I am curious to know why, when you first start increasing your water intake, you spend most of your time in the toilet and then it settles down. I would have thought that if your body is in need of extra fluid, and you decide to increase your water intake on a regular basis, then it would "soak up" all the water and only release a certain amount. However, the reverse seems to happen, and it appears it all flushes out quickly and regularly. If extra fluid intake is maintained, then it settles down, and you don't appear to go to the toilet as much. This is something that has always puzzled me.
When fluids are ingested, the volume of blood and interstitial fluid (the fluid between the cells in the body) increases. The kidneys control the level of fluid in the body by controlling urine production and the reabsorption of electrolytes (i.e., sodium, potassium, etc.). A large increase in water intake without a subsequent increase in sodium (i.e., you are not drinking salt water) will change the ratio between the electrolytes and fluid in the body (osmolality). Generally speaking, sodium causes the kidneys to maintain a greater fluid volume in the body to maintain this ratio between sodium and water. The average American gets a good dose of sodium daily with a moderate to low intake of fluid. Increased water will result in increased urine production. If sodium was not increased proportionately, which is likely given the low amount of sodium even in tap water, the kidneys can excrete much of the extra water plus some of the fluid present in the body before the increased intake. The reason for the extra trips to the bathroom is a product of the extra fluid consumed plus the “extra” stored interstitial fluid and possibly some blood volume. When the “extra” fluid is excreted, the trips to the toilet decrease.
FYI: Often therapy for hypertension involves the use of diuretics that alter the amount of sodium and other electrolytes reabsorbed by the kidneys. With less sodium in the body, blood volume decreases from the increased urine production and blood pressure decreases.