Calcium homeostasis Flashcards
Describe the roles of calcium in the body
- Ca is an important signalling molecule
- Important in blood clotting (Clotting cascade)
- Apoptosis - Programmed cell death
- Skeletal strength 99% - Calcium in body is wrapped up in bone
- Membrane excitability - Ca2+ decreases Na membrane permability
Describe the effects of hypocalcemia ?
- Increases neuronal Na+ permeability and leads to hyperpolarisation and hyper-excitation of neurons
- In extreme cases causing tetany and asphyixation
Describe the effects of hypercalcemia?
Decreases Na permeability causes hypopolarisation of cells
- This can lead to cardiac arrhythmias
Describe calciums disruption throughout the body?
- 99% is stored in bones, mostly in the form of hydroxyapatite
- 0.9% is stored intracellularly, in the mitrochondria and endoplasmic reticulum
- 0.1% is stored extracellularly
Describe the extracellular calcium?
0.1% of calcium is stored extracellularly (2.4mM)
Exists in 3 forms
- 50% of the (0.1%) is free in solution and physioloically active
- 40% bind to large neagtivley charged plasma proteins
- 10% neither bound to plasma proteins or free in solution, they bind to plasma anions (Bicarbonate, Phosphate)
Describe the effects of pH on calcium concentration
- During alkolsosis (Decreased pH) more calcium binds to plasma proteins, potential tetany
- During acidosis (Increased pH) less calcium binds to plasma protein and more is physiologically active, potential cardiac arrythmia
Describe the relationship between bone intergrity and Ca plasma balance
- Calciums main role is to provide mechanical support however plasma Ca takes precedence over this
Describe osteoblasts, osteoclasts and osteocytes
- Osteoblasts are used in the build up of new bone
- Osteoblasts differentiate to form osteocytes, which regulate the activity of osteoblasts and osteoclasts
- ## Osteoclasts are used in the breakdown of bone, secrete H ions
Describe the two hormones involved in control of calcium homeostasis
- Parathyroid hormone produced by parathyroid gland
- Calcitriol a product of Vit D3 produced by liver and kidneys
When is parathyroid hormone released and the function of PTH?
- PTH is released in decreased calcium plasma levels
- It acts to increase plasma calcium levels
What are the functions of PTH
- Stimulates osteoclasts to increase reabsorption at the bone
- Inhibits osteoblasts to reduce to reduce Ca bone deposition
- Increases Ca reabsorption at the kidneys, therefore decreasing excretion
- Increases renal excretion of phosphate, this elevates free calcium by preventing it from being deposited back into the bone
- Stimulates kidney to synthesise calcitriol from vitamin D3
Describe the production of calcitriol
- Vitamin D obtained from the diet or sunlight is activated to 25 - hydroxycholecalciferol in the liver
- This is converted to active vitamin D3 or calitriol in the kidney in response to PTH or prolactin by the kidney with the enzyme 1 alpha hydroxylase
Describe the actions of calcitriol
Calcitriol is a steroid hormone
- Increases absorption of Ca from the gut
- Facilitates reabsorption of Ca
- Mobilises calcium stores in the bone by stimulating osteoclasts activity
Describe normal, deficient and pregnancy, lactation, growth spurts effect on calcitriol level
- Healthy individual - 30% reabsorption from gut
- Vitamin D deficant individual - 10-15% reabsorbed from gut
- Pregnancy, lactation, growth spurts - 50% reabsorptoin from gut
Describe vitamin D3 deficiency and its consequences
- PTH will work hard to maintain Ca plasma levels
- This means stimulating osteoclasts and removing Ca from the bone
- It children still growing this can result in bending - rickets
- In adults this can result in bones that easily fracture - Osteomalacia
