To overcome this, your heart rate increases. Because less blood reaches the skin, heat loss is reduced and the body overheats. Just like your car, your body can quit running. If you fail to replace the water you lose, you run the risk of becoming dehydrated. Not only that, the impact of dehydration on the cardiovascular system can produce heart problems in people with coronary heart disease and diabetes.
Dehydration is also hard on the kidneys.
You need to put in as much water as is going out. Sweating is not the only way you lose water. This water loss increases as your physical activity increases because you breathe more. So if you are just replacing how much you are sweating out, it's not enough. Also if you just drink when you are thirsty, you aren't getting enough water because thirst alone isn't the best measure of a body's fluid needs.