The only energy required is that needed to drive the fluid through the transportation channels. Another biological example of the counter-current pattern is found in the kidney. This transfer is accomplished in the kidney. One mystery was the purpose of the Loops of Henle . Sodium ions and other ions move through the semi-permeable wall of the blood vessels into the intercellular zone near the urine ducts and thence into the urine ducts by osmotic pressure.
Ions move down the density gradient.
The counter-current process operates along the Loops of Henle such that high concentrations of salts are possible at the turn of the loops. In this fashion high concentrations of uric acid and salts are possible in the urine of land animals . A passive energy gradient is maintained by the shape or arrangement of these loops and the system satisfies one requirement our bodies must meet in order to stay out of the salty sea from whence we came. Simplified schematic of the kidney. The operation of the kidney is very intricate and more complicated than I indicate here.