Caffeine is a bitter-tasting stimulant found in coffee, tea, soda and energy drinks. It temporarily acts to stimulate the central nervous system, affecting memory, alertness, sleep patterns, behaviour, heart rate and even blood pressure. These effects can last as long as seven hours.
Blood pressure is defined as the force exerted by blood on the vessel walls. It's measured by two main figures: systolic and diastolic pressures. Systolic pressure refers to the pressure of the blood when the heart is beating. Diastolic pressure refers to the pressure of the blood when the heart is at rest. Both types are measured by a unit known as mmHg, or millimetres of mercury. This is equal to the pressure exerted by fluid a millimetre deep in mercury.
According to Dr. Sheldon G. Sheps, a hypertension specialist with the Mayo Clinic, two or three cups of coffee can raise systolic pressure by 3 to 14 mmHg and diastolic pressure by 4 to 13 mmHg. This is true both at rest and during exercise. However, the distribution of pressure throughout the body may be uneven. For example, the pressure in the aorta, an artery originating in the heart and extending down the abdomen, can be higher than the pressure in the brachial artery of the arm after the ingestion of caffeine.
The mechanism behind the effect of caffeine upon blood pressure is unknown as of May 2011. Caffeine may block hormones that help widen arteries or induce the adrenal gland to release more adrenaline, which increases blood pressure. A 2001 study published in the journal "Hypertension" lends credence to the idea that caffeine increases arterial stiffness. When the size of the arteries shrinks and the volume of blood remains the same, pressure increases because the blood is forced into a smaller area. Normally the size of the blood vessels is dynamic and constantly changing to account for the changes of blood flow throughout the body.
The line between normal and high blood pressure is thin—hypertension is defined by a systolic pressure of at least 140 mmHg or a diastolic pressure of at least 90 mmHg—so caffeine can have a significant effect. If your blood pressure readings are straddling two different lines, then caffeine may be the difference between normal readings and a state of pre-hypertension or even between a state of pre-hypertension and a state of hypertension. It is true that some people who regularly drink caffeinated beverages develop a tolerance to it and no longer experience any of its influences. However, people who are worried about its effects on high blood pressure should limit their amount of caffeine to about 200mg per day, or the same amount of two 355 ml (12-oz.) cups of coffee.