Bones -- Effects of Space Flight on Human Physiology

We are designed to live on Earth , Our bodies have been developed to live in the earth’s gravity, radiation, etc . Unless we find another planet with the exact size, shape, gravity and at a close proximity to us , we are stuck here… As we can see the farther we go from the Earth, The weaker our bodies get… Its like our bodies our yearning to go back. In this series of posts we will discussing in detail about the effects of Space Flight on the Human Physiology . SO... lets get started!

BONES

Bone is a living tissue. It is dynamic, responsive to disease and injury, and self-repairing. It is literally the thing that is holding us together! Bone has both an organic component and an inorganic component. The organic component is composed of mainly collagen, long chains of protein that intertwine in flexible and 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.

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. 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). 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.

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.

The most severe loss occurs between the second and fifth months in space, Astronauts regain most of their bone mass in the months following their return from space, but not all of it.

In the microgravity environment of space, astronauts lose on average 1% to 2% of their bone mineral density every month. This loss may not hinder astronauts while they are in orbit, but upon return to Earth, their weakened bones will be fragile and at an increased risk of fractures.

Bones are not unchanging calcium structures; they constantly reshape themselves in relation to the stress that is put on them. Just like muscles, if you don't use your bones, they will weaken. Bone loss occurs in the weightless environment of space because bones no longer have to support the body against gravity. (Lack of stress on bones among sedentary Earthlings, such as those confined to beds due to illness or old age, also contributes to bone loss.)

The actions of these two cell types -- called "osteoblasts" and "osteoclasts" -- usually balance each other out. But when stresses on bones are reduced (or during the onset of osteoporosis), removal outpaces replacement, leading to too little bone which can more easily break.

Bone loss is hardly such a far-off concern, though. Right here on our own planet millions suffer from osteoporosis. While in space, immobilization and illnesses occur at a much higher rate than normal aging, but both conditions are due to bone resorption (loss) occurring faster than formation of new bone.

Crew members engage in physical exercise for two and a half hours a day, six times a week (fifteen hours a week) while in orbit to avoid these issues. Nevertheless, the risks of these problems occurring cannot be completely eliminated through physical exercise alone.

The recovery after returning to Earth taking at least three or four years. Bisphosphonate is a therapeutic agent that has been used to treat osteoporosis patients for more than a decade, with a proven efficacy to increase bone mass and decrease the occurrence of bone fracture. Bone loss is also observed in bedridden older people