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Chapter 1: How Your Body Controls Blood Pressure
To understand why high blood pressure is a serious health problem, you first need to become familiar with the concept of blood pressure. This chapter reviews how and why the body maintains blood pressure and what can go wrong in the many systems involved.
Your heart and blood vessels work together to deliver oxygen and nutrients throughout the body and to remove waste products, such as the carbon dioxide you exhale with each breath. Blood is the medium through which these substances are transported. Blood pressure is the result of two opposing forces in the body: the force created by the heart when it pumps blood out and the force of the arteries (blood vessels that carry oxygen and nutrients to the tissues) as they resist this blood flow. The force generated by the heart when it contracts is called the systolic blood pressure, the force against the walls of the arteries when the heart relaxes is called the diastolic blood pressure. If the force of the heart pumping blood or of the arteries resisting blood flow (or both) are too great, you have hypertension.
A CARDIOVASCULAR PRIMER
You need oxygen and food to survive. Your body has a sophisticated method for delivering oxygen and nutrients to each of the millions of cells that make up your tissues and organs. Blood is kept in constant circulation by the pumping action of your heart. The heart sends blood first to the lungs to pick up oxygen, then to the rest of the body, and back to the heart through a system of tubes known as the vascular system. Because the heart and blood vessels (the vascular system) are all joined in one continuous and interdependent circuit, you will usually hear this called the cardiovascular system.
Your heart is the small, powerful pump that moves blood through the cardiovascular system. Even though it is not much larger than a clenched fist, your heart is the hardest working muscle in your body. Your heart beats approximately once each second and is responsible for pushing 10 to 11 pints of blood through 60,000 to 100,000 miles of blood vessels.
Your heart is divided into four chambers separated by valves that prevent backflow and intermingling of blood among the chambers. Blood that has circulated through the body is received by the first heart chamber, the right atrium. Once this oxygen-poor blood has been collected in the atrium, it is pushed into the next heart chamber, the right ventricle. The right ventricle sends this "used" blood straight to the lungs through the pulmonary artery (the only artery to carry oxygen-poor blood). The lung tissues filter out the carbon dioxide and supply oxygen to the blood. This oxygen-rich blood returns to the next heart chamber, the left atrium, through the pulmonary veins (the only veins to carry oxygen-rich blood). When this oxygen-rich blood has collected in the left atrium, it is pushed into the final heart chamber, the left ventricle.
The left ventricle is the main pumping chamber of your heart. The largest and most powerful chamber, the left ventricle sends oxygen-rich blood through the aorta, the body's largest artery, to the entire body. The left ventricle works so hard that its muscular walls can be over one-half inch thick, more than three times thicker than those of the right ventricle.
Like any other organ, your heart needs oxygen and nutrients to function. The heart supplies blood to itself through two coronary arteries (right and left) that branch off the base of the aorta. The "used," oxygen-poor blood drains through the coronary veins directly into the right atrium. High blood pressure puts you at risk for coronary artery disease, so you can now see why these particular arteries are so important to keep healthy. When blood flow through these vessels is reduced or blocked, parts of your heart become weak and may even die. This is a heart attack.
So that it can separate the oxygen-rich blood from the "used" blood, the vascular system is divided into two components. Arteries deliver oxygen and nutrients from the heart to the body, and veins return oxygen-poor blood back to the heart, where the cycle begins again. Arteries and veins branch off into ever smaller and thinner walled vessels as they get farther from the heart and
deeper into the tissues. The arterioles conduct blood from the arteries to the capillaries. Capillaries are the body's smallest blood vessels, which transmit oxygen and nutrients to individual cells and collect waste products. From the capillaries, blood low in oxygen and full of waste products flows first to venules (the smallest veins), which then pass blood along through veins back to the heart.
A healthy person's arteries are muscular and elastic. They stretch when the heart pumps blood through them. They can also change their diameter as needed to regulate the flow of blood. To raise blood pressure, the arterioles constrict or become narrow, to lower it, they dilate or widen. You will learn how and why in the sections ahead.
BLOOD PRESSURE CONTROL
You might think that your heart is responsible for your high blood pressure, but many organs and the chemicals they produce are involved. Because so many body systems are involved, researchers still have not determined the exact cause of hypertension. Blood pressure can be altered by anything that affects total cardiac output (the amount of blood your heart pumps) or total peripheral resistance (the degree to which blood vessels resist blood flow) or both.
Your heart, controlled by the brain, works to circulate blood continuously at a steady rate of about 5 quarts per minute. If you have excess fluid in your body, your blood volume goes up, causing both your cardiac output and your blood pressure to increase as well. The greater the volume of fluid, the harder your heart and blood vessels must work to circulate it. Diuretics, a common type of drug used to treat hypertension, work by reducing the volume of fluid in the body.
Your kidneys, the two bean-shaped organs in your lower back area, regulate the amount of fluid circulating in the body. They control fluid volume by either retaining salt and water or eliminating them in your urine. Normally, if you have eaten too much salt, your kidneys eliminate the excess sodium along with a certain amount of water. However, if your kidneys cannot get rid of the extra sodium, your body retains water, which raises blood volume and therefore blood pressure.
Two important chemicals are involved in maintaining the proper balance between sodium and water in the body. One is an enzyme produced by the kidney called renin. (Enzymes are proteins that speed up various chemical reactions in the body.) The kidney decides when to release renin on the basis of the amount of fluid in the body (which in turn is determined in part by the amount of salt you eat) and on the level of blood pressure in the arteries that supply the kidney. The lower the pressure, the more renin released. Renin acts by speeding up the conversion of angiotensinogen, another protein in the blood, to angiotensin. This reaction and its consequences are described in the next section.
