Articles on Medical Diseases and Conditions

Category: Guide to Preventing and Treating Heart Disease

Essential Information You and Your Family Need to Know about Having a Healthy Heart

  • Eating to Control Your Cholesterol

    Choosing foods that are low in saturated fats, trans fats, and cholesterol can lower your cholesterol. You might think that cholesterol in food is the major contributor to elevated blood cholesterol, but that is not the case. The biggest culprits are saturated fats and trans fats. The ?rst step toward lowering your cholesterol through diet is to understand the different types of fats in foods and their impact on blood cholesterol.

    Fats That Raise Cholesterol

    Two types of fats are known to raise your cholesterol: saturated fats and trans fats. If you have high cholesterol, current guidelines recommend that you limit your intake of saturated and trans fats to total no more than 7 percent of the total calories you consume in one day. Saturated fats, which your body uses to make bad LDL cholesterol, mostly come from animal products. Beef, veal, lamb, pork, and whole-milk dairy products including butter, cream, milk, and cheeses are all high in sat- urated fat. Plant sources of saturated fats include tropical oils (coconut, palm, and palm kernel oils) and cocoa butter. These foods are also high in dietary cholesterol. However, the fat in cocoa butter appears to be more neutral and less likely to raise LDL levels.
    Trans fat or trans fatty acid is an unsaturated fat, but it can also raise your LDL levels and lower your HDL levels. Trans fats are made when hydrogen is added to vegetable oils to make them solid and longer last- ing. Trans fats are widely used in commercial baking (crackers, cookies, and cakes) and in restaurants, particularly for frying. They also occur naturally in some foods such as meat and whole milk. Recently the Food and Drug Administration mandated that the amount of trans fatty acids in any prepared food product be spelled out on the food label (see sam- ple label on page 94). Also recently, the American Heart Association recommended that people limit their consumption of trans fatty acids to no more than 1 percent of their total calories each day. However, a label may state “0 g trans fat” but still contain up to 0.5 g of trans fats per serving, so to be con?dent you are controlling the amount of trans fats, make sure the label says the product contains no hydrogenated oil or “partially hydrogenated oil.” The New York City Department of Health recently banned the use of trans fats in restaurants in the city.

    Fats That Lower Cholesterol

    Some fats may actually lower your cholesterol. Both polyunsaturated and monounsaturated fats alike have qualities that help lower your cho- lesterol. They are both good substitutes for saturated or trans fats, but you still need to moderate your intake of fats in order to keep down your total calorie intake. To lower your cholesterol, your intake of all fats combined should be 25 to 35 percent of your total calorie intake per day.
    Monounsaturated fats are found in oils and fruits, such as olive oil and avocadoes. In your body, these fats help your body’s cells resist absorption of fat and cholesterol and slow the buildup of plaque in your arteries. Polyunsaturated fats are found in many nuts and seeds, corn, and soybeans and their oils. It is important to recognize that canola oil has the lowest content of saturated fat among the various pressed oils that are available.
    Foods rich in omega-3 polyunsaturated fats may be especially health- ful, reducing your risk of coronary artery disease, high triglycerides, blood clotting, abnormal heart rhythms, and sudden death. The American Heart Association recommends that you eat at least two servings of baked or grilled ?sh, preferably fatty ?sh, each week. Omega-3 fats or fatty acids are found in ?sh, especially fatty ?sh such as sardines, mackerel, lake trout, salmon, and albacore tuna. However, concerns about the high levels of mercury in mackerel, sword?sh, and tuna have led experts to recommend that adults limit themselves to eating no more than one serving of these ?sh per week. A fetus may be especially vulnerable to mercury, so doctors often recommend that pregnant women limit their consumption of mercury-containing ?sh even more. As an alternative to ?sh, several plant sources are rich in omega-3 fats, including ?axseed and ?axseed oil, soy- bean oil, and walnuts. Soy, though high in total fat, is very low in saturated fat and might have a bene?cial effect on lipids. Soy may be consumed in various forms including tofu, soy milk, and edamame beans.

    Dietary Cholesterol

    Cholesterol is found exclusively in animal-based products. Red meat, whole-milk dairy products, egg yolks, and organ meats are especially high in cholesterol. To lower cholesterol, current guidelines recom- mend that you limit your cholesterol intake to less than 300 milligrams per day, on average. Keep in mind that plant-based foods— fruits, vegetables, grains, nuts, and seeds—don’t raise your cholesterol level, so you can eat more of them.

