The loop of Henle is a part of a nephron, a tiny tube inside the kidneys that filters solutes. Each kidney contains hundreds of thousands of individual nephrons, which pass between the cortex of the kidneys and the medulla, connecting to collecting ducts which route urine to the ureter so that it can be expressed. The loop is an important part of the whole system, as it allows the kidneys to filter out salt and maintain the correct balance of water in the body.
As the name would suggest, the structure is a large loop that dips down from the nephron to create a descending arm that connects with the medulla, and an ascending arm that moves back into the cortex of the kidney. As fluids move down the loop, water is filtered out, moving into the interstitial fluid of the medulla. When the fluid moves up the ascending arm, salts are pulled out, and the cells are impermeable to water, so no additional water is lost.
Because salt and water are filtered out at different points, the loop of Henle contributes to the formation of an area of high salt concentration in the medulla. Meanwhile, the filtrate in the nephron passes through to the collecting ducts. The ducts pass through the medulla, which means that they move through an area of increased salt concentration.
Water moves from areas of low concentration to areas of high concentration, and it will naturally want to pass from the filtrate to the saltier surrounding tissue. The body controls the amount of water that passes through with a hormone called anti-diuretic hormone (ADH). When there is a great deal of salt in the blood, more of this hormone is produced, allowing water to flow freely through the collecting tubes and into the medulla and creating concentrated urine because less water ends up in the ureter. If the salt level in the blood is low, the collecting tubes become less permeable, allowing less water through and generating dilute urine.
Using the combination of ADH and the loop of Henle, the body can control the levels of salt and water present. When someone drinks a big glass of water, diluting salt in the blood, the kidneys express the water quickly to keep the balance level. If someone spends a few hours without drinking water, the kidneys will conserve the available water, which explains why morning urine tends to be very concentrated, because people don't usually drink during the night.
The kidneys and the body can adjust their activity to compensate for things like a large intake of salt or increased water consumption to keep levels of salt and water in the body stable. In people with kidney failure, dialysis is used to replace the function of the kidneys, filtering the blood in a machine that stands in for the huge numbers of nephrons that normally do all the work.