Final answer:
The first event when ADH is increased is the temporary insertion of aquaporins into kidney tubules, allowing more water reabsorption, reducing urine volume, and helping regulate blood pressure and osmolarity.
Step-by-step explanation:
When the posterior lobe of the pituitary is stimulated to increase its secretion of ADH (Antidiuretic Hormone), the first event to occur is the temporary insertion of aquaporins into the kidney tubules, making the kidneys more permeable to water. This allows water to move back into the capillaries and reduces urine volume. As a result, the water reabsorbed reduces blood osmolarity and helps in maintaining fluid balance. The negative feedback mechanism will then reduce ADH secretion as the osmolarity of blood decreases.
ADH, also known as vasopressin, is a hormone produced by the hypothalamus and released from the posterior pituitary gland in response to high blood osmolarity. It plays a critical role in regulating the body's retention of water by affecting the tubules in the kidneys. Its primary action is to conserve body water and help to regulate the blood pressure.