To optimize your hydration status, you must be an active participant in your hydration regime by consuming adequate fluids and electrolytes on a daily basis - as well as before, during and after exercise. Leaving hydration to chance or ignoring dehydration symptoms can be hazardous to health and performance.
Sadly, once you are dehydrated, you can't 'make-up' for fluids lost.
Going into a workout dehydrated and/or not replenishing fluids and electrolytes lost during exercise can cause great strain on your cardiovascular system. As dehydration reduces plasma volume, blood becomes thicker and retains more sodium. This makes blood harder to circulate through your body. To compensate, your heart beats faster, which increases heart rate and blood pressure. Central venous pressure decreases which reduces the amount of blood returning to the heart. Less blood entering the heart decreases the amount of blood leaving the heart. An increase in core temperature leads to an increased rate of glycogen breakdown (carbs stored in the muscles), which causes an intracellular increase in acids. As lactic acid is produced, pH decreases causing skeletal muscle fatigue. Dehydration may also increase levels of the stress hormone cortisol - which reduces testosterone levels and impacts muscle growth.
Keeping your body well-hydrated on a daily basis - as well as before/during/after exercise helps your heart pump blood more easily, protects your body from heat stress and allows oxygen to reach your muscles to help the muscles work efficiently.
To learn more about mastering or fine-tuning your hydration needs during exercise, check out my book Essential Sports Nutrition. Not only do I discuss how to best hydrate before, during and after workouts and races but I also provide many guidelines on how to best fuel your incredible body in motion in training and on event day.