Why does the blood flow to the kidneys decrease during exercise?

This happens because there's only so much blood to go
around in the body. When skeletal muscles are at rest, the capillary bed that supplies
the muscle tissue is relatively inactive, with only 20-25% of the capillaries open. When
the muscle becomes physically active, additionally capillaries begin to open, and
eventually 100% of the muscle capillaries are open. This means that during exercise a
much larger percentage (as much as 50% of the total volume) of blood is shunted to the
muscles than is the case at rest. Blood flow to the brain has to be maintained in order
to remain conscious, but blood flow to the other visceral organs, including the kidneys,
can be temporarily reduced in order to shunt toward the muscles.