Introduction: to organs and tissues. Zero gravity called weightlessness,

Introduction:

The human circulatory
system consists of three independent systems that work together: the heart
(cardiovascular), lungs (pulmonary), and arteries, veins, coronary and portal
vessels (systemic). An
average adult has 5 to 6 quarts (4.7 to 5.6 litres) of blood, which is made up
of plasma, red blood cells, white blood cells and platelets.

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The system of blood vessels in the human
body measure about 60,000 miles (96,560 kilometres).

Arteries carry oxygen-rich blood from
the heart through the body. Veins carry oxygen-poor blood back to the heart.

The superior vena cava carries
oxygen-poor blood into the heart. The aorta carries oxygenated blood from the
heart to organs and tissues.

Zero gravity called weightlessness, is
the absence of gravity.

Is best illustrated by astronauts
floating in their spacecraft.

Can be experienced by everyday
activities like jumping off dividing boards.

It is the “free fall” period of these
activities when the microgravity occurs and only lasts for a short period of
time.

Effects in
fluids:

v  The absence of
gravitational acceleration on the body results in a removal of hydrostatic
gradients.

v  This results in
a thoracic fluid shift. Upon arrival to microgravity, about two litres of fluid
shifts from the lower body to the chest and head.

v  The astronauts
describe this as puffy-face and bird-legs-the face and neck swell, the neck
veins protrude, and the legs get skinner.

v  Over time, the
body adapts somewhat and fluid distribution becomes uniform.

v  Venous pressure
equalizes throughout the body and directly reflects right atrial pressure.

v  Having increased
fluid in the head often results in nasal congestion, headaches their eyesight
prescription.

 

Affected by the muscle
atrophy and fluid shift (from lower to upper body) as a result of gravity:

v  Heart muscles do
not have to work as hard in space. Over time, the heart shrinks, blood volume
de crease, and blood pressure decreases. However, exercise has been shown to
counteract this atrophy- in particular, rowing seems to be very effective.
Studies of this condition in astronauts has helped doctors identify the roles
gravity plays in illnesses like postural Orthostatic Tachycardia Syndrome.

v  A microgravity environment
leads to changes in fluid distribution, muscle loading, and altered signaling
pathways. Some basic changes include alterations in blood pressure and the
quantity of blood that is pumped by the heart with each beat. The human heart
is designed to force blood to the body, and the most difficult organ to perfuse
is the brain since it is above the heart. In space your heart does not have to
work against gravity to pump blood to your brain and blood accumulates in the
upper body because gravity is not there to pull it toward your feet. The human
body takes advantage of this lack of work and begins to be less efficient as
demonstrated by the lower stroke volume. The heart generates slightly higher
systolic and diastolic pressures because large muscle groups (like the legs) are inactive and do not demand blood, resulting
in vasoconstriction. Also, since the heart is less efficient some blood remains
in the heart after each contraction which slightly increases the pressure
during the relaxation phase known as diastole. Taken together, the amount of
blood that is being pumped out of the heart (stroke volume) will change. As the
flight duration increases, these changes become slightly more dramatic, and may
affect an astronaut’s other physiological functions. There could even be
permanent changes in the way organs and blood vessels behave.

v  Table 1 demonstrates a
comparison of percent changes obtained from pre-flight and post-flight taken on
17 male astronauts, with an age range of 34-48 years. Thirteen subjects had
flown on short-duration flights of 4-17 days versus four subjects that had
flown long-term missions of 129-144 days.

                                              (Table 1)

 

Short-duration
n=13  (Mean ±SE)

Long-duration
n=4  (Mean ± SE)

Systolic blood
Pressure (mm Hg)

        6.69±2.47

           8.93±3.12

Diastolic blood pressure (mm Hg)

        6.64±4.12

          16.60±3.90

Stroke volume+ (mL)

       -5.00±0.03

         -17.40±0.05

Cardiac output*(L/min)

       -2.30±0.07

         -12.20±0.09

Percent Changes from
Pre-Flight to Landing of Echocardiographic     
Parameters (Mean ± SE)

v  In Table 1 blood pressure is
not presented in the usual format of    systolic/diastolic.
Each data set in Table 1 is presented as the percent change of the values
measured before and immediately after flight.

