Learn more about Hypertension.

High blood pressure, termed "hypertension," is a condition that afflicts more than 50 million Americans and is a leading cause of morbidity and mortality. Hypertension is much more than a "cardiovascular disease" because it affects other organ systems of the body such as kidney, brain, and eye. Tens of millions of Americans are not even aware of being hypertensive because it is usually asymptomatic until the damaging effects of hypertension (such as stroke, myocardial infarction, renal dysfunction, etc.) are observed.

The term "hypertension" can apply to elevations in mean arterial pressure, diastolic pressure, or systolic pressure. Hypertension is often defined as a diastolic pressure of 90 mmHg or above, or a systolic pressure of 140 mmHg or above. A diastolic pressure of 80 to 89 mmHg, and a systolic pressure of 120-139 mmHg is termed prehypertension. Elevations in either diastolic or systolic pressure represent a significant risk factor to a patient.

Mean arterial pressure is usually not discussed in the context of hypertension because it is not normally measured in a patient. However, changes in either cardiac output or systemic vascular resistance will increase not only diastolic and systolic pressures, but also mean arterial pressure. The term "mean arterial pressure" is usually spoken in the context of the arterial pressure that is responsible for organ perfusion.

In most patients (90-95%) presenting with hypertension, the cause is unknown. This condition is called essential (or primary) hypertension. The remaining 5-10% of hypertensive patients have hypertension that results secondarily from renal disease, endocrine disorders, or other identifiable causes. This form of hypertension is called secondary hypertension.

Regardless of the origin of hypertension, the actual increase in arterial blood pressure is caused by either an increase in systemic vascular resistance (SVR) or an increase in cardiac output (CO). Therefore, in order to understand how arterial blood pressure can become elevated, it is first necessary to understand the mechanisms that regulate both SVR and CO.