What Causes Hypertension?
Two forms of high blood pressure or Hypertension have been described: essential (or primary) hypertension and secondary hypertension.
Essential hypertension is a far more common condition and accounts for 95 percent of the population with hypertension. The cause of essential hypertension is unknown.
In secondary hypertension, which accounts for 5 percent of all cases, the high blood pressure is secondary to or caused by a specific abnormality in one of the organs or systems of the body.
Essential hypertension affects a major part of the population, yet, its basic cause or underlying defect is not known. Nevertheless, certain associations have been recognised in people with essential hypertension. For example, the condition develops only in groups or societies that have a fairly high intake of salt, exceeding 6.8 grams daily. In fact, salt intake may be a particularly important factor in relation to essential hypertension in a number of varied situations. Thus, excess salt may be involved in the hypertension that is associated with advancing age, black racial background, obesity, hereditary (genetic) susceptibility and kidney failure (renal unsufficiency).
Genetic Factors and Essential Hypertension
Genetic factors are thought to play a prominent part in the development of essential hypertension. However, the genes for hypertension have not yet been identified (genes are tiny portions of chromosomes that produce the proteins that determine the characteristics of individuals). The current research in this area is focused on the genetic factors that affect the Renin-AngiotensinAldosterone system. This system helps to regulate blood pressure by controlling salt balance and the tone (state of elasticity) of the arteries.
Approximately 30 percent of causes of essential hypertension are attributable to genetic factors. For example, in the United States of America, the incidence of high blood pressure is greater among blacks than among whites or Asians. Also, in individuals who have one or two parents with hypertension, high blood pressure is twice as common as in the general population. Rarely, certain unusual genetic disorder affecting the hormones of the adrenal glands may lead to hypertension.
As mentioned above, the underlying causes of essential hypertension is unknown. Nevertheless, it has been found that the vast majority of patients with essential hypertension have in common a particular abnormality of the arteries. That is, they have an increased resistance (stiffness or lack of elasticity) in the tiny arteries that are most distant from the heart (peripheral arteries or arterioles). The peripheral arteries supply blood containing oxygen and nutrients to all of the tissues of the body. Just what makes the peripheral arteries become stiff is not known. Yet, this increased peripheral arteries, resistance is present, as well, in those people whose essential hypertension is associated with genetic factors, obesity, lack of exercise, overuse of salt, and aging.
What are the causes of secondary hypertension? Five percent of people with hypertension have what is called Secondary Hypertension. This means that the hypertension in these individuals is secondary to (caused by) a specific disorder of a particular organ or blood vessel, such as the kidneys, adrenal glands or aortic artery.
Renal (Kidney) Hypertension
Diseases of the kidneys can cause secondary hypertension. This type of secondary hypertension is called Renal Hypertension, because it is caused by a problem in the kidneys. One important cause of renal hypertension is the narrowing (stenosis) of the artery that supplies blood to the kidneys (renal artery). In younger individuals, usually women, the narrowing wall of the arteries going to the kidneys (Fibromuscular Hyper Plasia) is caused by the thickening of the muscular wall. In older individuals, the narrowing generally is due to hard fat containing (atherosclerotic) plaques that are blocking the renal artery.
How does narrowing of the renal artery cause hypertension?
First, the narrowed renal artery impairs the circulation of the blood to the affected kidney. This deprivation of blood then stimulates the kidney to produce the Hormones, Renin and Angiotensin. These hormones along with aldosterone from the adrenal gland, cause a constriction and increased stiffness (resistance) in the peripheral arteries throughout the body, which finally, as mentioned previously, results in high blood pressure.
Renal hypertension is usually first suspected when high blood pressure is found in a young individual or a new onset of high blood pressure is discoverd in an older person. Screening for renal artery narrowing then may include renal isotope (radioactive) imaging, ultrasonographic (sound wave) imaging, or Magnetic Resonance Imaging (MRI) of the renal arteries. The purpose of these tests is to try to determine whether there is a restricted blood flow to the kidneys.