The mechanisms shown here are traditionally called the 'counter-current multiplier' and the 'counter-current exchanger'. The first takes place in the region of the nephron called Henle's loop; the second occurs in a region of the peritubular capillary bed called the 'vasa recta'. Both are involved in establishing an osmotic gradient throughout this region.
The portion of the nephron called the 'Henle's loop' is shown in the right half of the illustration. It consists of a descending limb that has a down arrow in it and an ascending limb with an up arrow in it. The ascending limb has a thick and a thin segment. The dots represent various solutes that become more concentrated in both limbs toward the bottom of the loop.
The thick walls of the ascending limb indicates that this region is impermeable to water.
This capillary bed is shown in red at the left of the illustration. It also also consists of a descending limb and an ascending limb identified by the arrows in each. Again, the dots represent various solutes that are more concentrated at the bottom of the loop.
Counter-currents exist when fluids flow in opposite directions in parallel and adjacent tubes. The two limbs of Henle's loop are a counter-current. The two limbs of the vasa recta are also a counter-current. It is apparent that these two sets of tubes are parallel and adjacent. Not apprent in the mind map is the fact that the descending limb of Henle is also counter-current with the ascending limb of the vasa recta; the same is true of the ascending limb of Henle and the descending vasa recta.
Examination of both limbs of the vasa recta shows the concentration of solutes (number of dots) is the same at any horizontal level. However, imagine the fluid flowing through the vasa recta for a short distance then stopping. Now compare the concentration of solutes at any level and they will not be the same. At any level the solute concentration in the descending limb will be less than in the ascending limb! But, because both limbs are freely permeable, sodium chloride will diffuse from the ascending into the descending while water will diffuse from the descending to the ascending...see block arrows. When equilibrium is reached both limbs will, once again, have the same concentration of water and solutes. Water is exchanged for sodium chloride...the counter-current exchange mechanism.
The above described counter-current exchanger would not exist if there were not some mechanism to initially make the vasa recta more concentrated at the bottom of the loop. This is accomplished by the loop of Henle.
The ascending limb of Henle and the early distal tubule are impermeable to water as indicated by their thick wall. These regions actively transports sodium chloride (NaCl) out of the filtrate and into the surroundings. (The asterisk (*) in the block arrow indicates that a more complex mechanism is involved but the net effect is that only NaCl is moved out of the filtrate.)
The NaCl diffuses into the descending limb of the vasa recta...block arrow. Any that might diffuse into the descending limb of Henle will only get pumped back out when it enters the ascending limb so this is not shown in the mind map. It will not diffuse into the ascending vasa recta because that fluid is already highly concentrated. This is the mechanism that 'multiplies' the concentration of NaCl in the descending vasa recta making the counter-current exchanger possible!
Both blood and filtrate descending into their respective loops have low solute concentrations. Both flow beside upcoming columns having higher solute concentrations. As they move past one another, water from the down-flowing fluid columns will diffuse into the more concentrated up-flowing columns. However, only the up-flowing vasa recta is permeable to water meaning all is returned to the blood and none to the filtrate...see both block arrows. This also insures that a high solute concentration will be maintained at the bottom of both loops.
Urea passively diffuses among and between the lower portions of both the blood and the filtrate loops as indicated by the large block arrow spanning this region. The result is that half the urea is excreted and half kept in the body.
Last Updated: 7/19/2005