    Fiber

    Eaten as part of a diet low in fat and saturated fats, ?ber can help lower your cholesterol. A high-?ber diet is linked to lower death rates from coronary artery disease and heart attack.

    Soluble ?ber (a type of ?ber that is partially broken down in your intestine) effectively lowers cholesterol about 5 percent by chemically binding to cholesterol-based substances to remove them from the bloodstream. Adding more ?ber to your diet is one means of enhancing the effects of your overall cholesterol-lowering diet. Soluble ?ber is found in oatmeal and oat bran, beans, peas, barley, citrus fruits, straw- berries, and apples. By contrast, the insoluble ?ber found in wheat products has no cholesterol-lowering effects.

    Plant Stanols and Sterols

    Your doctor may recommend that you start using soft margarines con- taining plant stanols and sterols. These substances are the plant equiva- lent of cholesterol, and they may significantly reduce your body’s absorption of dietary cholesterol from other sources. Margarines con- taining these substances are available at most grocery stores. Liquid mar- garine, spray margarine, or soft margarine in tubs are recommended over hardened margarines in sticks, because those contain hydrogenated fat or trans fat.

    Alcohol and Cholesterol

    You may have read about some studies suggesting that moderate use of alcohol may actually raise your good HDL cholesterol. However, the bene?ts are not clear enough to recommend that you start drinking alcohol if you don’t drink now. People who drink in moderation—one drink a day for women, two drinks a day for men, on average—have a lower risk of heart disease than nondrinkers. But drinking in higher amounts is dangerous to your cardiovascular health in many ways, con- tributing to your risk of developing high blood pressure, obesity, and stroke. Also, for women, more than one alcoholic drink per day increases the chances of breast cancer.

  • Interpreting Cholesterol Test Results

    Cholesterol readings are expressed in milligrams per deciliter of blood (mg/dL), and the numbers are classi?ed by level of health risk. Your reading can change somewhat from day to day, and the classi?cations are based on ranges. Of course, being in a high range doesn’t guarantee that you will develop heart disease, nor does being in a low range assure that you will not. The impact of your cholesterol readings on your over- all risk for heart disease or stroke depends on other factors, including your family history, conditions such as diabetes or high blood pressure, and other health habits such as smoking or physical inactivity.

    Total Cholesterol Level

    The total cholesterol level is the most common screening measurement. About half the adults in the United States have a reading in the desirable range of 200 mg/dL or less, indicating a lower risk of developing heart disease. Even if you are at this level, it’s still a good idea to eat foods that are relatively low in saturated fats and trans fats, and to exercise regu- larly. Continue to get a full lipoprotein pro?le every 5 years.
    The total needs to be interpreted, along with how the cholesterol is packaged in the bloodstream. A total level of 200 to 239 mg/dL is called borderline high and places you at up to two times the risk of heart attack

    The total needs to be interpreted, along with how the cholesterol is packaged in the bloodstream. A total level of 200 to 239 mg/dL is called borderline high and places you at up to two times the risk of heart attack as someone with a reading below 200 mg/dL. Your
    doctor will discuss this reading and the rest of your pro?le with you, as well as other factors that con- tribute to your risk. Some people, such as menstru- ating women before menopause or young, active men, may have an elevated total cholesterol reading but may not be at high risk for heart disease. The reason is that although the total cholesterol is ele- vated, it is the HDL, or good cholesterol, that is ele- vated, and LDL, the bad cholesterol, is within target range. Talk to your doctor to interpret your results.

    If your total cholesterol is above 240 mg/dL, you are more than twice as likely to have a heart attack as someone with a borderline high reading. You are also at a higher risk of stroke. Again, you need to dis- cuss your overall pro?le with your doctor and get started trying to bring your level down to a healthy target.

