There’s one final cardiovascular trick in response to stress, involving the kidneys. As that zebra with its belly ripped open, you’ve lost a lot of blood. And you’re going to need that blood to deliver energy to your exercising muscles. Your body needs to conserve water. If blood volume goes down because of dehydration or hemorrhage, it doesn’t matter what your heart and veins are doing; your ability to deliver glucose and oxygen to your muscles will be impaired. What’s the most likely place to be losing water? Urine formation, and the source of the water in urine is the bloodstream. Thus, you
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