<P> In the example shown in the image, water enters at 299 mg / L (NaCl / H O). Water passes because of a small osmotic pressure to the buffer liquid in this example at 300 mg / L (NaCl / H O). Further up the loop there is a continued flow of water out of the tube and into the buffer, gradually raising the concentration of NaCl in the tube until it reaches 1199 mg / L at the tip . The buffer liquid between the two tubes is at a gradually rising concentration, always a bit over the incoming fluid, in this example reaching 1200 mg / L . This is regulated by the pumping action on the returning tube as will be explained immediately . </P> <P> The tip of the loop has the highest concentration of salt (NaCl) in the incoming tube--in the example 1199 mg / L, and in the buffer 1200 mg / L. The returning tube has active transport pumps, pumping salt out to the buffer liquid at a low difference of concentrations of up to 200 mg / L more than in the tube . Thus when opposite the 1000 mg / L in the buffer liquid, the concentration in the tube is 800 and only 200 mg / L are needed to be pumped out . But the same is true anywhere along the line, so that at exit of the loop also only 200 mg / L need to be pumped . </P> <P> In effect, this can be seen as a gradually multiplying effect--hence the name of the phenomena: a' countercurrent multiplier' or the mechanism: Countercurrent multiplication, but in current engineering terms, countercurrent multiplication is any process where only slight pumping is needed, due to the constant small difference of concentration or heat along the process, gradually raising to its maximum . There is no need for a buffer liquid, if the desired effect is receiving a high concentration at the output pipe . </P> <P> A circuit of fluid in the Loop of Henle--an important part of the kidneys allows for gradual buildup of the concentration of urine in the kidneys, by using active transport on the exiting nephrons (tubules carrying liquid in the process of gradually concentrating the urea). The active transport pumps need only to overcome a constant and low gradient of concentration, because of the countercurrent multiplier mechanism </P>

Where does countercurrent exchange occur in the nephron