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How does physics integrate biology in this?

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I have a misunderstanding in human biology...

as its known, when the blood pressure drops in human body, the autonomic nervous system (the sympathetic) stimulates the body to decrease blood vessel diameter, to increase the blood flowing speed, and then return the pressure...
but why when this process occurs during blood clotting (spasm), the injured blood vessel diameter decreases to PREVENT more blood wasting, although physically it should increase the blood flow!!!???