The Skeletal System

• American astronaut Scott Kelly and Russian colleague Mikhail Kornienko began a one-year mission on the International Space Station


  March 2015
• This was the longest tour of duty ever served on the ISS
• Being in microgravity for a long time


  Causes damage to astronauts' bones
• Bones


  Dynamic, living tissue that is constantly breaking down, regenerating, and repairing itself
• An average human body contains 206 bones
• Types of bones by location

  • Axial bones (skull, vertebral column, rib cage)
  • Appendicular bones (limbs, pelvis, shoulder blades)
• Types of bones by shape


  • Long bones
  • Short bones
  • Flat bones
  • Irregular bones
• Compact bone


  Dense, smooth-looking external layer of bone
• Spongy bone


  Porous, honeycomb-looking area of bone made up of trabeculae
• Red marrow


  Produces blood cells
• Yellow marrow


  Stores energy as fat
• Structure of long bones


  • Flared ends (epiphyses) bookend the shaft (diaphysis) which surrounds a hollow medullary cavity
• Osteons


  Cylindrical, weight-bearing structural units of bone composed of concentric tubes (lamellae) filled with collagen fibers
• Lacunae


  Tiny spaces between lamellae that house osteocytes
• Osteoblasts


  Bone-building cells that construct bone tissue
• Osteoclasts


  Bone-breaking cells that resorb old bone tissue
• Bone remodeling


  1. Osteocytes detect stress/strain and signal osteoclasts to resorb old bone

  2. Osteoblasts then rebuild new bone
• Exercise stimulates bone remodeling and strengthens bones
• Microgravity in space


  Increases osteoclast activity and decreases osteoblast activity, leading to net bone loss
• Astronauts need to exercise at least 15 hours per week to slow bone degradation in space