Counter heat current<unk> : Note the gradually declining differential and that the once hot and cold streams exit with a reversed temperature difference; the hotter entering stream becomes the exiting cooler stream and vice versa.<unk>  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 chemical, 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<unk> ng both heat and mass. The maximum amount of heat or mass transfer that can be obtained is higher with<unk>  than co-current (parallel)<unk>  because<unk>  maintains a slowly declining difference or gradient (usually temperature or concentration difference). In cocurrent<unk>  the initial gradient is higher but falls off quickly, leading to wasted potential. For example, in the adjacent diagram, the fluid being heated (exiting top) has a higher exiting temperature than the cooled fluid (exiting bottom) that was used for heating. With cocurrent or parallel<unk>  the heated and cooled fluids can only approach one another. The result is that<unk>  can achieve a greater amount of heat or mass transfer than parallel under otherwise similar conditions. See: flow arrangement.<unk>  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<unk>  multiplier, enabling a multiplied effect of many small pumps to gradually build up a large concentration in the buffer liquid. Other<unk>  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.<unk>  circuits or loops are found extensively in nature, specifically in biologic systems. In vertebrates, they are called a rete mirabile, originally the name of an organ in fish gills for absorbing oxygen from the water. It is mimicked in industrial systems.<unk>  is a key concept in chemical engineering thermodynamics and manufacturing processes, for example in extracting sucrose from sugar beet roots.<unk>  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.<unk>  multiplication has been found in the kidneys as well as in many other biological organs.
Countercurrent exchange
Bioconcentration
Selective reabsorption
Isobaric counterdiffusion
Infiltration 
Reabsorption
Clonal selection
Exeresis