Theoretically, if you inject litres of water / blood into a person's veins, you could fill up his circulation so much that he might actually get a high pressure.
But in real life, this is not the actual cause of the usual high blood pressure. It's due to the imbalance of the nervous control of blood vessels, hormone levels etc. In many cases, scientists aren't even sure about the exact trigger of high blood pressure yet, although the mechanism and the control method are well known.
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