Lecture 7 - calcium balance

Question 1

What are the main functions of calcium in the body?

Answer 1

• cell signalling (vessel exocytosis, muscle fibre contraction, altered enzyme function)
• blood clotting
• apoptosis
• skeletal strength
• membrane excitability (controls sodium permeability).

Question 2

What does an hypercalcaemia do to cell excitability?

Answer 2

• Decreases Na permeability.
• takes cell further away from threshold
• less likely to contract
• can lead to cardiac arrthymias if heart stops beating

Question 3

What does a hypocalcaemia do to cell excitability?

Answer 3

• Increases Na permeability.
• brings cell closer to threshold
• more likely to excite cell
• leads to tetany (contraction) of muscles
• can lead to asphyxia if larynx and respiratory muscles tetany.

Question 4

How much of the bodies calcium is stored in the bones?

Answer 4

99%

its stored with phosphate (hydroxyapatite)

Question 5

How much of the bodies calcium is stored in the intracellular fluid?

Answer 5

0.9%
mostly stored in the mitochondria and sarcoplasmic reticulum
only a tiny amount is free in the cytosol

Question 6

How much of the bodies calcium is stored in the extracellular fluid/plasma?

Answer 6

0. 1%
1. 05% is free/unbound
2. 04% is bound to plasma proteins
3. 01% is bound to anions (bicarbonate or phosphate)

Question 7

How much percentage of the bodies calcium is physiologically active?

Answer 7

0.05%

Question 8

How many millimolars of calcium is physiologically active in the body?

Answer 8

1.2mM

Question 9

How much does the calcium in skeletal muscle weigh?

Answer 9

1kg.

Question 10

What happens to calciums binding to plasma proteins when the pH rises?

Answer 10

• more calcium binds to the plasma proteins, decreasing the calcium concentration in the plasma.
• there’s less h+ ions competing to bind to the plasma protein.

Question 11

What happens to the calciums binding to plasma proteins when the pH falls?

Answer 11

• the plasma concentration of calcium will increase.
• less calcium will bind to the plasma proteins
• because there is more acidic H+ ions to bind to the plasma proteins

Question 12

What will hypocalcaemia cause?

Answer 12

Muscle tetany.

asphyxiation.

Question 13

Where do we get our calcium from?

Answer 13

entirely from the diet.

Question 14

Where does the calcium leave the body?

Answer 14

by the kidney and faeces

Question 15

What controls calcium concentration in the ECF/plasma?

Answer 15

Stores of calcium in the bone.
The bone favours maintaining calcium concentration over providing bone strength (it will give up calcium even if bones get weaker).

Question 16

Where in the body acts as a storage place for calcium?

Answer 16

The bone.

- you can add to the stores or take calcium out of it

Question 17

Which cells control bone turnover?

Answer 17

• osteoblasts = build new bone
• osteocytes = these are differentiated osteoblasts and controls the osteoblasts and osteoclasts
• osteoclasts = breakdown old bone

Question 18

what sort of environment aids calcium breakdown, releasing bone calcium to the plasma?

Answer 18

An acidic environment.

Question 19

How do osteoclasts break down old bone?

Answer 19

• secrete H+ cells
• creates acidic environment
• acid breaks down calcium salts
• also creates proteolytic enzymes to digest the extracellular matrix
• the acidic environment dissolves the calcium salts and releases it into the plasma

Question 20

Which hormones increase plasma calcium concentrations?

Answer 20

Parathyroid

calcitriol

Question 21

Which hormone decreases calcium concentrations?

Answer 21

Calcitonin

Question 22

How many parathyroid glands are there?

Answer 22

4

Question 23

Where are the parathyroid glands usually located?

Answer 23

on the posterior surface of the thyroid gland

1 in 10 people have them in different places eg - the arm

Question 24

What happens to the parathyroid glands during thyroid surgery?

Answer 24

You should try to preserve the parathyroid glands when removing the thyroid gland.
Even if 1 or 2 is left this will be enough.
The parathyroid gland is essential for life.

Question 25

What causes the secretion of parathyroid hormone?

Answer 25

A decrease in calcium concentration in the plasma

Question 26

What effects does parathyroid hormone have?

Answer 26

• inhibits osteoblasts from laying down new bone
• stimulates osteoclasts to break up bone
• stimulates production of calcitriol
• increases excretion of phosphate
• increases reabsorption of calcium from kidneys

Question 27

What step does parathyroid hormone stimulate in the calcitriol production?

Answer 27

Step 2 in the kidney

Question 28

What action does calcitriol have?

Answer 28

increases blood glucose:

• absorbs more Ca from the gut
• increases reabsorption of Ca from the kidneys

If there’s leftover calcium then it promotes bone remodelling/strengthening:

• increases reabsorption of phosphate
• increases mobilisation of calcium in the bones

Question 29

What is calcitriol made from?

Answer 29

Vitamin D

Question 30

What is vitamin D Ade from?

Answer 30

Comes from the diet

or sunlight binds with precursors in the skin to create it

Question 31

What stimulates calcitriol?

Answer 31

Prolactin - increased demand for Ca in the plasma to be used in milk production in lactation

Question 32

How much calcium is absorbed from the diet?

Answer 32

• healthy individuals - 30%
• vitamin D deficient - 10-15%
• pregnancy/lactation/growth spurts - 45-55% (because there’s more vitamin D).

Question 33

When are you classed as having vitamin D deficiency?

Answer 33

If levels fall to less than 20ml

Question 34

How does vitamin D deficiency result in bone weakness?

Answer 34

Because parathyroid hormone has to do all of the work.
Since it can’t increase absorption like calcitriol can, it takes all of the calcium from bones.
It also promotes phosphate secretion instead of reabsorption
This leaves the bones soft.

Question 35

What is rickets?

Answer 35

Soft bones that bend in children - their bones are still growing. especially the femur. from vitamin D deficiency.

Question 36

What is osteomalacia?

Answer 36

Softening of bones in full grown adults.

Question 37

What is phytate?

Answer 37

found in chapatti flour - binds to calcium making it harder to digest

Question 38

Can pigmented skin make vitamin D?

Answer 38

they are less able to make vitamin D in response to UV light

Question 39

What creates calcitonin?

Answer 39

The parafollicular C (Clear) cells in the thyroid.

Question 40

What does calcitonin do?

Answer 40

Decreases calcium plasma concentration:

- binds to osteoclasts to inhibit their bone resorption - no calcium can be taken and released into the blood

Question 41

what happens with increased calcitonin?

Answer 41

Nothing

the parathyroid hormone can counteract this so calcium concentration levels in the plasma don’t get too low.

Question 42

What other hormones alter calcium balance?

Answer 42

• cortisol - inhibits osteoblasts, increases renal excretion of calcium and phosphate, reduces absorption of intestinal calcium = reduces calcium

Insulin - increases bone formation and antagonises cortisol (diabetics with not enough insulin might have significant bone loss)

Oestrogen - promotes bone formation via oestrogen receptors on osteoblasts (post menopausal osteoporosis is a major problem)

growth hormone - stimulates bone formation

Prolactin - stimulates calcitriol formation which promotes calcium absorption from the gut