Bone metabolism

Question 1

What is bone remodelling?

Answer 1

The removal of old bone which is replaced by new bone, which has an appositional growth where new growth is on the outer edges and newer bones are in the centre.

Question 2

What is the process of bone remodelling?

Answer 2

Osteoclasts begin bone remodelling by inducing bone resorption in the centre which frees calcium stores which is utilised by quiescent mesenchymal cells to differentiate into osteoblasts and secrete osteoids which become calcified and form a mineral that traps osteoblasts to form osteocytes.

Question 3

How do osteoblasts become osteocytes?

Answer 3

They secrete osteoids that bind to calcium and become mineralised to entrap the osteocytes.

Question 4

What happens to bone as we age?

Answer 4

Bone growth is rapid until age 30 where it declines gradually. This process is faster post menopause because of the drop in oestrogen levels which promote bone deposition.

Question 5

What does bone store?

Answer 5

Bone stores calcium and phosphate as hydroxyapatite crystals.

Question 6

How are bone mineral levels regulated?

Answer 6

By PTH, oestrogen and androgens which responds to low calcium levels in the blood and calcitonin which responds to high calcium levels. Vitamin D contributes to increasing calcium levels in blood.

Question 7

How does phosphate affect calcium levels?

Answer 7

In the serum, phosphate and calcium form a crystal to be taken up.

Question 8

How does PTH affect the bones?

Answer 8

It increases the formation of osteoclasts from osteocytes for bone resorption to increase the release of calcium for absorption in the small intestine.

Question 9

How does PTH affect the kidneys?

Answer 9

It decreases reabsorption of phosphate in the PCT and increases reabsorption of calcium in the DCT to be taken up in the serum. This is due to phosphate forming a salt crystal with calcium that limits its absorption.

Question 10

What is bone metabolism?

Answer 10

The constant production and degradation of bone which is influenced by Parathyroid hormones.

Question 11

What is the role of calcium?

Answer 11

Calcium is important for nerve conduction, muscle contraction and blood clotting. It is stored along with phosphorus as hydroxyapatite crystals. PTH increases calcium reabsorption in the DCT to increase blood serum levels for reabsorption in the small intestine.

Question 12

What is the role of phosphorus?

Answer 12

Phosphorus is stored with calcium as hydroxyapatite crystals in the bone. PTH responds to low calcium levels by increasing kidney excretion of phosphorus in the PCT.

Question 13

What is the role of vitamin D?

Answer 13

Responsible for increasing calcium and phosphate levels in the serum via promoting bone resorption for absorption in the small intestine.

Question 14

What is the process of Vitamin D formation in the body?

Answer 14

Synthesised by sunlight/taken by diet as Vitamin D3 calciferol along with D2 eciferol. This is hydroxylated by the liver to form 25 (OH) d3 and hydroxylated again by the kidney via PTH to form 1,25 (OH)2 D3.

Question 15

What is the action of 1,25 hydroxyvitamin D?

Answer 15

It increases bone resoprtion via osteoclast formation to release calcium and phosphate in the serum for absorption in the small intestine

Question 16

What is parathyroid hormone?

Answer 16

Hormone released by the parathryroid glands in a negative feedback cycle as a response to blood calcium levels to increase by promoting osteoclasts and bone resorption.

Question 17

What is the action of the parathyroid hormone?

Answer 17

It activates vitamin D via hydroxylation in the liver to form 1,25 (OH)2 vitamin D which increases serum calcium and phosphate levels and intestinal absorption. It also increases osteolysis and bone resoprtion.

Question 18

What is calcitonin?

Answer 18

Secreted by parafollicular cells in the thyroid gland in response to high calcium levels which decreases bone resorption and binds to calcitonin receptors on osteoblasts to increase bone deposition.

Question 19

What is the role of oestrogen?

Answer 19

Responds to low calcium levels by decreasing osteolysis and increasing action of osteoblast. It has a high anabolic effect.

Question 20

What is the role of androgens?

Answer 20

Responds to low calcium levels by decreasing osteolysis and increasing osteoblast action

Question 21

What is the effect of vitamin D on osteoblasts?

Answer 21

No effect

Question 22

What is the effect of vitamin D on osteoclasts?

Answer 22

Reduces osteoclast activity in response to high serum calcium

Question 23

What is an anabolic effect?

Answer 23

When a hormone promotes bone remodelling, haematopoiesis and protein synthesis such as oestrogen

Question 24

What is the effects of hormones on bone metabolism?

Answer 24

PTH and calcitonin increase calcium and phosphate levels, oestrogen and androgen decrease calcium and phopshate levels.

Question 25

What is metabolic bone disease?

Answer 25

When there is a disruption of bone remodelling caused by reduction of bone quality or bone quantity or kidney disease

Question 26

What is osteoporosis?

Answer 26

Reduction of bone quantity which leads to increased risk of fractures with simple falls that reduces confidence. Treatment is usually with PTH hormones and calcium and vitamin D supplements. Determined by T value where your standard deviation of bone density is compared to a healthy person of the same sex and it is greater than -2.5

Question 27

What are the risk factors for osteoporosis?

Answer 27

Elderly, female, post-menopausal, low BMI, smoking, drinking.

Question 28

What is osteopenia?

Answer 28

Low bone density which makes you at risk for osteoporosis. T score will be between -1 and -2.5.

Question 29

How can osteoporosis occur?

Answer 29

Due to greater rate of bone resorption than bone deposition.

Question 30

What is osteomalacia?

Answer 30

Reduction of the bone quality due to Vitamin D deficiency caused by malabsorption or not enough exposure to sunlight that leads to thick but fragile bones.

Question 31

What is the cause of osteomalacia?

Answer 31

Vitamin D deficiency, which is treated through exposure to sunlight and taking vitamin D supplements.

Question 32

What is paget’s disease?

Answer 32

Disruption of bone remodelling that causes deformed irregularly joined bones
which are weakened. Individuals may be symptomatic or suffer from pain.

Question 32

How is paget’s disease treated?

Answer 32

Drugs to induce osteoporosis.

Question 33

How can kidney disease affect the bones?

Answer 33

Kidney is responsible for the hydroxylation of vitamin D, stimulated by PTH for the release of calcium and phosphate to increase serum levels via bone resorption of old bone for uptake of these nutrients in the small intestines. Kidney disease will lead to increased bone resorption and result in calcium deposits in the heart.