In order to understand hypertension, it is first necessary to understand what blood pressure is and how it occurs.
The body's tissues depend on nourishment from the blood in order to survive. Blood circulates to all body tissues through a network of blood vessels and organs known as the circulatory system. The blood is entirely contained by the circulatory system. It stays within the various vessels, arteries and organs (the lungs and heart) comprising the circulatory system and fills the space within that system pretty much completely.
Blood does not move through the circulatory system by itself. Rather, it is forced through the circulatory system by the heart. The force of the heart's muscular contractions exert pressure onto the blood, forcing it to flow through the blood vessels. The blood vessel walls contain the force exerted by the heart, pushing back on the blood and providing it only one avenue of forward movement; through the circulatory system in a continual loop.
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Blood pressure is thus the result of two sets of forces – the force of the heart and the resistance of the vessel walls – coming together to push the blood through the body's circulatory system.
Blood pressure is not constant or flat; rather it is cyclical or wavy. The heart beats and relaxes again and again in a steady rhythm. If one was to graph the force exerted by the heart as it plays out over time, a waveform would result, with peaks and troughs corresponding respectively to heart beats and resting periods. As the heart beats, it exerts force on the blood, increasing the overall blood pressure. As the heart relaxes between beats, there is less force exerted onto the vessels and the blood pressure lowers. This moment to moment up and down cycling of blood pressure is normal and continues throughout a person's life.
In addition to the blood pressure being wavy due to the alternation of heart beats and resting periods, the total average blood pressure is also dynamic and always changing in response to the amount of arousal people experience. During exercise, for example, the heart beats faster and with more force in each beat, compared to the resting state and the total average blood pressure changes accordingly. If you go from sitting to standing your blood pressure will change slightly. If you go from walking to running your blood pressure will change slightly. Therefore your blood pressure may vary from one measurement to the next.
The body's blood vessels are strong and elastic and designed to accommodate a variety of blood pressures, but even good materials have their limits. It is important that the blood pressure never get too strong. What is needed is just enough pressure to move the blood through the body, and not any more than that optimal amount. If the pressure becomes too strong, the vessels themselves can be damaged with long term consequences for the health of the tissues and organs that depend on those vessels. When damage does occur, it generally happens slowly over time and not as any single 'blowout' event.