Elevated Blood Pressure or BP as in Essential Hypertension is a diagnosis. The disease or rather the pathophysiology is in reality, a vascular disease that produces an elevated resistance to blood flow known as systemic vascular resistance or SVR. This is the total resistance that the flow of cardiac output pushes against. In most clinical situations, BP can be measured but SVR generally requires invasive catheters in the central circulation of the heart. The technology to measure SVR is not available in the usual situation. The abnormality or abnormal pathophysiology is elevated SVR but the indicator used for diagnosis and treatment is blood pressure. This is in preference to practicality.
Although the generalized vascular disease in Essential Hypertension cannot be imaged, we know of its existence because of the physics of blood flow. This generalized increase in the resistance to blood flow is unnoticed except when blood pressure is measured. The increases in blood pressure tend to overcome even the effect of stress on the cardiovascular system. Even the spikes in resistance are unnoticed as blood pressure rises to over come the increase in resistance with various situations. When SVR rises enough to be symptomatic then anaerobic metabolism will develop as the impaired delivery of oxygen develops.
The increase in systemic vascular resistance in aging and with other causes is due to vascular changes including narrowing of the vessels and irregularities of the lumen that produce turbulence. Generally as people age these changes become more significant and the resistance to blood flow increases. This is apparent as an increased incidence and severity of asymptotic hypertension.
Elevated Blood Pressure to preserve flow is a compensation for the pathological increase in SVR
Elevated blood pressure is a response to the real abnormality, the rise in SVR. The rise in blood pressure is a compensation to preserve flow. If the blood pressure did not rise to preserve flow the downstream tissue would suffer the lack of blood flow. The most critical problem is the delivery of oxygen and the support of metabolism. The rise in blood pressure in this sense is a good thing. It allows individuals to continue to live with the vascular disease that would otherwise cause noticeable illness or be fatal.
It is possible to measure, track and monitor blood pressure. In other words, BP is observable. Systemic Vascular Resistance is not. Essentially all research on Essential Hypertension for practical reasons is done using observations of BP not SVR. BP is what is measured, and indexed against outcomes. There are no doubt important considerations that would be different, if we could monitor SVR to track the changes in the vascular system.
It has been demonstrated that diuretics while lowering blood pressure do not reduce the risk of vascular disease or reduce the complications of asymptomatic hypertension. Since diuretics do not act directly on the vascular system, this is not surprising. Hypertension is a vascular disease. There are also some studies that show that diuretics do reduce the risk of vascular disease. We should be doubtful of such studies.
Essential Hypertension is often considered a risk factor for illness such and stroke and myocardial infarction. With stress such as sepsis or drug overdose the resistance to the flow of blood will become evident and an illness that is asymptomatic at baseline will become symptomatic as the flow of blood is limited by the high resistance.
Vascular disease in Essential Hypertension is mostly irreversible. Although the blood pressure may be controlled by medication the vascular disease remains. Elevated blood pressure in this condition will return when the medication is discontinued.
It is beneficial that blood pressure rises to compensate for the generalized vascular disease.