Water reabsorption is the process by which the kidneys reclaim water from the filtrate back into the bloodstream, primarily occurring in the proximal convoluted tubule and the loop of Henle, regulated by hormones such as antidiuretic hormone (ADH). This mechanism is crucial for maintaining fluid balance, electrolyte homeostasis, and blood pressure. Impaired water reabsorption can lead to conditions like diabetes insipidus, characterised by excessive urination and thirst.