When you or your doctor takes your blood pressure, it is truly the measure of the pressure of your blood. As the heart pumps blood through the body, there is pressure or force exerted against the arterial walls. The systolic blood pressure is when the heart muscle contracts in the cardiac cycle; the diastolic pressure is the relaxation or dilation phase of the heart muscle in the cardiac cycle. Blood pressure is not strictly a function of how the heart pumps. It is also somewhat controlled by hormones secreted by the kidneys and by the sympathetic nervous system, which controls the dilation and constriction of blood vessels through the body.
When your blood pressure is taken with a manual instrument, a cuff is placed around your upper arm and inflated until the blood in your artery stops. Gradually the air is let out of the cuff. The person taking the blood pressure reading listens through a stethoscope (placed over the artery at your inside elbow) for a thumping sound, which represents the systolic pressure. When the thumping starts to fade, that is the diastolic pressure. At both these points, the level of the mercury (in millimeters) in the instrument is read. Your blood pressure reading is represented by the two numbers – the first number (and normally the larger number) is the systolic reading, and the second number is the diastolic reading.
In some circumstances, the blood pressure might vary through the day. For example in say 20% of the patients, their blood pressure taken at the doctors office could be significantly higher than what it normally is. This is caused by anxiety and stress,; some patients suffer from a condition called white coat hypertension which is the result of anxiety due to the examination by a health care professional.
Generally, when diagnosed, there are two bands of blood pressure: high and low. So what do these mean, and what do they cause? Lets start with high blood pressure, what does this mean to your body?
Quite simply high blood pressure, means your heart has a bigger work load, think of it in a way where you are in a very crowded place (such as a musical concert/subway) and you need to squeeze past all those people to reach your destination. This is the same for the heart, except that it’s the one pushing you along through all those people. High blood pressure also stresses your arteries, causing unhealthy tissue growth on the inside, there are several other factors such as increased risk of a heart attack, a stroke, headaches, convulsions, confusion and difficulty concentrating.
On the other hand, low blood pressure can be just as bad. Symptoms of low blood pressure are confusion, shock, fainting and dizziness. These are mostly cause when the blood supply in the arteries drops below a certain point, leading to a short fall of blood supply to the brain. These situations occur mostly when the patient stands up or sit up; due to gravity the circulation of blood is now more focused on the lower of the body and there’s not as much blood left to reach the upper parts such as the brain etc.
Aleksandr Lukas (Article Writeup)
The New York: Science Desk Reference