The renal corpuscle is comprised of the glomerulus and Bowman's capsule, and is responsible for creating the primary filtrate that will enter the tubules of the nephron. This is accomplished via filtration. The proximal convoluted tubule is primarily responsible for reabsorption and secretion, and the loop of Henle is the site where filtrate is concentrated.

ATP is needed to help power the excretion of urine through the nephron. The thick ascending limb, for example, is thicker because it contains more mitochondria than other portions of the loop of Henle. The changing environment of the nephron is a product of the continual reabsorbtion of water and ions throughout the excretion process.

The renal medulla contains the loops of Henle and collecting ducts. As filtrate travels toward the interior of the kidney, the increased ion concentration (hypertonic environment) aids in the reabsorption of water. This ion gradient is established by ion flow from the concentrated filtrate in the descending limb toward less concentrated blood and interstitial fluids.

When the filtrate enters the bottom of the loop of Henle, it is at its highest concentration. As the filtrate travels up the thin ascending limb, sodium is passively reabsorbed as it flows down its concentration gradient to exit into the less concentrated interstitium.

Sodium is then actively pumped by the sodium-potassium-chloride co-transporter in the thick ascending limb, which transports all three ions out of the filtrate into the interstitial fluid (reabsorption). Later, in the collecting duct, a sodium/potassium transporter is used to further reabsorb sodium, while excreting potassium into the urine.

As the nephron dips into the medulla in the descending limb of the loop of Henle, water passively diffuses out of the filtrate. This concentrates the solutes in the filtrate. As the filtrate enters the ascending limb of the loop of Henle, the tube becomes impermeable to water and ions are pumped into the interstitium. This creates a gradient of higher ion concentration in the medulla and dilutes the filtrate.

The diluted filtrate enters the distal convoluted tubule, where water and ions are reabsorbed. This slightly increases the filtrate concentration before it enters the collecting duct. As the filtrate flows down the collecting duct into the renal medulla, the ions in the interstitium act to draw water out of the duct (dependent on the presence of antidiuretic hormone). The result is a highly concentrated urine product after the filtrate travels down the collecting duct, all due to the ion gradient established by the loop of Henle.

The loop of Henle is essential for creating an ion gradient in the renal medulla (the inner part of the kidney). In the descending limb of the loop of Henle, water is removed from the filtrate by aquaporin proteins (water channels). The result is a highly concentrated filtrate at the bottom of the loop.

The filtrate then enters the thick ascending limb, which is permeable to sodium ions. The sodium ions rush out of the filtrate into the interstitium, which now has lower concentration than the filtrate. When the collecting duct later travels through the high concentration of sodium ions that have accumulated in the renal medulla from the thick ascending limb, water is pulled out of the filtrate into the interstitium. This allows for the final step in concentrating the urine before it travels to the bladder.

Diuretic drugs promote the production of urine. One drug, known as furosemide, inhibits transport of both sodium and chloride in the ascending limb of the loop of Henle. This mechanism of action prevents the maintenance osmolarity gradients that promote the reabsorption of water, resulting in more dilute urine. If the gradient is lost, then water is not drawn out of the filtrate as it travels through the collecting duct and the result is a larger volume of dilute urine. Antidiuretic hormone, secreted from the posterior pituitary, works to promote the reabsorption of water in the collecting duct, which concentrates the urine and conserves water.

The loop of Henle serves the crucial function of creating an ion gradient in the renal medulla by fluctuating the reabsorption of water and ions. In the descending limb, water is removed from the filtrate and enters into the interstitium, resulting in a highly concentrated filtrate. This filtrate then enters the ascending limb. The ascending limb is not permeable to water, but is permeable to sodium ions. The result is a massive efflux of sodium ions, which exit the filtrate and enter the interstitium. Sodium pumps amplify this process by continuing to remove sodium from the filtrate. The filtrate becomes more dilute, but the interstitium in the renal medulla is highly concentrated. When the filtrate enters the collecting duct, this gradient helps pull water out of the filtrate, allowing it to reach a maximum concentration before being transported to the bladder.

If the ascending limb of the loop of Henle were permeable to water, this process would be impossible and the filtrate would not be concentrated in the collecting duct.          

Water is reabsorbed at various times during the excretion process as it passes through the nephron, in order to maintain proper ion levels. It is not, however, reabsorbed as urine ascends through the thin and thick ascending limbs in the loop of Henle. Rather, this region only involves ion reabsorption and urea secretion.

Sodium is reclaimed through passive transport in the thin ascending limb and is reclaimed by active transport in the thick ascending limb, distal tubule, and collecting duct. Each location of sodium resorption uses a different transport protein and mechanism.

The filtrate and surrounding interstitial fluid are at their highest osmolarities at the bottom of the loop of Henle. As the filtrate continues on, it enters the thin ascending limb of the loop of Henle, which is impermeable to water. As the thin ascending limb moves up through the nephron into areas with a lower osmolarity, sodium flows down its concentration gradient to exit the filtrate. At this point, the filtrate is at a lower osmolarity than the surrounding interstitial fluid due to sodium flowing out and water being barred from flowing in.

As filtrate travels down the descending limb of the loop of Henle, water passively leaves the filtrate as the descending limb passes through portions of the nephron that contain a more concentrated interstitial fluid. Water always travels from places of high water concentration (low osmolarity) to low water concentration (high osmolarity); thus, the water will passively flow out of the filtrate and into the interstitium.

As the loop of Henle turns, the filtrate passes through the thin ascending limb, which is impermeable to water, but permeable to ions. As the limb passes through less concentrated areas of the nephron, sodium passively flows down its concentration gradient from the filtrate to the interstitial fluid. At no point during this process is water actively transported.