Calcium

Question 1

What is the daily calcium intake

Answer 1

500 - 1500 mg/day

About 30% of dietary calcium is absorbed

Question 2

Where is calcium stored

Answer 2

99% of total calcium is stored in the skeleton

Remainder distributed between intracellular and extracellular fluid

Question 3

What is calcium required for

Answer 3

Physiological processes
- skeletal growth and maintenance
- Normal blood clotting
- Muscle contractility
- Nerve function

Question 4

What is the plasma concentration off calcium

Answer 4

Controlled to narrow limits

2.2-2.6 mmol/L

approx. 45% bound to plasma proteins - biologically unavailable

45% circulating ionised form

10% bound to other proteins
- citrate
- phosphate

Question 5

What % of plasma calcium is bound to plasma proteins

Answer 5

45%

Question 6

What % of plasma calcium is circulating in its ionised form

Answer 6

45%

Question 7

What % of plasma calcium is bound to compounds such as citrate and phosphate

Answer 7

10%

Question 8

When is calcium metabolically active

Answer 8

In ionised state

Question 9

What organ regulates the amount of calcium excreted in the body

Answer 9

Kidney

Question 10

Approximately what % of serum calcium filtered by the kidneys is reabsorbed

Answer 10

98%

Question 11

Where is calcium reabsorbed in the kidneys

Answer 11

Passively - proximal tubule and ascending loop of Henle

Actively - Distal tubes of the kidneys

Question 12

What happens when plasma calcium falls

Answer 12

Bone mineral reservoir acts by increasing the rate of bone resorption

Leads to a decrease in bone mineral density

Question 13

What acts as a readily accessible source of exchangeable calcium

Answer 13

Skeleton

Question 14

What are the most important hormones in calcium homeostasis

Answer 14

PTH

Calcitriol (1,25 dihydroxy Vitamin D)

Question 15

What is the role of PTH in calcium homeostasis

Answer 15

Release is increased in response to low serum ionised calcium

Parathyroid gland senses changes through calcium sensing receptors

Acts in three ways to increase serum calcium
1. Decreasing renal calcium excretion
2. Increasing bone reabsorption
3. Enhancing dietary calcium absorption by stimulating the production of calcitriol in the kidney

Question 16

What is the role of calcitonin in calcium homeostasis

Answer 16

Secreted by cells in the thyroid glands

Reduces serum calcium concentration

Question 17

How much calcium is in the skeleton

Answer 17

1200g

Question 18

How much calcium is in extracellular space

Answer 18

1g

Question 19

How much in mmol/L is the ionised serum calcium

Answer 19

1.1. mmol/L

Question 20

What is the affect of alkalosis on calcium

Answer 20

Higher pH albumin binds strongly to calcium

Total serum calcium = unchanged (2.4 mmol/L)
Ionised serum calcium = decrease (0.9mmol/L)

Extracellular fluid space calcium is in equilibrium with ionised calcium

Question 21

What is low ionised calcium associated with

Answer 21

Contraction of the small muscles of the feet and hands = tetany

Due to depolarisation of the long nerves of the upper limb

Question 22

What are the three ways homeostasis is maintained

Answer 22

Calcium absorption (from intestines) / endogenous faecal Ca

Calcium reabsorption (from kidneys) / Ca excretion

Bone resorption / bone formation

Question 23

What are the source of dietary calcium

Answer 23

Major source = dairy products 2/3

Minor sources
- vegetables e.g. broccoli
- cereals e.g. white bread
- oily fish e.g. sardines

Question 24

What % of dietary calcium do we absorb

Answer 24

30%

Question 25

When do we absorb a higher fraction of calcium

Answer 25

When put on a low calcium diet Mediated by calcitriol - the active form of vitamin D Most calcium is absorbed by active transport

Question 26

When can calcium be released rapidly

Answer 26

Exchangeable calcium on the bone surface

Question 27

When can calcium be released slowly

Answer 27

By osteoclasts during bone resorption

Question 28

What does the amount of calcium filtered by the glomerulus depend on

Answer 28

Glomerular filtration rate Ultra filterable calcium - ionised - complexed

Question 29

What increases/decrease the amount of calcium reabsorbed

Answer 29

More - PTH level is high Less - filtered sodium is high High sodium diet decreases Claudin-2 expression, resulting in increased TRPV5 and calcium transport

Question 30

Describe calcitonin

Answer 30

Hormone produced by C cells (parafollicular cells) in thyroid Secretion is stimulated by an increase in serum calcium Effect is to lower bone resorption Importance in man is uncertain

Question 31

What is the action of the parathyroid hormone

Answer 31

Acts on - bone - kidney Uses second messengers such as cyclic AMP, calcium Positive and negative signs relate to actions in osteoblasts

Question 32

What is the response to a low calcium diet

Answer 32

Low dietary calcium = Less calcium is absorbed = Results in low serum ionised calcium = Results in higher PTH = Results in fast and slow responses Returns serum ionised calcium to normal

Question 33

What is low dietary calcium an example of

Answer 33

Negative feedback