Weightlessness and Osteoporosis

Question 1

What is the composition of bones?

Answer 1

• 70% inorganic: hydroxyapatite crystals (Ca and PO4)

- 30% organic: 2% cells, 93% collagen, water

Question 2

What is the role of hydroxyapatite crystals in bone?

Answer 2

-Helps resist compression

Question 3

What is the role of collagen in bone and where is it?

Answer 3

• Provides tensile strength
• Resists stretching as it is flexible
• Sitting embedded in ground substance

Question 4

What are the bone cells?

Answer 4

• Osteoclasts
• Osteoblasts
• Osteocytes

Question 5

What is the rate of bone and muscle turnover?

Answer 5

-Around 20% in young/ year and around 2% in elderly

Question 6

In a young adult what % body mass is made up by bone and muscle?

Answer 6

• Bone around 15%

- Muscle around 30% in women and 40% in men

Question 7

What does the mass of muscle and bone reflect?

Answer 7

-A complex interaction of mechanical demands, genetic and endocrine factors

Question 8

When does bone remodel?

Answer 8

-In response to mechanical load and microdamage

Question 9

Describe the process of bone remodelling?

Answer 9

1-Activation: preosteoclasts stimulated and differentiate under the influence of cytokines and growth factors into mature active osteoclasts
2-Resorption: osteoclasts digest mineral matrix (old bone)
3-Reversal: end of resorption
4-Formation: osteoblasts synthesise new bone matrix
5-Quiescence: osteoblasts become resting bone lining cells on the newly formed bone surface

Question 10

What does bone remodelling rely on?

Answer 10

• An adequate diet to provide protein, mineral and vitamins

- Balance of PTH, calcitonin and vitamin D

Question 11

What does PTH do?

Answer 11

-Increases plasma Ca by releasing it from bone

Question 12

What does Vit D do?

Answer 12

-Increases plasma Ca and PO4 absorption from the gut and recovery from renal filtrate

Question 13

What does calcitonin do?

Answer 13

-Lowers plasma Ca by reducing osteoclast activity

Question 14

What can dsiturb the breakdown/ growth of bone?

Answer 14

-Diet and lifestyle: during rapid growth, low intake, too little sunlight etc
Solution: food supplements, clean air, more time outside
-Mechanical loading: activity to promote growth, in young and elderly
-Endocrine factors: changes over the life cycle
Endocrine disorders (pituary hormones, acromegaly, dwarfism)

Question 15

What are some of the physiological consequences of prolonged space flight?

Answer 15

• Fluid shifts, fluid and electrolyte loss
• Negative energy balance
• Bone loss
• Skeletal and cardiac muscle atrophy
• Radiation exposure
• These are similar processes to ageing

Question 16

What are the condition in space station?

Answer 16

-Not 0g conditions, more like free fall conditions

Question 17

How may food intake change in space and why is this significant?

Answer 17

-Less appetite and motion sickness and food being presented unapeallingly may decrease intake.
-Could lead to muscles decondition and leg volume decreases, due to reduced load muscles in microgravity.
Some reduction is fluid and some muscle mass.

Question 18

What does loss of muscle do in space and what happens to O2 uptake in space?

Answer 18

• Loss of muscle, leads to reduction in exercise capacity

- 28% reduction in peak O2 uptake after 20 days in space

Question 19

What happens to bone remodelling in spaceflight?

Answer 19

-Induces loss of bone due to increased bone resorption and decreased bone formation.

Question 20

What are possible solutions to changes in bone remodelling from spaceflight?

Answer 20

• Reduce resorption with alendronate (bisphosphonate)
• Maintain formation with heavy resistance exercise
• Maintains nutrients: Vit D supplementation, other vitamins and minerals

Question 21

How can body mass loss be restricted in space?

Answer 21

• Limit extra vehicular activity and volume of exercise (not possible due to volume of work and use of exercise as a countermeasures for bone and muscle loss.
• Increase food intake
• Increase calorie content of diet (not possible if astronauts suffering from motion sickness).

Question 22

How can bone and muscle mass loss be restricted?

Answer 22

-Exercise; anchoring to treadmills for endurance exercise

Lifting weights against springs for resistance exercise

Question 23

Why do astronauts sit down when they come back?

Answer 23

• Hypotension (reduced blood volume)
• Weakness (sarcopenia)
• Bone demineralisation (osteopenia)

Question 24

What happens to functional capacity with increasing age and why is this important?

Answer 24

• Reduces
• Low muscle strength is the main cause of preventable falls in elderly
• Falls are main cause of accidental death in elderly