Calcium Homeostasis Flashcards
What are the roles of calcium in the body?
Signalling Blood clotting Apoptosis Skeletal strength Membrane excitability
Why is calcium important in short term homeostasis?
Calcium plays a role in membrane excitability as it decreases sodium permeability
How does hypocalcaemia affect membrane excitability?
Hypocalcaemia increases neuronal sodium permeability leading to hyperexcitation of neurons. In extreme causes can cause tetany, which can spread to respiratory muscles and cause asphyxiation
How does hypercalcaemia affect membrane excitability?
Hypercalcaemia decreases neuronal sodium permeability which will reduce excitability and depress neuromuscular activity and in extreme cases trigger cardiac arrhythmias
Where is calcium stored intracellularly?
Mostly stored inside mitochondria and sarcoplasmic reticulum
How is calcium bound in the ECF?
~50% of calcium in ECF is bound to protein so very little body calcium is free in solution and physiologically active
How is calcium stored in bone?
Calcium is stored in the calcified extracellular matrix of bone, mostly in the form of hydroxyapatite (calcium-phosphate complex) so phosphate homeostasis is also important in determining calcium balance
What is the given value for calcium in ECF?
~2.4mM
How does pH affect binding capacity of calcium?
Binding capacity is increased under alkalotic conditions and decreased under acidotic conditions as pH has affect on binding capacity of plasma proteins
Where is calcium drawn from when ECF [Ca] is low?
Stores in the bone
What is the role of osteoblasts within bone?
Osteoblasts are the bone-building cells. They are highly active cells that lay down a collagen extracellular matrix which they then calcify
What is the role of osteocytes within bone?
Osteoblasts differentiate to form osteocytes in established bone. Osteocytes are less active than osteoblasts but appear to regulate the activity of osteoblasts and osteoclasts
What is the role of osteoclasts within bone?
Osteoclasts are the cells that are responsible for mobilising bone. They secrete hydrogen ions to dissolve the calcium salts and also provide proteolytic enzymes to digest the extracellular matrix
What are the hormones involved in endocrine control of calcium homeostasis?
Parathyroid hormone
Calcitriol
What effects do parathyroid hormone and calcitriol have on [Ca2]plasma?
They increase plasma calcium
What are the characteristics of parathyroid hormone and calcitriol?
Parathyroid hormone is a polypeptide hormone produced by the parathyroid glands
Calcitriol is a steroid hormone produced from vitamin D by the liver and kidneys
How does parathyroid hormone act to increase free [calcium] plasma?
- Stimulates osteoclasts to increase release of calcium and phosphate in bone
- Inhibiting osteoblasts to reduce calcium deposition in bone
- Increasing reabsorption of calcium from the kidney tubules, decreasing its excretion in urine
- Increasing renal excretion of phosphate, elevating free calcium by preventing it from being deposited back into bone (process requires a phosphate)
- Stimulates the kidney to synthesise calcitriol from vitamin D which promotes calcium absorption at the gut and kidney
What are the actions of calcitriol?
Increase of absorption of Ca2+ from the gut
Facilitates renal absorption of Ca2+
Mobilises calcium stores in bone by stimulating osteoclast activity
These actions complement those of parathyroid hormone to increase [Ca2+]plasma
How does calcitriol increase absorption of calcium from the gut?
Active transport system that moves calcium from the interstitial lumen to the blood is under the control of calcitriol.
How does vitamin D deficiency lead to a calcium and phosphate deficiency?
Vitamin D3 deficiency causes intestinal malabsorption of calcium, triggering an increase in parathyroid hormone, which promotes phosphate deficiency and aggravates calcium loss from bone
What is the main hormone that decreases free calcium in the plasma?
Calcitonin
What are the characteristics of calcitonin?
Peptide hormone produced by the thyroid gland in response to increases free calcium in the plasma
What are all the hormones that alter calcium balance?
Parathyroid hormone Calcitriol Calcitonin Cortisol Insulin Oestrogen Growth hormone Prolactin
