Astronauts regain most of their bone mass in the months following their return from space, but not all of it. Extended stays on Mir have resulted in losses of bone mass of as much as 20%. The most severe loss occurs between the second and fifth months in space, although the process continues throughout the entire time spent in microgravity. This bone loss begins within the first few days in space. This drop in density, known as disuse osteoporosis, leaves bone weak and less able to support the body's weight and movement upon return to Earth, putting the astronaut at a higher risk of fracture. The high amount of calcium found in astronaut's blood during spaceflight (much higher than on Earth) reflects the decrease in bone density, or bone mass. Over time, calcium normally stored in the bones is broken down and released into the bloodstream. At the same time, many bones that aid in movement are no longer subjected to the same stresses that they are subjected to on Earth. In space, the amount of weight that bones must support is reduced to almost zero. Some of the processes and functions of bones change after the astronaut has lived in microgravity for several days. Many bones, especially the ribs, also protect the internal organs. Protection: The skeleton houses and protects the brain, spinal column, and nerves.Marrow is important for the formation and development of red and white blood cells and platelets. Production of blood: In addition to essential minerals, bone is also the storage site of marrow.Like calcium, these minerals are used by many systems of the body for a wide range of purposes. Bone is also a good source of phosphate, hydrogen, potassium, and magnesium. When the body absorbs too much calcium from bones the skeleton can become thin and weak. From the bloodstream, the calcium is taken up by different organs and systems of the body. This constant calcium level in the bloodstream allows proper neural, muscular, and endocrine (hormone) functioning, as well as other cellular activities (e.g., blood clotting). ![]() ![]() Between meals, the body maintains a constant concentration of calcium by absorbing it from bone and releasing it into the bloodstream. The calcium is stored in hydroxapatite (the principal bone salt which provides the compressional strength of vertebrate bone).
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