What happens to the filtrate in the loop of Henle?

The point of the loop of Henle is to reduce the quantity of water in the filtrate and so reduce water loss. The loop is split into three key areas: The descending limb, the thin ascending limb and the thick ascending limb. First the filtrate passes through the descending limb where water leaves the filtrate and diffuses through aquaporins into the surrounding tissue fluid due to osmosis.This happens because the surrounding tissue fluid contains a very high concentration of sodium chloride. The remaining filtrate enters the thin ascending limb. The thin ascending limb is impermeable to water but permeable to ions. Due to the high concentration of ions in the filtrate and the low concentration of them in the tissue fluid they passively diffuse out of the thin ascending limb. However, there are a number of ions that are important to us and that we don't want to lose. Here's where the thick ascending limb comes in. This part of the loop of Henle is, like the thin ascending limb, impermeable to water but it actively transports sodium and potassium ions into the surrounding tissue fluid. This produces an electrochemical concentration gradient where the outside is more positive than the inside of the loop. As a result of this chlorine ions (which are negative) passively leave the loop. The thick ascending limb has one final feature, it has potassium ion channels which allow the potassium ions to leak back into the loop. This creates an electrochemical gradient which makes other important ions like calcium and magnesium to leave the thick ascending limb. In summary, the loop of Henle is surrounded by tissue fluid which has a high concentration of ions in it. This causes water to move out of the descending limb by osmosis. Because of this concentration gradient ions move into the loop in the thin ascending limb. The important ions (Na+,Cl-,Ca2+ & Mg2+) are then actively transported out of the filtrate in the thick ascending limb to keep them in the body and maintain the high concentration gradient which causes water to be removed from the filtrate in the descending limb.

In summary, the loop of Henle is surrounded by tissue fluid which has a high concentration of ions in it. This causes water to move out of the descending limb by osmosis. This produces a more concentrated filtrate because of this, ions move out of the loop in the thin ascending limb. Any remaining important ions(Na+,Cl-,Ca2+ & Mg2+) are then actively transported out of the filtrate in the thick ascending limb to keep them in the body and maintain the high concentration gradient which causes water to be removed from the filtrate in the descending limb.