Editing Countercurrent exchange (section)

{{Short description|Mechanism occurring in nature and mimicked in industry and engineering}}
{{More citations needed|date=May 2020}}

[[File:Échange de chaleur contre-courant.svg|thumb|263x263px|Counter heat current exchange: 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.]]
'''Countercurrent exchange''' is a mechanism between two flowing bodies flowing in opposite directions to each other, in which there is a transfer of some property, usually heat or some chemical. 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. It occurs in nature and is mimicked in industry and engineering. It is a kind of exchange using counter [[flow arrangement]].

The maximum amount of heat or mass transfer that can be obtained is higher with countercurrent than co-current (parallel) exchange because countercurrent maintains a slowly declining difference or [[gradient]] (usually temperature or concentration difference). In cocurrent exchange 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 exchange the heated and cooled fluids can only approach one another. The result is that countercurrent exchange can achieve a greater amount of heat or mass transfer than parallel under otherwise similar conditions.

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 multiplication|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.

Countercurrent exchange circuits or loops are found extensively in [[nature]], specifically in [[biology|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. Countercurrent exchange is a key concept in [[chemical engineering]] [[thermodynamics]] and manufacturing processes, for example in extracting [[sucrose]] from [[sugar beet]] roots.

[[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<ref>Both countercurrent exchange and countercurrent multiplication systems have been found in the kidneys. The latter in the loop of Henle, the first in the [[Straight arterioles of kidney|vasa recta]]</ref> as well as in many other biological organs.