A nephron is one of the working parts of the kidney involved in urine production. There are two types of nephrons, referred to as cortical nephrons and juxtamedullary nephrons. A cortical nephron sits in the outer part of the kidney, known as the cortex, while a juxtamedullary nephron sits closer to the inner part of the kidney, called the medulla. Part of the juxtamedullary nephron, a U-shaped tube called the loop of Henle, is longer than that of a cortical nephron and dips right down into the inner medulla. This plays an important part in the formation of urine.
There are many nephrons inside a kidney, probably around a million in total. Each nephron consists of a knot of minute blood vessels called glomeruli, enclosed in a capsule that is connected to a series of tubes. Blood enters the glomerulus and filters through into the capsule and then the tubes, eventually forming urine. As the filtered blood makes its way through the nephron, water and molecules are added to it and lost from it, until the final product is what we know as urine. The extra-long loop of Henle found inside each juxtamedullary nephron enables the kidney to remove more water from the filtered blood, making the urine more concentrated.
Each juxtamedullary nephron has another feature that is different from a cortical nephron. A special elongated and U-shaped network of blood vessels runs alongside each loop of Henle. These blood vessels are involved in the process of urine concentration, as they move sodium out into the medulla of the kidney. It is important for the inner medulla to have a high concentration of particles such as sodium, because this tends to draw water out of the loop of Henle, leading to more concentrated urine.
The U-shaped loop of Henle in each juxtamedullary nephron is described as having a descending section, which dips down into the kidney's medulla, and an ascending section which rises up to the tubes that collect urine. Sodium and chloride leak out of the ascending section, and are also pumped out, making the urine slightly more dilute. Once removed, the sodium and chloride increase the concentration of dissolved particles in the inner medulla, helping to draw water out of the loop of Henle. This concentrates the urine. After urine leaves the ascending portion of the loop of Henle and enters the collecting tubes, these carry it back down into the inner medulla, so that even more water is drawn out.