    HDL Level

    HDL is the good cholesterol that, at higher levels, appears to reduce your risk of heart disease. People with a low level of HDL are at increased risk for heart disease. In the average man, HDL levels range from 40 to 50 mg/dL. In an average woman, they are higher, 50 to 60 mg/dL, because the female hormone estrogen raises HDL. After menopause, a woman’s HDL levels may fall, increasing her risk of heart disease. For a man or a woman, a reading below 40 mg/dL is considered too low. People who are overweight or physically inactive are more likely to have a low HDL reading.
    If your reading is low, your doctor may recommend that you get more exercise, lose weight if you are overweight or obese, and quit smoking if you are a smoker. Although treatment for high
    cholesterol usually focuses on lowering LDL

    cholesterol, doctors are placing increasing emphasis on the importance of raising HDL as well. A key strategy for raising your HDL choles- terol levels is eating more ?sh and less red meat. Further, consuming omega-3 fish oil reduces triglycerides and raises HDL.
    The ratio of total cholesterol to HDL is a more meaningful indicator of risk than is total

    HDL Guidelines

    HDL Level (mg/dL) Category

    Less than 40 Low
    40 to 59 Desirable; the higher the better
    60 and above High

    cholesterol alone. This is especially true because a normal total choles- terol number (less than 200 mg/dL) may pose increased risk if it is asso- ciated with a low HDL level. To calculate that ratio, divide your total cholesterol by your HDL value. A number greater than 5 shows a higher risk level. The lower your ratio is, the lower your risk of heart disease is. Try to keep your ratio lower than 4 to 1.

    LDL Levels

    LDL is the harmful cholesterol that can slowly build up plaque in the arteries. Of your lipid readings, it is the single most important indicator of your risk of cardiovascular disease. A reading of less than

    100 mg/dL is considered optimal, but not everyone needs to be that low if their other risk factors are under control.
    You and your doctor will talk about your target LDL reading level in the context of other aspects of your cardiovascular health and other risk factors. Medical conditions that increase your risk include high blood pressure (120/80 mm Hg or higher) or being on med- ication for high blood pressure, other vascular disease, type 2 diabetes (fasting blood glucose of

    •126 mg/dL or higher), or having had a heart attack. Other risk factors are:
    • Smoking cigarettes or long-term exposure to second-hand smoke
    • An HDL reading of less than 40 mg/dL

    • A family history of early heart disease (heart disease in your father or brother before age 55 or in your mother or sister before age 65)
    • Age (45 years or older if you are a man; 55 years or older or menopausal if you are a woman)
    • Menopause, at any age
    • A sedentary lifestyle

    To lower your LDL reading, a diet low in saturated fat, trans fat, and cholesterol and high in ?ber is your ?rst step (see below and pages
    30–32). You also need to lose weight if you are overweight, and get more exercise. If your level of LDL is high, however, drug therapy may be started while you work on lifestyle changes. Lifestyle measures will go a long way to improve your LDL level and condition your heart and blood vessels. But if they do not bring your reading down to your tar- get, your doctor may prescribe medication (see box page 244). A com- bination of cholesterol-lowering drugs and lifestyle changes will bring LDL levels down in most people.

    Triglycerides

    A high triglyceride level may contribute to your risk of developing heart disease, but it is not clear to what degree high triglycerides alone are a risk factor. Doctors do know, however, that a combination of high LDL level and high triglyceride level raises the risk of a heart attack to a greater extent than either one does on its own. People with high triglycerides are often obese or have low levels of HDL, high blood pressure, or diabetes. Extremely high triglyc-
    erides (more than 500 mg/dL) can lead to a life threatening in?ammation of the pancreas called pancreatitis.
    If you have an elevated triglyceride reading, you will benefit from staying at a healthful weight, eating a diet low in saturated fat and trans fat, limiting intake of sugar and other car- bohydrates, drinking in moderation if at all (see page 97), and exercising regularly. You should also have a fasting blood sugar test to monitor for early signs of diabetes. Elevated triglycerides can be a sign of meta- bolic syndrome, also called insulin resistance syndrome.

