The hypothalamus sends signal to the kidneys to absorb more water if the blood is very diluted with water
Step-by-step explanation:
If the blood is very dilute with too much of water then the hypothalamus region in the brain gets triggered to release the ADH harmone and absorb more water from the blood by sending signal to kidneys.
If the blood is too dilute then it becomes difficult for the heart to pump the blood and this thereby increases the blood pressure and causes hypertension, to avoid this the water is absorbed from blood.