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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.
If you have high blood pressure then this can result in chronic flushing in the face. High blood pressure can be caused by various things  such as cholesterol which is fatty deposits clogging our veins and arteries. This essentially thins the space available in the blood vessels thereby increasing blood pressure.