[[1]] is the pressure exerted by the blood against the walls of the vessels. 

Blood pressure is the pressure exerted by the blood against the walls of the vessels. Blood pressure is important to maintain adequate blood flow to the body's cells and organs. Abnormal blood pressure, either high or low, can cause problems and is a major risk factor for heart disease.

Blood pressure is expressed as the ratio of two pressures: systolic pressure (the pressure when the heart beats) and diastolic pressure (the pressure when the heart is resting between beats). Normal blood pressure is typically around 120/80 mmHg.