Final answer:
Antidiuretic hormone (ADH) promotes the reabsorption of water in the kidneys through the insertion of aquaporins in the collecting ducts, reducing urine volume and adjusting blood osmolarity while not directly affecting sodium ion excretion.
Step-by-step explanation:
Secretion of antidiuretic hormone (ADH), also known as vasopressin, promotes the reabsorption of water in the kidneys. When ADH is released from the posterior pituitary gland, it travels to the kidneys and influences the collecting ducts to become more permeable to water. This occurs through the temporary insertion of water channels called aquaporins into the cells of the collecting ducts. As a result, water is reabsorbed from the urine back into the bloodstream, thereby reducing urine volume and decreasing blood osmolarity towards normal levels.
High levels of ADH can lead to vasoconstriction, which increases blood pressure. Moreover, underproduction of ADH can lead to improper kidney functioning. However, ADH does not directly influence the excretion of sodium ions; that is the role of another hormone called aldosterone, which promotes Na+ reabsorption and K+ excretion, influencing water balance and blood pressure differently than ADH.