Blood pressure (BP) is the force or pressure that carries blood to all parts of the body. A blood pressure reading is the pressure that blood puts on the walls of arteries. There are 2 parts to a blood pressure. One is called systolic (sis-tall-ik) and is the top or the first number in a blood pressure reading. The other number is called the diastolic (di-uh-stall-ik) and is the bottom or.