Weightlessness and Osteoporosis Flashcards
What is the composition of bones?
- 70% inorganic: hydroxyapatite crystals (Ca and PO4)
- 30% organic: 2% cells, 93% collagen, water
What is the role of hydroxyapatite crystals in bone?
-Helps resist compression
What is the role of collagen in bone and where is it?
- Provides tensile strength
- Resists stretching as it is flexible
- Sitting embedded in ground substance
What are the bone cells?
- Osteoclasts
- Osteoblasts
- Osteocytes
What is the rate of bone and muscle turnover?
-Around 20% in young/ year and around 2% in elderly
In a young adult what % body mass is made up by bone and muscle?
- Bone around 15%
- Muscle around 30% in women and 40% in men
What does the mass of muscle and bone reflect?
-A complex interaction of mechanical demands, genetic and endocrine factors
When does bone remodel?
-In response to mechanical load and microdamage
Describe the process of bone remodelling?
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
What does bone remodelling rely on?
- An adequate diet to provide protein, mineral and vitamins
- Balance of PTH, calcitonin and vitamin D
What does PTH do?
-Increases plasma Ca by releasing it from bone
What does Vit D do?
-Increases plasma Ca and PO4 absorption from the gut and recovery from renal filtrate
What does calcitonin do?
-Lowers plasma Ca by reducing osteoclast activity
What can dsiturb the breakdown/ growth of bone?
-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)
What are some of the physiological consequences of prolonged space flight?
- Fluid shifts, fluid and electrolyte loss
- Negative energy balance
- Bone loss
- Skeletal and cardiac muscle atrophy
- Radiation exposure
- These are similar processes to ageing
What are the condition in space station?
-Not 0g conditions, more like free fall conditions
How may food intake change in space and why is this significant?
-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.
What does loss of muscle do in space and what happens to O2 uptake in space?
- Loss of muscle, leads to reduction in exercise capacity
- 28% reduction in peak O2 uptake after 20 days in space
What happens to bone remodelling in spaceflight?
-Induces loss of bone due to increased bone resorption and decreased bone formation.
What are possible solutions to changes in bone remodelling from spaceflight?
- Reduce resorption with alendronate (bisphosphonate)
- Maintain formation with heavy resistance exercise
- Maintains nutrients: Vit D supplementation, other vitamins and minerals
How can body mass loss be restricted in space?
- 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).
How can bone and muscle mass loss be restricted?
-Exercise; anchoring to treadmills for endurance exercise
Lifting weights against springs for resistance exercise
Why do astronauts sit down when they come back?
- Hypotension (reduced blood volume)
- Weakness (sarcopenia)
- Bone demineralisation (osteopenia)
What happens to functional capacity with increasing age and why is this important?
- Reduces
- Low muscle strength is the main cause of preventable falls in elderly
- Falls are main cause of accidental death in elderly
