Life in the microgravity environment of space brings many changes to the human body. The loss of bone and muscle mass, change in cardiac performance, variation in behavior, and body-wide alterations initiated by a changing nervous system are some of the most apparent and potentially detrimental effects of microgravity. Changes to bone are particularly noticeable because they affect an astronaut's ability to move and walk upon return to Earth's gravity.
Structure and Function of Bone
Bone is a living tissue. It is dynamic, responsive to disease and injury, and self-repairing. Bone has both an organic component and an inorganic component. The organic component is composed mainly of collagen, long chains of protein that intertwine in flexible, elastic fibers. Hydroxyapatite, the inorganic component, is a calcium-rich mineral that stiffens and strengthens the collagen. Together, the interwoven organic and inorganic components of bone create a sturdy yet flexible skeletal structure.
Image to right: Bone loss is one example of how the body changes in microgravity. Credit: NASA
The body is constantly breaking down old bone, and replacing it with new bone. Bone is formed by cells called osteoblasts. These cells lay down new mineral along the surface of bone. Osteoclasts, large multinucleate cells, breaks down old bone, and are in part responsible for releasing calcium into the bloodstream. In a healthy individual on Earth, bone is formed at the same rate at which it is broken down, so there is never an overall loss of bone mass. This process changes as a person grows older, or enters microgravity for an extended period of time.
On Earth, bones perform four basic functions:
Mechanical support: The skeleton supports soft tissue and the body's weight. Many bones also act as levers for muscles, enabling movement.
Storage of essential nutrients: Bone stores much of the calcium received from the diet. The calcium is stored in hydroxapatite (the principal bone salt which provides the compressional strength of vertebrate bone). Between meals, the body maintains a constant concentration of calcium by absorbing it from bone and releasing it into the bloodstream. 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). From the bloodstream, the calcium is taken up by different organs and systems of the body. When the body absorbs too much calcium from bones the skeleton can become thin and weak. Bone is also a good source of phosphate, hydrogen, potassium, and magnesium. Like calcium, these minerals are used by many systems of the body for a wide range of purposes.
Production of blood: In addition to essential minerals, bone is also the storage site of marrow. Marrow is important for the formation and development of red and white blood cells and platelets.
Protection: The skeleton houses and protects the brain, spinal column, and nerves. Many bones, especially the ribs, also protect the internal organs.
Bone and Microgravity
Some of the processes and functions of bones change after the astronaut has lived in microgravity for several days. In space, the amount of weight that bones must support is reduced to almost zero. 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. Over time, calcium normally stored in the bones is broken down and released into the bloodstream. 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. 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.
This bone loss begins within the first few days in space. The most severe loss occurs between the second and fifth months in space, although the process continues throughout the entire time spent in microgravity. Extended stays on Mir have resulted in losses of bone mass of as much as 20%. Astronauts regain most of their bone mass in the months following their return from space, but not all of it.
The exact mechanism that causes the loss of calcium in microgravity is unknown. Many scientists believe that microgravity somehow causes bone to break down at a much faster rate than it is built up. However, the exact trigger for this rate change has not been found. Researchers are currently pursuing multiple lines of research, including hormone level, diet, and exercise, in order to determine exactly what causes -- and may control or prevent -- osteoporosis during space flight.
Another type of osteoporosis is a problem on Earth. As we grow older, the body begins to absorb bone much faster than it produces new bone. This leads to a lowered bone density, the same effect that microgravity has on astronauts. As a result, bones become more fragile and are more susceptible to fractures, especially in the hip, spine, and wrist. In many cases, people do not know that they have
osteoporosis until their bones become so weak that an accidental bump or fall causes a fracture.
Just as astronauts eat a careful diet and get plenty of special exercise in space to prevent disuse osteoporosis, steps can be taken to prevent osteoporosis on Earth. A balanced diet rich in calcium and vitamin D, exercise, a lifestyle free of smoking and alcohol, bone density testing, and medication all prevent or alleviate osteoporosis.
Excerpted from Virtual Astronaut's Bag of Bones