Hypertension Effects on the Cardiovascular System

Introduction to Hypertension

High blood pressure, or hypertension, is defined as a consistent recording of systolic blood pressure of 140 mmHg or greater and a diastolic blood pressure of 90 mmHg or greater. High blood pressure is the most common circulatory disorder among human beings, occurring all over the world — most commonly in middle-aged or elderly men and women. Nearly half of all Americans who reach the age of 74 develop high blood pressure. As indicated in Table 1 below, hypertension can be classified as normal, pre-hypertensive, Stage 1, or Stage 2 (National Institutes of Health). Blood pressure is determined by cardiac output and total peripheral resistance. Cardiac output is the total volume of blood pumped by the heart per minute, while total peripheral resistance is the force the heart must work against to pump blood. Cardiac output is a central factor determined by heart rate and stroke volume, whereas total peripheral resistance is a peripheral factor determined by vascular resistance.

It is estimated that 50 million Americans have high blood pressure. The prevalence of high blood pressure is higher for African Americans than any other racial group. In addition, hypertension is more prevalent in lower socioeconomic status (SES) groups and in men. The prevalence of high blood pressure also increases with age. Hypertension is often associated with obesity, increased salt intake, heavy alcohol intake, decreased physical activity, and psychosocial stress.

Hypertension can be categorized as either primary or secondary. Primary hypertension accounts for approximately 90% of all cases and is idiopathic in nature. Secondary hypertension accounts for the remaining 10% of cases and is normally caused by kidney disorders, adrenal tumors, drugs, or toxemia in the final trimester of pregnancy.

Table 1. Categories for Blood Pressure Levels in Adults (in mmHg, millimeters of mercury)

Normal: Systolic less than 120 / Diastolic less than 80
Prehypertension: Systolic 120–139 / Diastolic 80–89
High Blood Pressure, Stage 1: Systolic 140–159 / Diastolic 90–99
High Blood Pressure, Stage 2: Systolic 160 or higher / Diastolic 100 or higher

Hypertension can have widespread effects on multiple organs, including the brain, kidneys, and eyes. However, the effects of hypertension on these organs are secondary to its effects on the cardiovascular system. That is, the changes that occur in the cardiovascular system due to hypertension lead to the deleterious effects observed in the brain, kidneys, and eyes. This paper therefore focuses on the effects of hypertension on the cardiovascular system.

Hypertension and the Arterial System

Hypertensive damage to the cardiovascular system occurs in either the myocardium (heart) or the arterial system. In general, hypertension increases the risk of heart attacks, congestive heart failure, peripheral vascular disease, and stroke. Even moderately elevated blood pressure can be dangerous. The effects of high blood pressure on the arterial system are examined first, followed by a discussion of its effects on the myocardium.

High blood pressure accelerates the hardening of arteries and arterioles, leading to an increased incidence of atherosclerosis along with increased calcium deposits in the coronary arteries (Lu et al.). For reasons that are not completely clear, hypertension hastens hardening of the arteries, a process known as arteriosclerosis or atherosclerosis — terms that are largely synonymous. Atherosclerosis refers to a buildup of plaque on the inside lining (intima) of an artery.

Although the underlying mechanisms linking high blood pressure to atherosclerosis are not fully understood, one theory holds that the increased pressure of blood flow damages the intima, making it more susceptible to narrowing through deposits of plaque and cholesterol. More specifically, the endothelial lining of the artery is damaged, and because of this damage, fats, cholesterol, platelets, cellular waste products, calcium, and other substances are deposited in the artery wall. These deposits may stimulate artery wall cells to produce additional substances that cause further cellular buildup.

These cells and surrounding material significantly thicken the endothelium. The artery's diameter shrinks, blood flow decreases, and the oxygen supply is reduced. The end result is a narrowing of the arterial lumen with a subsequent reduction in blood flow and an aggravation of the pressure problem. Often, a blood clot may form near the plaque and block the artery completely, stopping blood flow altogether.

In the elderly population, atherosclerosis also results from aging processes. With increasing age, arterial compliance (elasticity) decreases, resulting in stiffer arterial walls. During systole — the pumping of blood from the heart into the arterial tree — the arteries do not expand sufficiently to accommodate the volume of blood being pumped, causing increased peripheral resistance to blood flow. The heart must then exert additional force to push blood through the narrowed arteries, and the resulting high blood pressure further injures the arterial walls and worsens atherosclerosis.

Over time, the constant pressure of blood flowing through a weakened artery can cause a section of its wall to enlarge and form a bulge, called an aneurysm. An aneurysm can rupture and lead to death from significant internal bleeding. Aneurysms can form in any artery throughout the body, but they are most common in the aorta. The majority of aortic aneurysms occur in the abdominal aorta or the descending thoracic aorta.

Atherosclerosis can thus lead to disease states across multiple organ systems. In the coronary arteries, the result can be angina or a heart attack. If atherosclerotic narrowing occurs in the arteries of the brain, stroke can result. In addition, narrowing of the arterial lumen increases total peripheral resistance, which increases the afterload on the heart and results in an elevated workload on the myocardium.