The kidney uses all three of the following processes: filtration, secretion and reabsorption. All three of these processes aid in allowing the body to filter waste products from the blood while retaining nutrients, salts, and water when needed.

Filtration occur when filtrate is separated from blood in the renal corpuscle. Reabsorption is the removal of ions from the filtrate to retain salts. Secretion is the input of salts to the filtrate to eliminate them. All of these processes occur in the nephrons.

The ascending and descending limbs of the Loop of Henle are responsible for creating a countercurrent multiplier system, which concentrates urine and allows water and electrolytes to passively diffuse down their concentration gradients.

All the other options are part of the nephron, but are not responsible for the process of urine concentration. The glomerulus and Bowman's capsule are responsible for collecting and producing initial filtrate from the blood, and form the renal corpuscle. The proximal convoluted tuble is the initial site of reabsorption.

The ascending limb of the loop of Henle is permeable to salt, not water. As salt is removed from the urine, the urine becomes less concentrated. The urine will ultimately be concentrated in the collecting duct prior to excretion. The ascending limb of the loop of Henle helps to establish the salt gradient of the nephron, ensuring that water will be removed from the urine as it travels down the collecting duct, ultimately increasing the final concentration, even by decreasing the immediate concentration.

Antidiuretic hormone (ADH), also known as vasopressin, increases the permeability of the collecting duct to water. This allows a more concentrated urine to be excreted, because water is being lost from the urine to the kidney tissue before excretion. The gradient created by the reabsorption of ions from the loop of Henle means that the interstitium is hypertonic to the collecting duct. If the permeability of the collecting duct to water is increased, we would expect water to flow out of the collecting duct. We would expect ADH levels to increase with dehydration in order to preserve water.

The proximal tubule of the nephron directly follows after Bowman's capsule, and is the first site of reabsorption. Any glucose and proteins that were able to enter the filtrate are removed here via active transport. Most glucose and proteins are blocked from the filtrate by the structure of the glomerulus wall and Bowman's capsule, but those that are able to pass must be removed quickly to maintain the proper oncotic pressures in the nephron. Sodium is perhaps the most important electrolyte in the body; though large quantities of sodium may enter the filtrate, over half of it is reabsorbed in the proximal tubule.

The proximal tubule also serves as a site for reabsorption for potassium and phosphate. Other regions of the nephron closely regulate the reabsorption of bicarbonate and protons, as well as fine-tune the balance of sodium and potassium.

The thick ascending limb of the loop of Henle is impermeable to water, though sodium is actively transported out of this region. By pumping sodium out of the thick ascending limb, the urine becomes less concentrated that it was after the descending limb of the loop of Henle.

Antidiuretic hormone (vasopressin) is produced by the posterior pituitary, and is responsible for the increased rate of water reabsorption in the collecting duct of the kidney. The collecting duct serves as the last site of blood volume and blood pressure regulation before the urine flows down the kidney ureter into the bladder for excretion. The collecting duct is usually relatively impermeable to water. Action of antidiuretic hormone causes the insertion of aquaporins in the collecting duct, allowing water to exit the filtrate into the extracellular space.

Filtrate from Bowman's capsule enters the proximal tubule and flows down the descending loop of Henle, where water is reabsorbed from the tubule. Once through the descending limb of the loop, the filtrate enters the thin and thick ascending limbs of the loop of Henle, where sodium is reabsorbed, but water is not. Additional sodium can be reabsorbed in the collecting duct. Water could only be reabsorbed in the collecting duct if antidiuretic hormone were present.

We need to look for the point where sodium concentrations are the highest and water concentrations the lowest in the filtrate. This corresponds to the descending limb of the loop of Henle. At the bottom of the descending loop of Henle the filtrate is the most concentrated. Water is drawn out of the filtrate as it travels down the descending limb, but ions remain in the tubule, causing the osmolarity to increase. As the filtrate begins to ascend the loop, ions are pumped out of the filtrate, reducing the concentration again.

When antidiuretic hormone is present, higher osmolarity levels could be found in the collecting duct; however, we are told that antidiuretic hormone is not present, thus additional water will not be reabsorbed in the collecting duct.

Aldosterone acts in the distal tubule to increase the rate of sodium reabsorption by increasing the number of sodium-potassium pump proteins implanted in the cells of this region. To maintain electrical neutrality, when sodium is reabsorbed, potassium is secreted.

Too much aldosterone can thus make someone hypernatremic (high blood sodium) and hypokalemic (low blood potassium). High blood pressure results from the hypernatremia, and cardiac arrhythmias can result from the hypokalemia. 

The glomerulus is the capillary bed that feeds filtrate into the nephron via Bowman's capsule. Under normal circumstances, the glomerulus functions to create the urine filtrate. One of its main functions is to inhibit the filtration of red blood cells and large proteins. The glomerulus is the only part of the nephron that has any intimate contact with red blood cells. None of the other portions of the nephron will be in close proximity to red blood cells; thus, the glomerulus is the only nephron structure that could cause the lysis of red blood cells.