If blood becomes more dilute or more concentrated, osmoreceptors detect this change and feedback to the hypothalamus (supraoptic and paraventricular nuclei and lateral preoptic area). The supraoptic and paraventricular nuclei control the release/suppression of ADH (antidiuretic hormone) from the posterior pituitary which controls the permeability of the collecting duct in the kidney.
ADH travels in the blood and binds to a receptor on the basolateral membrane of the collecting duct. This sets off a cascade of enzyme reactions that leads to the increased insertion of Aquaporin 2 in the apical membrane . This will lead to an increase in the water permeability of the collecting duct and hence more water will be reabsorbed back into the blood. The concentration of urine will therefore increase.