If our body's blood pressure is 120 mm-Hg then why blood goes out of our body in 760 mm -Hg pressure if we have cuts?
That's actually the gauge pressure that we measure. Gauge pressure is the amount by which the pressure measured in a fluid exceeds that of the atmosphere. So, when we say that our blood pressure is 180mmhg, it's wrong. In reality, the blood pressure is (180+760)mmHg inside the blood vessel, and our blood pressure exceeds atmospheric pressure by 180mmhg.