1. Countercurrent flow mechanism

2. Countercurrent exchange is a mechanism occurring in nature and mimicked in industry and engineering, in which there is a crossover of some property, usually heat or some component, between two flowing bodies flowing in opposite directions to each other. The flowing bodies can be liquids, gases, or even solid powders, or any combination of those. For example, in a distillation column, the vapors bubble up through the downward flowing liquid while exchanging both heat and mass.

3. Countercurrent exchange when set up in a circuit or loop can be used for building up concentrations, heat, or other properties of flowing liquids. Specifically when set up in a loop with a buffering liquid between the incoming and outgoing fluid running in a circuit, and with active transport pumps on the outgoing fluid's tubes, the system is called a countercurrent multiplier, enabling a multiplied effect of many small pumps to gradually build up a large concentration in the buffer liquid.
Other countercurrent exchange circuits where the incoming and outgoing fluids touch each other are used for retaining a high concentration of a dissolved substance or for retaining heat, or for allowing the external buildup of the heat or concentration at one point in the system.

4. Countercurrent multiplication is a similar but different concept where liquid moves in a loop followed by a long length of movement in opposite directions with an intermediate zone. The tube leading to the loop passively building up a gradient of heat (or cooling) or solvent concentration while the returning tube has a constant small pumping action all along it, so that a gradual intensification of the heat or concentration is created towards the loop. Countercurrent multiplication has been found in the kidneys[1] as well as in many other biological organs.

5. Countercurrent flow—almost full transfer
In countercurrent flow, the two flows move in opposite directions.
Two tubes have a liquid flowing in opposite directions, transferring a property from one tube to the other. For example, this could be transferring heat from a hot flow of liquid to a cold one, or transferring the concentration of a dissolved solute from a high concentration flow of liquid to a low concentration flow.
The counter-current exchange system can maintain a nearly constant gradient between the two flows over their entire length of contact. With a sufficiently long length and a sufficiently low flow rate this can result in almost all of the property transferred. So, for example, in the case of heat exchange, the exiting liquid will be almost as hot as the original incoming liquid's heat.

6. example
In a countercurrent heat exchanger, the hot fluid becomes cold, and the cold fluid becomes hot.
In this example, hot water at 60 °C enters the top pipe. It warms water in the bottom pipe which has been warmed up along the way, to almost 60 °C. A minute but existing heat difference still exists, and a small
amount of heat is transferred, so that the water leaving the bottom pipe is at close to 60 °C. Because the hot input is at its maximum temperature of 60 °C, and the exiting water at the bottom pipe is nearly at that temperature but not quite, the water in the top pipe can warm the one in the bottom pipe to nearly its own temperature. At the cold end—the water exit from the top pipe, because the cold water entering the bottom pipe is still cold at 20 °C, it can extract the last of the heat from the now-cooled hot water in the top pipe, bringing its temperature down nearly to the level of the cold input fluid (21 °C).
The result is that the top pipe which received hot water, now has cold water leaving it at 20 °C, while the bottom pipe which received cold water, is now emitting hot water at close to 60 °C. In effect, most of the heat was transferred.

7. Countercurrent multiplication loop
A countercurrent multiplication loop is a system where fluid flows in a loop so that the entrance and exit are at similar low concentration of a dissolved substance but at the far end of the loop there is a high concentration of that substance. A buffer liquid between the incoming and outgoing tubes receives the concentrated substance. The incoming and outgoing tubes do not touch each other.