MEH - Calcium Metabolism

Question 1

Where is most calcium stored in the body?

Answer 1

In the skeleton

Question 2

How much calcium is stored in the average adult human?

Answer 2

Around 1000g - 99% of this is in the bones in hydroxyapatite crystals

Question 3

How does the skeleton help with calcium metabolism?

Answer 3

It helps to buffer serum calcium levels and releases calcium phosphate into the interstitium

Question 4

How does parathyroid hormone affect calcium levels? (It is active when serum calcium is low)

Answer 4

• stimulates bone resorption and release of calcium into circulation
• stimulates calcium resorption in kidney

Question 5

How does dietary vitamin D affect calcium levels?

Answer 5

It increases intestinal absorption of dietary calcium and renal reabsorption of calcium, and increases bone resorption

Question 6

What is the role of calcitonin?

Answer 6

It counteracts the effects of PTH

Question 7

What do chief cells in the parathyroid gland do?

Answer 7

Produce parathyroid hormone

Question 8

How long is the half life of parathyroid hormone?

Answer 8

4 minutes

Question 9

What is the structure of PTH?

Answer 9

It is a straight chain polypeptide hormone

Question 10

Why must PTH be continually synthesised?

Answer 10

Chief cells degrade the hormone as well as synthesising it, so it is not stored

Question 11

What are the actions of PTH on bone?

Answer 11

• induces osteoblastic cells to synthesise and secrete cytokines on the cell surface
• cytokines stimulate differentiation and activity in osteoclasts and protect from apoptosis
• PTH decreases osteoblast activity
• resorption of mineralised bone and release of calcium and phosphate into extracellular fluid

Question 12

What is the typical dietary calcium intake per day?

Answer 12

1000mg/day

Question 13

What is vitamin D?

Answer 13

Lipid soluble vitamin which comes in two forms - D3 (made in skin and from dairy), and D2 (from yeast and fungi)

Question 14

Where does vitamin D come from?

Answer 14

Can be derived from plants or the action of sunlight on cholesterol in the skin

Question 15

What does vitamin D3 bind to in the circulation?

Answer 15

Transcalciferin

Question 16

What is the half life of vitamin D3?

Answer 16

0.25 days

Question 17

What is the main action of calcitriol?

Answer 17

Decreases the urinary loss of calcium by stimulating reabsorption in the kidney

Question 18

Where is calcitonin secreted?

Answer 18

Parafollicular cells of the thyroid gland

Question 19

How is calcitonin used clinically?

Answer 19

To lower serum calcium in cases of hypercalcaemia

Question 20

True or false - calcium is not involved in blood clotting?

Answer 20

False - it is factor IV in the clotting cascade

Question 21

Give some causes of hypercalcaemia in a hospital setting

Answer 21

Malignant osteocytic bone metastases, multiple myeloma

Question 22

Give some common cancers which can metastasise to bone causing hypercalcaemia

Answer 22

Breast, lung, renal, thyroid

Question 23

What causes primary hyperparathyroidism?

Answer 23

One of the four parathyroid glands develops an adenoma and secretes excessive parathyroid hormone, causing serum calcium to rise

Question 24

What are the symptoms of primary hyperparathyroidism?

Answer 24

Bones, (kidney) stones, abdominal groans and ‘psychic moans’

Question 25

What are the effects of hypercalcemia on neurones?

Answer 25

Lethargy, confusion, coma due to suppression of neuronal activity

Question 26

What are the effects of hypocalcemia on nerves?

Answer 26

Leads to ‘excitable’ nerves because it lowers the threshold for depolarisation

Question 27

Why is it important to keep patients who have severe hypercalcemia hydrated?

Answer 27

The polyuria can lead to dehydration which exacerbates the hypercalcemia

Question 28

What is osteomalacia?

Answer 28

The ratio of mineral to matrix is decreased, so there is not enough mineral content. It leads to soft bones which are prone to bending.

Question 29

What is osteomalacia referred to in children?

Answer 29

Rickets