GET A MOVE ON IT!
Gradients are a phenomenon that most likely results in movement of energy, fluids, or gases. Be it movement due to differences in an electrical current, molecule concentration, heat, or pressure, our body exploits gradients as part of its every day function. Two main types of gradients exist: passive and active gradients.
Passive gradients, also known as "naturally occurring" gradients, are the result of movement from a high value to a low value representing whatever is causing the gradient to exist. For example, a concentration gradient will result in the movement of molecules, via diffusion from the area with the high concentration of molecules to the area with the low concentration of molecules.
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Another example of a passive gradient would be the movement of heat from an area of high temperature to an area of low temperature. A third example would be movement of fluids from an area of high mechanical pressure to an area of low mechanical pressure (just like a garden hose). Passive gradients are called naturally occurring gradients since they so not require any energy investment in order to happen. Furthermore, they are "preferred" by the body since they are more efficient as you get movement for "free" 9no energy investment).
A passive gradient is nicely represented by a waterfall. The water fall as part of the waterfall without any energy investment on the water's behalf. Analyzing a waterfall, literally where water falls, the passive gradient is created by the differences in potential energy, originating from the differences in height between the upper end and the lower end of the waterfall. The higher the object, the greater its potential energy, and vice versa. Thus, potential energy is greater at the top of the waterfall, while potential energy is lower at the bottom of the waterfall. Thus, water falls from high potential energy towards low potential energy passively.
Active gradients, also known as "unnaturally occurring" gradients, are the result of movement from a low value to a high value representing whatever is causing the gradient to exist. For example, a soccer ball rolling up hill will not occur without the investment of energy in the shape of someone kicking the ball upwards or throwing it, etc. In this case, the ball is moving from low potential energy to high potential energy, against the occurrence of what would happen due to a naturally occurring gradient.
A biological example of an active gradient could include the entering to the cell through a biological membrane against the natural diffusion gradient. In such a case, diffusion according to the natural gradient is not possible, and energy must be invested for it to happen. This would be the case of a molecule that is too big to diffuse naturally into the cell, needing "help" in the form of the creation of an active gradient "pushing" it into the cell.
Another well-known active gradient created in the body is the flow of blood upwards, against the influence of gravity. Naturally, gravity would cause fluids to flow downward in light of its influence as a natural resistive force effecting movement downwards (towards the center of Earth). Yet, with the active work of smooth muscles, blood within veins is squeezed upwards.
As the smooth muscles squeeze the veins, they increase the mechanical pressure behind the blood being pushed upwards, creating an active gradient where in front of the blood (in the direction of blood flow upwards) the mechanical pressure is lower than behind the blood. Thus, blood finds itself moving upwards against the naturally occurring gradient that would cause it to flow downwards (similar to the waterfall).
The smaller the substance, the more it tends to move as a result of passive gradients, while the bigger the molecule, the greater the chances an active gradient will be required to make it move. A unique analysis of gradients of any kind relates to how a gradient is defined if both values are the same. In such a case, some would say the gradient does not exist, and some would say that the gradient is zero.
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The greater the differences between the two ends of the gradient (the two values that create the gradient), the greater the movement expected, and the faster movement expected. This is true to any gradient, yet as part of an active gradient, greater and faster movement requires more energy invested. This is similar to kicking a ball; the more energy applied to the ball, the farther away it will go, and the faster its movement will most likely be.
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