Where in the brain send impulses of hormones if the blood flowing through it has too much water?

Water levels and mineral salts in the blood are controlled to protect animal cells by stopping too much water from entering or leaving them. The concentration of water and salts is the same inside and outside of the cells. If body cells lose or gain too much water by osmosis, they do not function efficiently.

The hypothalamus sends signal to the kidneys to absorb more water if the blood is very diluted with water

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.