Control of extracellular calcium

Question 1

What is endocrine control of extracellular calcium often referred to?

Answer 1

Calcium homeostasis

Question 2

What does calcium homeostasis mean?

Answer 2

Keeping calcium at a constant level in the extracellular compartment (plasma and extracellular fluid).

Question 3

What is the relatively constant calcium level?

Answer 3

• Approximately 2.5mM in total calcium
• 1.25mM is free calcium

Question 4

Is it the total calcium or free calcium that needs to be kept at a constant level?

Answer 4

Free calcium

Question 5

What are the 3 main hormones involved in calcium homeostasis?

Answer 5

• Parathyroid hormone (PTH) - Most important role
• Calcitonin
• Vitamin D

Question 6

What is the major consequence of change in extracellular calcium?

Answer 6

The major consequence of change in extracellular calcium is the changes to excitability of excitable cells (e.g. neurons, muscle cells etc)

Question 7

Why is extracellular calcium levels important?

Answer 7

Calcium is important in controlling the permeability of the cell membrane to sodium

• Low Ca2+ increases Na+ permeability
• High Ca2+ decreases Na+ permeability

Question 8

What happens if we increase the Na+ permeability?

Answer 8

It will cause a depolarisation. Because if we increase sodium permeability that means more sodium can go into the cell therefore more positive charge going in, resulting in depolarisation.

Question 9

What happens if we decrease the Na+ permeability?

Answer 9

It will cause hyperpolarisation

Question 10

What is hypocalcaemia?

Answer 10

• When there is too little calcium outside the cell
• Will increase muscle/nerve excitability due to depolarisation
• Can cause muscle spasm -> fatal via spasm of the respiratory muscles

Question 11

What is hyercalcaemia?

Answer 11

• When there is too much calcium outside the cell
• Will decrease the muscle/nerve excitability due to hyperpolarisation
• Can cause cardiac arrhythmias -> fatal

Question 12

What are the different calcium compartments in the body?

Answer 12

• Free Ca2+ (1.1-1.4mM)
• Bound Ca2+
• Ca2+ reservoir (bones and teeth) 99%
• Ca2+ excretion kidneys
• Dietary Ca2+
• Free Ca2+ cytosole (100nM)
• Bound Ca2+ intracellular
• Intracellular Ca2+ stores (ER,SR,mitochondria)

Question 13

What are the 2 types of bone?

Answer 13

• Trabecular bone
• Compact/cortical bone

Question 14

What is the trabecular bone?

Answer 14

The spongy inner layer network of trabeculae is lighter and less dense than cortical bone

Question 15

What is the cortical bone?

Answer 15

The hard-outer layer, is strong and dense.

Question 16

What does the structure of the cortical bone consist of?

Answer 16

• Osteon (circular structures around the central canal)
• Central canal (where the blood vessel is found)
• Osteocytes (make up the osteon)
• Lamella (Network of cells in the osteocyte)
• Canaliculi (contain the extracellular fluids for the cells - bone fluid)
• Osteoblast (line the central canal and the outside of the bone)
• Osteoclast (sit on the outside of the bone)

Question 17

What does majority of the bone comprise of?

Answer 17

Hydroxyapatite crystals which precipitates around the collagen extracellular matrix (the osteoid) in the bone.

• There are high levels of Ca2+ and PO4^3- in the fluid within the bone