It is the force that guides your life on Earth, but you'll never see it, touch it or taste it. It is responsible for the formation of Earth itself. The force is gravity, and it is the reason you remain fixed on the surface of a planet traveling at 67,000 mph through space. That force has a profound impact on human physiology, and even simple variations in gravitational force, or g-force, affect all of the body processes, including the heart rate.
Significance
As you stand still on the surface of the Earth, the force of gravity exerted on your body is actually the mechanical force of the Earth's surface pushing back against you, preventing you from falling into the center of the planet, which would be your body's natural inclination. Inside your body, your heart and all of your blood has no surface pushing back at it like your feet do from the ground. The blood in your body is, in a sense, in a state of free fall, and it takes an outside force to keep it from puddling at your feet.
Function
Your heart provides the mechanical force to keep your blood flowing from your head to your toes. The farther away from the ground your blood goes, the greater the inclination toward free-fall. For this reason, your heart rate declines when you are lying down, because the heart does not have to work quite as hard to get blood all the way to your brain.
Types
On Earth, you can experience gravity, hypergravity and microgravity, given the right conditions. Parabolic flights by specialized aircraft soar high in the sky, peak and then descend sharply, creating conditions of hypergravity on the way up and down and a brief moment of microgravity at the apex. A roller-coaster ride can give the same effect. During periods of hypergravity, your heart must work harder to counteract the force. Fluid weighs more in such conditions, says Malcom Cohen, chief of the Human Information Processing Research Branch at NASA Ames. Similarly, fluid weighs less in periods of microgravity, so the heart needn't pump as hard to send blood to all corners of the body.
Considerations
Inversion therapy, a treatment method that suspends the patient either completely or partially upside down, presents a different effect on heart rate. Instead of increasing or decreasing the force of gravity on the blood, inversion therapy essentially reverses the direction. Short periods of inversion therapy produce no appreciable change in heart rate, according to a 1988 study in by M. Zito in the journal, "Physical Therapy," but the Mayo Clinic warns that prolonged inversion can lead to higher blood pressure and decreased heart rate, even in healthy adults.
Studies
NASA has a vested interest in understanding the effect of Earth's gravity on heart rate as it subjects its astronauts to long periods of microgravity, which could have an impact on heart function long-term. The heart is a muscle like any other, and in microgravity, it atrophies. Astronauts returning to Earth after long periods in space demonstrate measurable reductions in heart mass, according to Julie Robinson of the Johnson Space Center, which could cause low blood pressure and even fainting. What NASA scientists don't yet understand is whether the reduction reveals cellular damage or simply shrinkage, like your biceps when you stop working out. Integrated Cardiovascular, a NASA program begun in 2009, seeks to answer these and other questions related to the long-term effect of gravitational forces on heart rate and heart function.
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