EFFECT OF GRAVITY ON CIRCULATION

• Divided into
  • Effect of normal gravity.
  • Effect of  ‘G’ forces – (Positive G & negative G).

Effect of normal gravity:

• Both venous & arterial pressures are affected by gravity.
• Pressure inside any vessel below heart level (at right atrium level) is increased by effect of gravity.
• Pressure inside any vessel above heart level is decreased by effect of gravity.

• Magnitude of gravitational effect – 0.77 mm Hg/cm of vertical distance above or below heart.
• Ie., Above heart level – Pressure decreases by 0.77 mm Hg for each centimeter.
• Below heart level – Pressure increases by 0.77 mm Hg for each centimeter.

• Mean pressure in large artery in head (adult human) in upright position –  62 mm Hg.
• Mean pressure in large artery in leg in upright position –  105 mm Hg.
• Mean pressure in vessels In lying down position at all levels – Same (Due to nil effect by gravity).

Effect of G forces:

• G-force – Force due to gravity acceleration or deceleration.
  • G – Unit of gravity force.
  • lG – Force of gravity on earth’s surface.

Positive G:

• Force of acceleration acting on body during upward movement.

Negative G:

• Force of deceleration acting on body during downward movement.
• Eg: Experienced during landing & take off of airplanes & rockets, parachute jumping, elevators on up & down movement.
• Antigravity suit or ‘G’ suit used by astronauts – Effectively cushion gravitational effects.

Effects of positive G:

• Venous return decreases –> hence, cardiac output decreases.
• Blood rushes towards lower limbs –> hence increased pressure in lower limb vessels.
• Blood is drawn away from upper body –> hence decreased cerebral arterial pressure.

At very high positive G:

• Difficulty in blood flow to brain –> results in cerebral ischemia & unconsciousness.
• Moments before unconsciousness, there is gray-out.
• Everything appears gray – Due to ischemia of cone & loss of color vision.
• Eg: Gray-out serves as warning sign for pilots – Indicates need for slowing down aircraft & reduce positive’G’.
• Gray-out followed by black-out
• Black-out means total loss of vision.

Effects of negative ‘G’:

• Venous return increases —> Increases cardiac output.
• Pressure in lower-limb veins decreases.
• Increased cerebral arterial pressure.
• Increased congestion in blood vessels of head & neck –> Causes throbbing headache with red-out.
• Red-out means mental confusion.
• Ecchymosis appears around eyes.

Effects of zero gravity:

• The zero gravity’ situation occurs when astronauts in a spacecraft go out of earth’s gravitational effect.
• Eg: During orbital flights to other planets.
• Blood becomes weightless –> Flows very easily to brain.
• Circulatory changes are same as with supine position on earth’s surface.