  • Testing Cholesterol Levels

    High cholesterol does not cause any symptoms, so people can have excessively high levels without knowing it. High blood cholesterol (de?ned as a level of 240 mg/dL or higher) is among the most impor- tant risk factors for developing heart disease. In countries such as Japan, which until recently had cholesterol levels averaging only 150 mg/dL, heart disease has been very rare.
    All adults over the age of 20 should have their cholesterol measured at least once every 5 years with a blood test called a full lipoprotein pro- ?le. Children in families with premature heart disease may be screened starting at age 2. A full lipid pro?le measures not only the total choles- terol but also HDL and triglycerides. Your doctor looks at all of these numbers, as well as your other risk factors for heart
    disease, and then can use a risk assessment tool to estimate your chances of having a heart attack in the next 10 years. Knowing your risk enables you to take steps to improve your cardiovascular health and lower your chances of heart disease and stroke.
    To have your cholesterol tested, you need to go to your doctor or a medical laboratory. The results from tests performed at shopping centers or health fairs are not as reliable as having your blood sample analyzed at an approved labora- tory. Reliable testing requires a fasting blood sample. A nonfasting sample does not allow an accurate determination of LDL, which is the most important indicator of your heart attack risk. You should do the following to prepare for the blood test:
    • For 10 to 12 hours before testing (often overnight), you may not eat or drink any- thing except water.
    • You can eat as you usually do until 10 to 12 hours before the test.
    • You might be asked not to drink any alcohol for several days before testing, because alco- hol can affect triglyceride levels.

    Home Cholesterol Testing

    You may see home cholesterol testing kits for sale in some drugstores. Some measure only total cholesterol, or only total cholesterol and HDL. At least one is designed to test HDL, LDL, triglycerides, and total cholesterol. There is no harm in trying one of these devices, or having your choles- terol checked at a health fair, but these tests do not take the place of a laboratory-analyzed lipoprotein pro- file (which requires you to fast before the test).
    If you try one of these methods and the results indicate that your total cholesterol is 200 mg/dL or more, fol- low up with a full profile done by trained professionals.
    Most laboratories do not measure your LDL directly but calculate it by subtracting your HDL from your total cholesterol level, then subtracting one-fifth of your triglyceride level. This figure is your LDL. In cases of markedly elevated triglycerides, direct testing of LDL is needed.

    • If you are sick on the appointed day, the test should be rescheduled.
    • Before the day of the test, check with your doctor as to which of your regular medications, if any, you should take at home in the morning.

  • Managing Your Cholesterol Level

    Cholesterol, also called blood cholesterol, is a natural waxy sub- stance that occurs in all your body cells. It is one of several types
    of fats (lipids) that circulate in your bloodstream. Your body uses it to form cell membranes and to make certain hormones, and therefore at healthy levels it is an essential component of cells and blood. Your liver makes as much cholesterol as your body needs—about 1,000 milligrams (mg) per day.
    You consume cholesterol (dietary cholesterol) in the foods you eat, especially animal products such as meats, eggs, and dairy foods. If cho- lesterol levels are high, cholesterol can be deposited in the blood vessel walls as a major component of plaque (a fatty deposit). A buildup of plaque restricts the blood ?ow, a process called atherosclerosis, and puts you at greatly increased risk of heart disease.
    Triglycerides are the true blood fat that exists in the body as well as in food. Triglycerides circulate in your blood along with cholesterol and are fuel for your body’s energy production. Together, cholesterol and triglycerides in the blood are called plasma lipids. Excess calories that you consume in foods are converted to triglycerides and are carried to fat cells for storage. In between meals, hormones control the release of triglycerides to meet your body’s need for energy. Like cholesterol, triglycerides can build up in the blood and contribute to atherosclerosis.

    Because neither cholesterol nor triglycerides can dissolve in the blood, they have to be moved to and from cells by carriers called lipoproteins. The two most common types, the ones you hear the most about, are low-density lipoproteins (LDL), the “bad cholesterol,” and high-density lipoproteins (HDL), the “good cholesterol.” LDLs have a lower density of protein (about 25 percent) and more cholesterol. HDLs have a higher density of protein (about 50 percent) and less cholesterol.
    If too much LDL accumulates in your blood, it causes fatty plaque to collect on your arterial walls, and the process of atherosclerosis begins. If the buildup reduces blood ?ow in the arteries that supply your heart, you may experience the chest pains known as angina. If a blood clot forms near the plaque and stops the blood ?ow, you have a heart attack. If a clot blocks blood ?ow to your brain, you have a stroke. Your LDL levels increase when you consume foods that contain lots of fat, choles- terol, or both. Foods rich in saturated fats and trans fats, such as butter fat, cheese, red meat, processed meats, and bakery goods, are the most harmful, along with tropical oils found in some products like crackers (see also pages 27–29).
    High-density lipoproteins carry cholesterol back from your cells to your liver, where it can be passed out of the body. Most doctors think that HDL can actually slow down the development of plaque by remov- ing cholesterol from it. A high level of HDL seems to protect against heart attack, and a low level places you at greater risk of heart attack and stroke.