 

Effects
in blood pressure:

Some factors that influence arterial blood pressure and the associated
mechanisms.

factors

why blood pressure increase

Increase in blood volume

Increased total “filling pressure” in the semi-flexible
cardio-vascular system; increased venous return to the heart, leading to
higher stroke volume

Increase in heart hate

Increased cardiac output which, without a countering change in
peripheral resistance, increase pressure

Increase in stroke volume

Same as increased heart rate

Increase in peripheral resistance

Normally varied by changing vessel diameter, particularly in the
arterioles, increased constrictive resistance increase pressure in the
vessels leading up to it

Increased in blood viscosity

Increased resistance, as thicker blood does not flow as easily

Baroreceptors, or “pressure receptors”, are specialized
nerve endings located in both the arterial and venous systems, which are
stimulated when the blood vessels are stretched by increased pressure.

Effects in heart:

 

Gravitational transition (g)

Change in heart rate(beats/minute)

                    1   ?1.8

                          +13

                    1.8?0

                           -23

                    0   ?1.6

                           +19

v  When
astronauts spend long periods of time at zero gravity in space, their hearts
become more spherical and lose muscle mass, a new study finds, which could lead
to cardiac problems.

Images were taken before,
during and after the astronauts’ time in space.

v  The heart shrinks

v  The ventricles shrink and
cardiac output decrease

v  Stroke volume decrease

v  Total loss of fluid from the
vascular and tissue spaces of the lower extremities has been found to be 1-2L

v  The contraction
of skeletal muscles in the legs helps to pump toward the heart, but is prevented
from pushing blood away from the heart by closure of the venous valves.

v  Within 3-5
days in space, total body water stabilizes at about 2-4% below the normal level
and plasma volume decreases by about 22%

v  Increased arterial
pressure

v  The images revealed the heart
becomes 9.4 percent more spherical in space.  Mathematical models
predicted the change almost exactly, in earth as predicted by doctors.

v  The spherical shape of the heart
could mean the heart is functioning less efficiently. The condition appears to
be temporary — the astronauts’ hearts returned to a normal elongated shape
after they returned to Earth. Scientists don’t know if the change has any
long-term effects.

v  Upon
returning to Earth, astronauts commonly become lightheaded or pass out due to a
sudden drop in blood pressure after standing up, a condition known as
orthostatic hypotension. Space travel has also been known to cause irregular
heartbeats, and the increased radiation in space may speed up arterial
hardening, or atherosclerosis.

v  The researchers are now
adapting their models for conditions such as coronary artery disease (the most
common type of heart disease and leading cause of death worldwide), hypertrophic
cardiomyopathy (thickening of the heart muscle that limits its ability to pump
blood) and diseases of the heart valves.

v  Lower
gravity or a gravity-free environment lessens the effort your heart has to make
in order to pump blood through your body. And,
like any less-exercised muscle, it can lose strength after a long visit in space.

Conclusion:

ü  To stay healthy and
productive in space and after astronauts return to earth, they follow certain
procedures, strategies, medications, exercise routines, etc. known as
countermeasures. Resistance training (weight lifting) and cardiovascular
(aerobic) exercise to minimize muscle atrophy and cardiovascular
de-conditioning are very important countermeasures.

ü  Our bodies are
very complicated mechanisms that monitor hundreds of variables and adjust
processes when these variables are alter readings. For example, the sensors in
our sinuses are supposed to be at lower pressure than the heart (in gravity),
so if we flip upside down and the pressure goes up, the baroreceptors will send
signals to correct. In microgravity, the baroreceptors become a lot useful.

ü  The increased
fluid volume in the chest also results in a decrease in ADH (Anti-diuretic
hormone) and aldosterone release which results in increased urine production
and decrease in thirst.

 

 

 

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