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The extra red blood cells make your blood thicker than normal. As a result, blood clots can form more easily. These clots can block blood flow through your arteries and veins, which can cause a heart attack or stroke. Thicker blood also doesn't flow as quickly to your body as normal blood.
With pulmonary hypertension, the rise in blood pressure is caused by changes in the cells that line your pulmonary arteries. These changes can cause the walls of the arteries to become stiff and thick, and extra tissue may form. The blood vessels may also become inflamed and tight.