    Cholesterol transport
    5Your liver produces blood cholesterol, a waxy substance that your body uses to build cells and make hormones, and you consume additional cholesterol in foods. Cholesterol is trans- ported to cells through the bloodstream on carrier particles called lipoproteins. Low-density lipoprotein (LDL) consists of
    less protein and more cholesterol, and it can be harmful at high levels when it builds up on the walls of your arteries as plaque. Very low-density lipoprotein (VLDL) is another form of choles- terol that can convert to LDL and cause problems. High-density lipoprotein (HDL; shown as smaller units), which consists of more protein and less cholesterol, can absorb some of the cho- lesterol in plaque and return it to the liver to be excreted. So you need to try to reach a high enough level for HDL in your bloodstream to help protect against heart attack and stroke.

  • Risk Factors for Heart Disease

    Some risk factors for heart disease are within your control, while others are not. The number of risk factors that affect you may change over the course of your lifetime. Having one or more of the major, proven risk factors doesn’t mean that you will develop cardiovascular disease or die of it. But generally, the more of these factors that apply to you, the more likely you are to develop the disease at some point. By knowing your own constellation of risk factors, you can control as many as possible and reduce your risk. These are the factors you can’t control:

    •Gender. Men are more likely than women to have a heart attack at a younger age. Women are generally protected from heart disease by their sex hormones until menopause. Cardiovasculardisease is still the leading cause of death for women, however. After menopause, a woman’s risk of heart disease starts to rise. After the age of 65, a woman’s chance of having coronary artery disease is about the same as a man’s, and after 75, a woman is at even greater risk than a man is.
    • Increasing age. Your risk of disease increases as you grow older.
    More than 80 percent of people who die from heart disease are over 65. As you age, your heart’s function tends to weaken. The heart is less able to pump blood, the walls of the heart may thicken, and the walls of the arteries may stiffen and narrow. In addition to atherosclerosis, other conditions such as hypertension may compound the problem. Clearly this process is affected by lifestyle, including diet and exercise.
    • Heredity. Cardiovascular disease runs in families, and you are more likely to develop it if your parents or siblings have coronary artery disease. Increased risk is linked to a family history of death from heart disease at a young age. Speci?cally, this is de?ned as coronary artery disease in men before age 55 and women before age 65. Your racial or ethnic background is another aspect of your heredity. In the United States, blacks are at higher risk than whites, in part because of higher rates of high blood pressure. The risk of heart disease is also somewhat higher in Mexican Americans and native Hawaiians. You can’t change your heredity, but it gives you strong motivation to manage other factors that you can change.
    The major proven risk factors for heart disease that you can modify, control, or treat are:
    • High blood cholesterol. High blood cholesterol directly increases your risk of heart disease. Cholesterol is a fatlike sub- stance that is carried in your blood, but excess cholesterol enters your body through foods derived from animals (meat, eggs, dairy products).
    • High blood pressure (hypertension). High blood pressure increases your risk of several forms of cardiovascular disease: coro- nary artery disease, heart attack, kidney failure, congestive heart failure, and stroke. Other factors, such as obesity, alcohol abuse, unhealthy diet, or physical inactivity can contribute to high blood pressure, but you can also have it independent of those other in?u- ences. (See chapter 3.)

    Obesity and overweight. Excess body fat contributes to the risk of heart dis- ease, independent of other risk factors, because it increases the heart’s work- load. It also raises blood pressure, adversely affects cholesterol levels, and contributes to the development of dia- betes.
    • Physical inactivity. An inactive lifestyle increases the risk of becoming overweight and developing high blood cholesterol levels, high blood pressure, and diabetes. Even moderate amounts of regular exercise will lower your risk of heart disease.
    • Type 2 diabetes. Having diabetes puts you at serious risk; about 65 to 75 per- cent of people with diabetes die from some form of cardiovascular disease. Controlling your diabetes may help control your risk of heart problems.
    • Smoking. If you smoke, you are more likely to develop cardiovascular disease than a nonsmoker is—in addition to the

