma
Mostly by rapid exchange between bone and ECF
chronic control of calcium levels
- Maintain total level in the body long term
- Stimulate vitamin D activation
- Adjust GIT absorption and urinary excretion
how is plasma Ca2+ concentration regulated?
- is determined by:
- Net absorption of Ca2+ from the GIT
- Net excretion of Ca2+ in urine
- Exchange of Ca2+ with bone
what hormones control plasma Ca2+ concentration
- Parathyroid hormone
- Responsible for raising plasma calcium levels
- 1,25-dihydroxycholecalciferol
- Calcitriol (activated vitamin D)
- Inactivated form cannot regulate calcium
- Responsible for raising plasma calcium levels
- Calcitonin
- Responsible for lowering plasma calcium levels
what role does parathyroid gland have in regulation of calcium
- has chief cells
- has calcium sensing receptors
- responsible for synthesis and secretion of Parathyroid hormone during hypocalcium
characteristics of parathyroid hormone
where is it stored
when is it secreted & at what rate
where does it have effect
- peptide hormone
- stored within chief cells
- short half life - rapidly inactivated
- secreted continuously at a low rate
- released in response to low blood calcium
- exerts its effects on bone, git and kidneys
- has a direct effect on bone and kiney
- indirect effect on GIT
action of parathyroid hormone on kidney and bone
- increase reabsorption of calcium from the urine
- Fast acting; decrease in urinary calcium in minutes
- increase the expression of the enzyme 1α-hydroxylase (activates vitamin D)
- Bone: increase osteoclast activity (Indirectly) causing increase in bone resorption
how is the effect of parathyroid hormone on bone indirect?
osteoblasts have receptors for parathyroid hormone
osteoblasts then release factors which stimulate osteoclast activity
how is plasma calcium regulated by parathyroid hormone ??
- Low plasma calcium
- Parathyroid gland produces parathyroid hormone
- Activation of osteoclasts
- Increase in plasma calcium levels
- Also acts on kidney
- Stops secretion of calcium
- Reabsorbing more calcium in kidney nephron
- Stimulates expression of enzyme 1α-hydroxylase
- Activates vitamin D
- Increases plasma calcium levels also
- Activation of osteoclasts
- Parathyroid gland produces parathyroid hormone
simple negative feed back
how is plasma calcium regulated - overall
last column = chronic renal disease
radiographic presentation of hyperparathyroidism
effect of hyperparathyroidism in model of osseous structure
- Elevated PTH in HPT patients reduces cortical bone and protects trabecular bone.
- In combination with the biomechanical forces particular to the oral cavity, cortical bone loss and trabecular expansion result in an increased incidence of tori.
