If a person drinks very little water and is doing heavy exercise, it will result in dehydration in the body, leading to increase in blood osmolarity and decrease in blood volume.
Osmoreceptors in the body are activated by changes in blood volume, body fluid volume and ionic concentration. An excessive loss of fluid from the body can activate these receptors which stimulate the hypothalamus to release antidiuretic hormone (ADH) or vasopressin from the neurohypophysis. ADH facilitates water reabsorption from latter parts of the renal tubule, thereby preventing diuresis.
The Juxta glomerular apparatus also play an important role in regulation of kidney function by activating the release of renin (RAAS) due to fall in glomerular blood pressure/GFR , resulting in the formation of angiotensin II & aldosterone which causes the conservation of and water.
Atrial natriuretic factor (ANF) is released when atria are stretched by high blood volume; here blood volume is low, so ANF would decrease, not increase.