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How
Astronauts Feel in Mircogravity
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How Astronauts
Feel in Mircogravity
Astronauts
have the following feelings when they first encounter microgravity:
- Nausea
- Disorientation
- Headache
- Loss of appetite
- Congestion
The longer
they stay in microgravity, the more their muscles and bones weaken. These
feelings are caused by changes in various systems of the body. Let's discuss
how the body responds to microgravity.
Spacesick
Before dicussing spacesick, let's talk about how people sense position
and motion. Orientation and motion are sensed by using the vestibular
system, which is located in the upper portion of the inner ear.
How the vestibular system senses orientation with respect to gravity?
- It has otolithic organs that contain crystals of calcium carbonate
- The crystals are attached to hair-like sensory nerve cells in three
orientations (x-, y-, z-axes)
1. When you
bend your head in different directions (forward, backward, sideways),
gravity pulls on the crystals that are oriented in the direction of the
pull.
2. The affected crystals stimulate the attached hair cells to send nerve
impulses to the brain.
3. The brain interprets these signals to find out which way the head is
oriented in space.
How the vestibular
system sense motion?
- There are three semicircular canals for sensing motion, specifically
acceleration.
- They are oriented at right angles to one another, and each is in one
of the three directions (x-, y- or z-axis).
- They contain fluid called endolymph and hair-like sensory nerve cells.
1. As your head accelerates in a given direction, the endolymph lags behind
because of its initial resistance to change in motion (inertia).
2. The lagging endolymph stimulates the appropriate hair cells to send
nerve signals to the brain.
3. The brain interprets them to find out which way the head has moved.
The nausea and disorientation that you feel are like your experience in
a drop on a roller coaster ride, only you have that feeling constantly
for several days.
This is the feeling of space sickness, or space motion sickness, which
is caused by conflicting information that your brain receives from the
eyes and the vestibular organs located in the inner ear. The eyes can
see which way is up and down inside the shuttle. However, because The
vestibular system relies on the downward pull of gravity to tell the astronauts
which way is up versus down and in which direction they are moving, it
does not function in microgravity. So the eyes may tell your brain that
they are upside-down, but the brain does not receive any interpretable
input from the vestibular organs. Their confused brain produces the nausea
and disorientation, which in turn may lead to vomiting and loss of appetite.
Fortunately, after a few days, the brain adapts to the situation by relying
solely on the visual inputs, and they begin to feel better. And NASA has
produced medication patches to help astronauts deal with the nausea until
their bodies adapt.
Puffy Face
Normally, when someone standing upright, gravity pulls blood downward
so it pools in the veins of the legs. In microgravity, the blood shifts
from the legs into the chest and head, so the face will feel full and
the sinuses will feel congested, which may lead to headaches and space
motion sickness. Also, the face tends to get puffy and the sinuse swell.
When the blood shifts to the chest, the heart increases in size and pumps
more blood with each beat. The kidneys respond to this increased blood
flow by producing more urine. Moreover, the increase in blood and fluid
decease anti-diuretic hormone (ADH) secretion by the pituitary gland,
which makes people less thirsty. Therfore, astronauts do not drink as
much water as they might on the Earth. After returning to the Earth, gravity
will pull those fluid back down to the legs and away from the head, which
will cause the people to feel faint when standing up. But the astronauts
will also begin to drink more, and the fluid levels will return to normal
in a couple of days.
Space Anemia
As the kidneys eliminate the excess fluid, they also decrease their secretion
of erythropoietin, a hormone that stimulates red blood-cell production
by bone marrow cells. The decrease in red blood-cell production matches
the decrease in plasma volume so that the hematocrit (percentage of blood
volume occupied by red blood cells) is the same as on Earth. Upon the
return to Earth, the erythropoietin levels will increase, as will the
red blood-cell count.
Weak Muscles
and Bird Legs
When astronauts are in microgravity, their body adopts a "fetal"
posture -- they crouch slightly, with their arms and legs half-bent in
front of them. In this position, they do not use many of their muscles,
particularly those muscles that help them stand and maintain posture (anti-gravity
muscles). This will make their muscles change, leading to the decrease
in mass of the muscles, which contributes to the "bird leg"
appearance. The muscle fiber types change from slow-twitch to fast-twitch.
The body no longer needs slow-twitch endurance fibers, such as those used
in standing. Instead, more fast-twitch fibers are needed as they push
themselves quickly off of space station surfaces. The longer they stay
on the station, the less muscle mass they will have. This loss of muscle
mass makes them weaker, presenting problems for long-duration space flights
and upon returning home to Earth's gravity.
Brittle Bones
On Earth, the bones support the weight of the body. The size and mass
of the bones are balanced by the rates at which certain bone cells (osteoblasts)
lay down new mineral layers and other cells (osteoclasts) chew up those
mineral layers. In microgravity, as the bones do not need to support the
body, all of the bones, especially the weight-bearing bones in the hips,
thighs and lower back, are used much less than they are on Earth. In these
bones, the rate at which the osteoblasts deposit new bone layers is reduced
(but no one knows exactly why, though it is thought that changes in force
and stress are somehow involved), while the rate at which osteoclasts
chew up bone stays the same. As a result, the size and mass of these bones
continue to decrease as long as the astronauts remain in microgravity,
at a rate of approximately 1 percent per month. These changes in bone
mass make the bones weak and more likely to break upon they return to
Earth's gravity. It is not known how much of the bone loss is recoverable
upon return to Earth, although it is probably not 100 percent. These changes
in bones may limit the duration of space flights.
Kidney Stones
The blood's calcium concentration increases slightly as the bones get
chewed up by osteoclasts (which is described previously). The kidneys
must get rid of the excess calcium, which makes them susceptible to forming
painful kidney stones.
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