Renin's by-products also stimulate the adrenal glands, which sit right on top of the kidneys, to produce a second chemical involved in regulating sodium levels in the body. This second chemical is a hormone called aldosterone. Unlike enzymes, hormones enter individual cells and serve as chemical messengers to their target tissues. Aldosterone goes from the adrenal glands into the blood and then to the kidneys. Aldosterone's chemical message to the kidneys is to retain more sodium and water. This retention of sodium and water elevates blood pressure.
The adrenal glands produce other hormones that affect cardiac output and blood pressure. When you are in a stressful situation of any sort, your brain preps the body for emergency action. One of the ways it does this is to send messages via the sympathetic nervous system to several organs. The sympathetic nervous system is one part of a larger portion of your nervous system, the autonomic nervous system. This system is responsible for controlling involuntary functions, such as breathing, digesting food, and controlling blood pressure.
When the adrenal glands receive an emergency signal from the sympathetic nervous system, they produce adrenaline or, as most doctors call it today, epinephrine, and noradrenaline (or norepinephrine). These hormones have several effects on the body. Epinephrine makes the heart beat faster. Norepinephrine is described in the next section. As you might expect, this sudden elevation in heart rate raises cardiac output and therefore blood pressure.
Your arteries play a more active role in regulating blood pressure than your heart does. The brain learns about fluctuations in blood pressure from special sensors in the walls of blood vessels that are part of the sympathetic nervous system. These baroreceptors, as they are called, notice when the pressure against the arterial wall goes up or down and relay this information to the brain. When your brain receives a signal that your blood pressure is too high, it sends a message through a collection of nerve cells (called vasomotor nerves) in your arteries to expand (or dilate) so blood can flow through more easily and under less pressure.
The brain also monitors the amount of oxygen and nutrients needed throughout the body. It adjusts blood pressure and blood flow to ensure that each organ's needs are being met. When the body is resting, the brain lowers blood pressure; when the body is under stress of any sort (including the stress of waking up), the brain raises blood pressure. The vasomotor nerves assist in accommodating these changes by telling the arteries to open (lowering blood pressure) or close (raising blood pressure).
As described above, the brain also readies the body for emergency situations by telling the adrenal glands to release epinephrine and norepinephrine. These hormones fit like keys into locks on specific tissues. These locks are called receptors, and when the hormone slips into them, it launches a certain action. Your heart has locks called beta receptors. When epinephrine and norepinephrine enter these receptors, your heart beats faster. Your kidneys also have beta receptors. When epinephrine and norepinephrine enter these receptors, the kidneys are stimulated to produce renin. You probably know of a type of high blood pressure drug called a beta blocker, which does exactly that: it fits into the beta receptor and blocks epinephrine and norepinephrine from triggering a rise in blood pressure.
Epinephrine and norepinephrine also fit into special locks on the arteries called alpha receptors. When this occurs, the artery constricts, raising blood pressure. Whether the walls of arteries and arterioles constrict depends on the amount of calcium inside the muscle cell. The constriction is triggered by a small amount of calcium passing into the cell through tiny passages called calcium channels. You may have heard about drugs called calcium channel blockers. These prevent the constriction of blood vessel walls by blocking the passage of calcium into the muscle cell. Drugs called alpha blockers work by preventing epinephrine and norepinephrine from triggering the constriction in the first place.
As mentioned in the last section, renin, an enzyme produced by the kidney (and other tissues in the body, such as the liver), is released when the body needs to raise blood pressure. However, renin does not directly affect blood pressure. Rather, it starts a chemical chain reaction by allowing one protein to convert into another, namely, angiotensinogen into angiotensin. If this angiotensin interacts with angiotensin converting enzyme (ACE you have probably heard of ACE inhibitors, drugs that prevent this interaction), yet another substance is formed, called angiotensin II. Angiotensin II is an active chemical agent that causes arteries to contract and the adrenal glands to release aldosterone. The renin-angiotensin-aldosterone system is not completely understood yet, but research has shown that it is crucial to the development of high blood pressure. An even newer class of drugs called angiotensin blockers interfere with the action of this powerful chemical agent.
Many other factors affect resistance in the blood vessels. Too much fluid, besides increasing blood volume, makes tissues stiff. The arteries must contract harder to push blood into the tissues, so blood pressure goes up.
Blood pressure tends to rise with age because the arteries become harder and less flexible and thus put up more resistance to blood flow. The arteries in many people tend to become narrow and clogged with accumulated fatty debris over the years. This, too, raises blood pressure, owing to increased peripheral resistance. As you will learn in Chapters 2 and 6, smoking also causes the arteries to become stiffer and more resistant to blood flow.
WHAT IS HIGH BLOOD PRESSURE?
Although all these organs, nerve cells, and chemical messengers normally work together to maintain a healthy blood pressure, in some people something goes wrong that upsets this finely integrated system of checks and balances. Their blood pressure remains high even when the body does not need the additional force to distribute oxygen and nutrients. If the exact cause is unknown, doctors refer to the sustained high blood pressure as essential or primary hypertension, which is the most common type of high blood pressure. If the elevated blood pressure is caused by another disease or medication, doctors refer to it as secondary or disease-related hypertension.
Actually, there is no clear dividing line between normal and high blood pressure. The exact point at which sustained blood pressure is too high and can be classified as hypertension has instead been defined over the years on the basis of medical experience. Doctors have examined thousands of patients with different blood pressure levels and identified who was at highest risk for additional physical damage and disease. On the basis of many years of information, the joint National Committee on Detection, Evaluation, and Treatment of High Blood Pressure established blood pressure guidelines to help Americans monitor their risk of hypertension and its unhealthy effects. How doctors diagnose and classify hypertension is described in Chapter 4.
Copyright © 1998 by American Medical Association