    Heart Disease and Genetics

    Because heart disease tends to run in fami- lies, having parents or siblings with the dis- ease is a major risk factor. But there is no single gene for cardiovascular disease; in fact, geneticists think that more than a thousand separate genes may influence the overall cardiovascular system. There are separate genes for obesity, high blood pressure, and diabetes, all risk factors for heart disease. Scientists are still identifying these genes and studying how they interact with one another—and with other influences such as diet—in an individual or a family. Many geneticists believe that one of the most effec- tive approaches for a person at high genetic risk of heart disease is to ensure that the per- son follows a healthy lifestyle.
    Other avenues of research include devel- oping drugs that target a specific genetic pre- disposition, along with developing genetic tests that can screen for high-risk patients. The ultimate implications of genetic research for testing and treatment of heart disease are still far in the future.

    risk of lung cancer. Smoking increases your heart rate, constricts your arteries and contributes to their obstruction with plaque, and can cause irregular heartbeat. It also increases your risk of blood clots, which cause heart attack or stroke. Even exposure to other people’s smoke increases a person’s risk of heart disease.
    • Early menopause. Women who have early menopause, whether naturally or as a result of surgery, have a higher risk of coronary artery disease.
    Other in?uences, called contributing factors, are linked to heart dis- ease, but their signi?cance is not fully understood or measured yet. These factors are:
    • Stress. Stress, particularly in some people, appears to increase the risk of heart problems, perhaps because it raises your heart rate and blood pressure, damaging your arteries over time. It may also

    contribute to other harmful behaviors such as overeating, smok- ing, or drinking too much.
    • Alcohol. Drinking more than a moderate amount of alcohol can raise blood pressure, negatively affect cholesterol and triglyceride (blood fats) levels, and cause irregular heartbeats. However, mod- est amounts of alcohol may actually reduce the risk of heart dis- ease. Since so many Americans drink to excess, doctors are reluctant to recommend moderate drinking to improve heart health, for fear that “moderate” usage will change to “excessive” use. Alcohol, whether wine, beer, or liquor, but only in moderate amounts, may be helpful to your health.
    • Birth control pills. If you smoke or have high blood pressure, and especially if you are over 35, birth control pills may increase your risk of heart disease. Today’s birth control pills contain much lower levels of hormones than early ones and are generally consid- ered safe, independent of other risk factors. You should not smoke and take birth control pills, especially over age 35, due to the increased risk of heart attack and blood clots.

  • Preventing Heart Disease

    Cardiovascular disease is the leading cause of death of men and women in the United States. Cancer, the second most common killer, accounts for the deaths of only half as many people. Heart and blood vessel dis- ease takes many forms: high blood pressure, coronary artery disease, valvular heart disease, congestive heart failure, atherosclerosis, and stroke. Because of the enormous toll that the burden of these diseases has taken on the nation’s health, extensive research has focused on preventing these problems. Over a period of decades, numerous studies involving hundreds of thousands of people have identi?ed the major risk factors that indicate an individual’s chances of developing cardiovascu- lar disease. Understanding these risk factors and how you can control them gives you a good chance to prevent or modify heart disease in your own body. Even though cardiovascular disease is still a major threat, the death rates today are substantially lower than they were because so many people have been able to make effective changes in their lifestyle that prevent the development or the worsening of the disease.
    These preventive changes—including how we eat, how physically active we are, and how we approach risky habits like smoking or drinking—make common sense in part because of the nature of heart disease and its treatment. Cardiovascular disease develops slowly and often without symptoms. Factors such as cholesterol buildup or rising blood pressure can start in childhood but may not become apparent as disease for decades, so prevention is the best answer.
    About half the deaths from heart disease are sudden—an unexpected fatal occurrence that leaves little opportunity for intervention. Many treatments—for instance, the coronary artery bypass procedures that have become so common—can have side effects and are inappropriate to perform on every person at risk. Other technologies, such as balloon angioplasty or drugs, can treat a problem, but they cannot stop the underlying disease process.
    Most positively, the picture that emerges from decades of research is that the healthy lifestyle choices that prevent heart disease also reduce the risk of other major diseases such as cancer and diabetes.

