Blood pressure is usually expressed in terms of the systolic pressure (maximum during one heart beat) pressure over diastolic pressure (minimum in between two heart beats) and is measured in millimeters of mercury (mmHg), above the surrounding atmospheric pressure (considered to be zero for convenience).
Corrosion on the terminals is due to hydrogen gas being released from the acid in the battery. It mixes with other things in the atmosphere under the hood and produces the corrosion you see on the terminals. Generally, if the corrosion is occurring on the negative terminal, your system is probably undercharging.
What causes corrosive build-up on the battery terminals, as seen in the picture, is a combination of hydrogen gas produced by the chemical reactions in the battery, and the dirt and sediment on the terminals. For this reason keeping a clean battery before corrosion occurs will help prevent the problem from starting. keeping grease or petroleum jelly on the terminals will prevent battery acid build up as well. Other car battery maintenance may include putting water in the cells when needed.