Blood pressure is measured as two numbers: the systolic pressure over the diastolic pressure. Systolic pressure is the pressure achieved when the heart is contracting, and diastolic pressure is measured when the heart is relaxing. Changes in blood pressure regulate blood flow to the entire body, and local changes in the organs regulate blood flow to the individual organs. In addition, certain diseases change the blood pressure.
High Blood Pressure
The body can respond to increasing demands by increasing the blood pressure. For example, organs such as the muscles require more oxygen during exercise, so the body increases the heart rate, the force of contractions of the heart and changes the diameter of the blood vessels, according to "Physiology" by Linda S. Costanzo. This has the effect of increasing the blood pressure to deliver more blood and oxygen to the body.
Certain diseases can also increase blood pressure. Pain activates the fight or flight nervous system, known as the sympathetic nervous system, to increase blood pressure. Essential hypertension is chronically high blood pressure that can lead over time to such problems as heart disease, strokes and kidney damage. Malignant hypertension is dangerously high blood pressure that must urgently be lowered.
Low Blood Pressure
Low blood pressure can be a normal state. Generally, the blood pressure is slightly lower when lying flat than standing up, because the heart does not have to work against gravity. In addition, some people have lower blood pressure normally than others.
In addition, some diseases can dangerously lower blood pressure. Sepsis is a massive infection in the body, usually from bacteria or fungi. In this condition, factors released by the invading organisms and the body's response to the infection result in lowering of the blood pressure. Other conditions that damage the heart, such as a heart attack, impair its ability to deliver blood and therefore lower the blood pressure, according to "Harrison's Principles of Internal Medicine" by Anthony S. Fauci.
Isolated Changes in Systolic or Diastolic Pressure
Generally, the systolic and diastolic pressures change together, but certain conditions can cause isolated changes in these individual pressures. In aortic regurgitation, the heart valve connecting the heart to the main artery of the body, called the aorta, allows blood to flow back into the heart. When the heart is contracting, the systolic pressure will be unaffected or can even be higher than normal. However, when the heart is relaxing, the diastolic pressure is lower because of the abnormal flow of blood back into the heart. Thyroid disorders can also affect these pressures. High thyoid hormone levels increase the systolic pressure. Low thyroid hormone levels can increase the diastolic pressure.
References
- "Physiology"; Linda Costanzo; 4th Ed 2009
- "Harrison's Principles of Internal Medicine"; Anthony S Fauci et al; 17th Ed 2008
