When a human being who don't know how to swim drowns in water,then
It would primarily activate the mamillian diving reflex which would shunt as much blood as possible away from your extremities and towards you brain and heart. Your heart rate would also slow down and essentially create a heart-brain circuit that would allow you to survive longer underwater. Of course, none of this matters once you run out of oxygen and your lungs fill with water. This would cause atelectasis and block the exchange of oxygen and carbon dioxide in the lungs. At this point once your body begins to lose oxygen already in the blood, you would begin to blackout and then die.