What Is Gravity,How Gravity Change on The Earth.Every planet influences the mass of other objects by the property of gravity, which causes mass to have weight. Gravity may be compared to acceleration. The relationship of force, mass, and acceleration may be expressed by the equation
force — mass x acceleration
Since weight is a force, the equation may be expressed
weight = mass x gravity
Gravitational forces are perpendicular to the planet surface. Since man developed in the gravitational field of earth, the near earth environment is used as a standard gravitational unit or 1 g. The gravitational fields of other planets are expressed in relation to the 1 g field of earth. The moon, with a mass one sixth that of earth, has a one sixth g field.
The gravitational force perpendicular to the surface of the earth may be described as a vector. Its effect upon the body is described in terms of the orientation of the gravitational vector to the three mutually perpendicular axes of the body: vertical, lateral, and anterior-posterior. These effects have been designated by symbols. While a person is standing upright, the gravitational vector is parallel with the vertical axis of the body, and there is associated pooling of blood below the diaphragm.
This is designated +GZ. The effect of hanging head-down is designated -Gz. While a person is lying on his back, the gravitational vector is perpendicular to the anterior chest wall, creating a +GX effect. While he is lying prone (face down), the gravitational vector is perpendicular to the back and has a -Gx effect. If the subject is lying on his left side, the gravitational vector has a +GV influence, and if he is lying on the right side, the gravitational vector has a -Gy effect.
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The same symbols are applied to the orientation of the body in relationship to an acceleration vector; thus, forward acceleration in the seated position, as encountered during driving an automobile, produces a +GX effect. Acceleration upward in an elevator produces a +GZ effect. Application of g force or acceleration force in these directions is a major consideration in aerospace flight.Gravity is responsible for the property of body weight, the weight of organs, and the weight of fluid columns.
Movement of body mass in gravitational field requires work or energy expenditure. In this sense work is required to achieve simple standing or perform any body movement An absence of gravity would result in a decree workload for body movement. The weight of fluid columns also alters physiology by changing p sure gradients within closed spaces. This illustrated by the changes in intravascular pressure at different regions of the body assort with simple standing.
Vertical g ( GZ)
The illustration of standing explains many of the physiologic ef? of +GZ force. Additional energy must be expen to maintain the upright position, thereby creating work load. Certain adaptations must be made the circulatory system. The weight created upright columns of blood changes intraday pressure as compared with the pressure found the level of the aortic valve. In a male of ave height the intra-arterial and intravenous pre– at the ankle may be 100 mm. of mercury hi than that noted at heart level.
Conversely, in vascular pressure falls above the heart level may be mm. of mercury less within the v’ of the brain than noted at the level. of the a valve. The latter effect is due chiefly to the length. of the column of blood between the aortic valve the occiput, which is commonly 30 cm. A column of blood of this height creates a pressure of 25 of mercury. The changes in intravascular pressure be altered by changing body position.
In seated position the column of blood from here, ankle is shortened, and the changes in the static pressure at the ankle are lessened. if one leans forward while in the seated position the height of the column of blood between heart and the brain is decreased, diminishing changes in pressure between the heart and brain.
There are many circulatory adaptations to the force encountered in simple standing. Changes arterioles tone are affected by peripheral solar constriction, principally in regions below diaphragm. These changes control the distribute of blood flow or cardiac output in an effort to obtain adequate cerebral blood flow. The controlling peripheral arterioles tone complex and involve both reflex mechanisms humoral factors. Failure to increase perish arterioles tone while standing may result in adequate cerebral blood flow and loss of consciousness nessness.
Standing increases the amount of blood pooled below the diaphragm. Commonly, the blood volume in the lower extremities increases 15 per cent. This diminishes the available circulating blood volume. Since the venous reservoir is readily expansive, factors that are associated with increased tone of the venous wall or intravascular pressure in the form of tissue tension or muscle tone tend to control the amount of venous pooling.
In the absence of adequate intravascular pressure or diminished venous tone, venous pooling may be excessive, resulting in diminished venous return to the heart, and fainting. Methods of applying external pressure, such as elastic bandages, elastic stockings, or the anti-g pressure garment used in aerospace activities, all serve to limit the expansion of the venous reservoir and venous pooling. By adequate control of this factor, major deficiencies in arterioles tone can be tolerated.
