Blood pressure is the pressure exerted by the blood against the walls of the blood vessels, especially the arteries. It varies with the strength of the heartbeat, the elasticity of the arterial walls, the volume and viscosity of the blood, and the person’s health, age and physical condition. It is measured in units of millimetres of mercury (mmHg).
The English scientist Stephen Hales discovered blood pressure in 1733 while investigating the spurting of blood from torn blood vessels. Here is an animation of the experiment he performed on a female horse:
After this experiment, he proceeded to draw various quantities of blood from the same horse while measuring the heights at which the blood rose in the tube and observed that as the volume of blood in the horse decreased, the height at which the blood rose (i.e. blood pressure) in the tube decreased.
Here is an oil painting of Stephen Hales: