Blood pressure is the pressure of blood within the arteries of the body. Blood pressure is the driving force that causes blood to flow through the body from the arteries (where the pressure is high), through organs, and into the veins (where the pressure is low). Blood pressure is generated by the pumping of blood by the heart into the arteries as well as by the resistance to the flow of blood by the arteries.The systolic blood pressure (the top number) represents the pressure in the arteries as the muscle of the heart contracts and pumps blood into the arteries. The diastolic blood pressure (the bottom number) represents the pressure in the arteries as the muscle of the heart relaxes after it contracts.
Systolic blood pressure for most healthy adults falls between 90 and 120 millimeters of mercury (mm Hg). Normal diastolic blood pressure falls between 60 and 80 mm Hg. (By convention, an individual’s blood pressure is written as systolic/diastolic blood pressure, e.g., 120/80). Current guidelines define normal blood pressure as lower than 120/80. Blood pressures between 120/80 and 140/90, which used to be considered “pre-hypertension,” now are considered too high. High blood pressure increases the risk of developing heart disease, kidney disease, hardening of the arteries (atherosclerosis or arteriosclerosis), eye damage, and stroke.
Low blood pressure is blood pressure that is so low that it is causing symptoms or signs due to the low flow of blood through the arteries and veins. When the flow of blood is too low to deliver enough oxygen and nutrients to vital organs such as the brain, heart, and kidney; the organs do not function normally and may be permanently damaged. It should be noted that unlike high blood pressure, which is defined on the basis of blood pressure alone, low blood pressure is defined primarily by signs and symptoms of low blood flow. In fact, some individuals may have a blood pressure of 90/50 and have no signs or symptoms of low blood pressure, and, therefore, not have low blood pressure while others who normally have a blood pressure of 130/80 may develop symptoms and signs of low blood pressure if their blood pressure drops to 100/60.
Article Source: http://medicinenet.com