How the amount of urine in humans is regulated?

The body detects the increase in blood volume/pressure and releases hormones which signal the kidneys.  The kidney has a mechanism to reabsorb water from the filtrate. This depends on how much water is left in the body and in the filtrate. The comparative concentration of water gives a signal to the brain which then takes the required corrective action of either reabsorbing water or releasing more water. Thus the volume of urine produced is in proportion with the volume of fluid consumed (allowing for loss as a result of sweat and respiration). In this way the amount of urine formation is regulated by kidneys.