Physiology of Bone and Calcium Homeostasis

Question 1

What is osteoid?

Answer 1

A collagen and protein mix produced by osteoblasts to which hydroxyapatite binds

Question 1

What are the major preventable risk factors for osteoporosis?

Answer 1

Low dietary Ca

Lack of exercise

Smoking

Question 2

Which cells release calcitonin?

Answer 2

C cells of the thyroid

Question 2

How does PTH cause increased activation of Vitamin D3

Answer 2

Activating kidney enzymes required for activation of D3

Question 3

When is calcitonin released?

Answer 3

Only in extreme hypercalaemia

Question 4

What are the progenitor cells to osteoblasts?

Answer 4

Fibroblasts

Question 4

The effect of PTH on Ca levels that is seen within minutes is due to what?

Answer 4

The increase in reabsorption in the kidneys

Question 5

What are chondrocytes?

Answer 5

Collagen producing cells of the cartilage

Question 5

What are the effect of PTH hyposecretion?

Answer 5

Hypocalcaemia

Hyperphosphataemia

Rare by often fatal

Question 6

What are the effects of calcitonin?

Answer 6

Decrease Ca and phosphates by:

Decreasing bone resorption

Increase Ca excretion

Question 7

Through which organ systems is the effect of PTH mediated?

Answer 7

Bone

Kidneys

Small intestines

Question 8

Explain the process by which chondrocytes lengthen bones

Answer 8

Chondrocytes replicate in vertical lines

Old chondrocytes at the end of those columns die leaving space

New bone is laid down by osteoblasts in the space left by chondrocytes

Question 8

Why do you get hypophosphataemia in a PTH hypersecretion state?

Answer 8

Because PTH blocks phosphate reabsorption (despite increase bone resorption that provides phosphate

Question 9

Does calcitonin have an effect on the small intestine?

Answer 9

No

Question 10

What percentage of total body Ca is extracellular?

Answer 10

0.1%

Question 11

What is osteocalcin and osteonectin?

Answer 11

Proteins produced by osteoblasts that aid bone deposition

Question 12

What are the main controllers of Ca reabsorption in the kidneys?

Answer 12

PTH = stimulates reabsorption

Calcitonin = downregulates reabsorption

Question 14

What makes up the major non-cellular component of bone?

Answer 14

Calcium phosphate (most commonly hydroxyapatite) crystalising on a collagen matrix

Question 16

What do osteoclasts produce to dissolve bone matrix?

Answer 16

H+ and enzymes

Question 17

Where in the kidney is phosphate reabsorbed?

Answer 17

Proximal tubule

Question 19

On which aspect of a bone is new bone depositied?

Answer 19

The outer aspect

Question 20

Is PTH or vitamin D3 the mediator of increased Ca absorption in the gut?

Answer 20

Vitamin D3 - but PTH helps activate vitamin D3

Question 21

What effect does PTH have on phosphate levels?

Answer 21

Decreases plasma phosphate

Question 23

What are the major signals for Ca acquisition from bone?

Answer 23

PTH

Calcitriol

Cortisol

Question 24

What effect does PTH have on Ca absorption in the small intestine?

Answer 24

Increase indirectly increases it via activating vitamin D3

Question 25

What is PTH secreted in response to?

Answer 25

Low Calcium

Question 26

Calcitriol is another name for what?

Answer 26

Vitamin D3

Question 27

Describe the appearance of osteoclasts

Answer 27

Large, multinucleated, mobile cells

Question 29

Why isn’t blood Ca low in vitamin D deficiency?

Answer 29

Because Ca levels are maintained by PTH stimulated resorption of bones - therefore you get bone demineralisation

Question 30

What are the main functions of Ca?

Answer 30

Intracellular signalling

Structural support

Cofactor in blood coagulation

Required for normal nerve excitability and function