The increased intravascular pressure in both the arterioles and the venous circulatory components results in diffusion of water from the intravascular to the intravascular spaces. The normal reabsorption of fluid by the venous circulation due to the osmotic pressure of plasma proteins and the intravascular tissue tension is not effected as readily since the intravascular pressure gradient exceeds their combined influence. This results in a gradual increase in hydration of extravascular spaces and skeletal muscles.
Those muscles that have tight fascia sheaths increase their tension and apply external pressure to the venous reservoir, thereby eventually tending to balance the pressure gradients and deter loss of fluid from the intravascular compartment. The loss of fluid to the intravascular spaces results in gradual decrease in plasma volume, which contributes to fainting after prolonged standing. This adverse effect can be counteracted by body movements or muscular contraction that tends to massage the venous reservoir and decrease intravenous pressure.
The circulatory adaptations described above are frequently associated with increased heart rate, fall in systolic pressure, and narrowing of pulse pressure. When these are severe, they may be associated with symptoms of orthostatic intolerance or simple fainting. During aerospace flight or human centrifuge studies, advantage may be taken of body position and other factors that decrease the vertical height of the column of blood.
Normally, man cannot tolerate increased acceleration loads above 4 to 5 g. As the g load increases, the pressure created by the column of blood between the aortic valve and the base of the brain is gradually increased. At 2 g’s the column of blood exerts the pressure of approximately 50 mm of mercury At 4 g’s the 30 cm.
When +GZ loads are sufficiently increased (3 to 4 g’s), visual disturbances occur. These are noted prior to fainting because the naturally occurring intraocular pressure causes a lower pressure gradient than noted in the brain. Peripheral vision is lost first, and finally circulation to the eye may be sufficiently impaired that vision is lost, but the person may still be conscious. A further increase of approximately 1 g usually results in loss of consciousness; this is noted at levels between 4 and 5 +GZ. The lunar environment of l/& g has an effect similar to that of being in a bed tilted upright 10 degrees. The 30 cm. column of blood would exert a pressure of only 6 mm. of mercury, and the difference between the pressure at heart level and in the brain would be minimal.
Other minor alterations in circulation are noted during +GZ loads. These include clearing of blood from the
apices of the lung and diminished cardiac size. Upon cessation of the g load the cardiac rate may slow abruptly with a cardioinhibitory response. This may result in bradycardia or such disturbances as nodal rhythm with A-V dissociation.
Tolerance to +GZ loads may vary because of such factors as anxiety, fasting, fatigue, and level of physical fitness. Physical stature also influences g tolerance. The short, stocky person with a short neck commonly has a shorter column of blood between the heart and the base of the brain, and usually tolerates increased g loads better than a tall, thin person.
When an automobile starts from a standing position or an airplane accelerates for take-off, a person seated in it experiences +GX acceleration. The astronaut lying on his back in the space vehicle during rocket firing experiences forces oriented in the same direction. Under these circumstances the long axis of the major vessels of the circulatory system is perpendicular to the direction of g force application. The absence of any significant vertical height in the fluid columns prevents extensive weight change in long columns of blood. The application of acceleration forces in this direction allows man to tolerate forces of a much greater magnitude.
The major load is placed on the respiratory system. Breathing is difficult or impossible while high levels of transverse acceleration are sustained. Normally, for short periods of time man can tolerate +G« loads of 10 g’s. This principle has enabled man to tolerate acceleration forces of sufficient magnitude to achieve orbital velocity for space vehicles and manned space flight. At high g loads there is compression of the posterior region of the lungs and overextension of the alveoli in the anterior regions of the lungs.
These variations, however, have not been noted to be sufficiently great to create problems within the range of g loads encountered for manned space flight.
In addition, to the influence upon the lungs, +GX forces sustained through a period of time may precipitate intraventricular cardiac arrhythmias. Usually these are in the form of sporadic atrial premature contractions. In some instances ^short bursts of atrial tachycardia may be observed. The arrhythmias are thought to be secondary to distention of the atria associated with sudden increase in venous return secondary to pressure on the abdomen.