Diuretics are drugs that increase water excretion in the kidneys. They are useful for the treatment of conditions like heart failure. In heart failure the volume of blood in the body is often increased. This occurs to maintain blood pressure in the face of decreasing cardiac function, but it can have adverse effects in various tissues. There are a number of different diuretics that are available. For each of the following examples explain why they have their diuretic effect.
Loop diuretics decrease the reabsorption of sodium in the loop of Henle.
Spironolactone blocks the effect of aldosterone in the collecting duct.

By blocking the reabsorption of sodium in the Loop of Henle, the Loop diuretics reduce the osmolarity of the renal medulla. This decreases the ability to reabsorb water in the more distal parts of the tubule system. If less water is reabsorbed more water will remain in the filtrate and be excreted from the body.

Aldosterone controls the insertion of the sodium channels in the collecting duct. In the collecting duct water is reabsorbed when it follows sodium ions into the cell. So decreasing sodium reabsorption will decrease water reabsorption. Another role of sodium is to increase the activity of the sodium-potassium ATPase in the cell membrane. This is important because the pump is continuously removing sodium from the cell and returning it into the blood. This keeps the sodium concentration in the cell low and allows sodium and therefore water reabsorption to continue. Blocking aldosterone will decrease the activity in the pump and therefore decrease water reabsorption.