  • The Heart and Other Body Systems

    Your heart beats and your blood circulates with little or no conscious awareness on your part. Even though circulation is an involuntary function, it is a dynamic one. Your cardiovascular system is constantly adjusting to changes in the external environment or to demands you place on it. It adapts quickly, or directs other systems to adapt to chang- ing conditions in order to maintain a constant ?ow of blood to body tis- sues. Even the simple act of standing up requires increased blood ?ow to the legs, because the heart must work harder to counteract the effects of gravity. This means that either blood ?ow to other parts of the body must be decreased or the heart must pump blood faster or in greater volume to accommodate the activity.
    The two main systems that help regulate cardiac function are ?rst, the brain and the nervous system, and second, the kidneys.

    The Brain and the Nervous System

    Nervous system receptors throughout your body constantly gather information about factors such as stretching of the arterial walls or the amount of oxygen in the blood. This information is relayed to the brain by chemicals called neurotransmitters. In the brain stem, at the base of the brain, regulatory centers involved with automatic body functions including heart rate, blood pressure, and respiration receive the mes- sages and formulate a response. Neurotransmitters such as adrenaline carry messages back that direct a response in the target tissue, such as commands to constrict the blood vessels or increase the rate of respira- tion to deliver more oxygen to your lungs.

    The Kidneys

    The kidneys in?uence the volume of ?uids in the body, so they can change the volume of circulating blood. In this way, they signi?cantly affect blood pressure. They release enzymes that can raise blood pres- sure by constricting blood vessels, raising sodium levels, and increasing water retention. The kidneys can adapt to changing environmental conditions by, for instance, concentrating your urine if your body is dehydrated. If, on the other hand, you eat a lot of salty foods and start to retain water, your kidneys will produce less urine.

  • The Lungs and the Respiratory System

    The story of oxygen transport to body cells is not complete without a look at the respiratory system, which brings oxygen from the air into the body, transfers it to the blood, and then rids the body of the waste products of cellular energy. When you breathe, the organs of your res- piratory system perform the physical job of bringing air into the body and expelling it. The same organs are the site of the more complex biochemical process of respiration, the oxygenation of blood at a cellu- lar level.
    When you inhale air, it passes down your trachea, into the tubular bronchi that branch into your lungs, and through a system of subdivid- ing air passages that end deep in lung tissue as microscopic tubes called bronchioles. The bronchioles open into tiny, elastic air sacs called alveoli.
    Parallel to these branching air passages, a network of blood vessels brings blood into lung tissue. Minute capillaries cover the surface of the alveoli, and through the walls of these capillaries oxygen passes from the air sacs into the blood. Carbon dioxide molecules, carried in the blood from body tissues, pass into the alveoli. The oxygen-laden blood ?ows back into the heart, where it then can be circulated throughout the body, while the carbon dioxide moves back through the lungs to be exhaled.

    4

    When you inhale, you bring air into your lungs via the trachea, or windpipe. In the lungs, branching air passages (bronchi) end deep in lung tissue in microscopic clusters of air sacs called alveoli. In these clusters, networks of tiny blood vessels (capillaries) cover the air sacs. Oxygen exchange takes place through the walls of the alveoli and capillaries, as oxygen passes from lung tissue into the bloodstream and waste products (such as carbon dioxide) pass from the bloodstream into the lungs to be exhaled out of your body.

  • The Circulatory System

    Your systemic circulation is the vast highway system that carries blood from your heart to every part of your body, and then returns it to the heart. The vessels that carry blood away from the heart are the arteries; the vessels that carry blood back to the heart are veins. Like a system of roads, your circulatory system keeps branching off into successively smaller vessels that carry blood to and from the smallest structures and ?nally individual cells in body tissues. At a cellular level, single red blood cells exchange oxygen and nutrients with single body cells through the walls of microscopic capillaries.

    The Arteries and the Capillaries

    The aorta, the largest artery in your body, emerges from the left side of your heart. About 1 inch in diameter, it ascends from your left ventri- cle engorged with oxygen-rich blood, then arches down the chest into the abdomen. Major arteries branch off it to supply different areas of your body. The carotid and vertebral arteries travel to your head and neck. The subclavian arteries supply the arms. The abdominal (descending) aorta provides branches to your stomach, liver, kidneys, and intestinal tract. The aorta then divides into the iliac arteries and then the femoral arteries of the legs.
    The pulmonary artery carries blood from your heart to your lungs. Exiting from your right ventricle, it transports oxygen-depleted blood into your lungs to replenish the oxygen. This pulmonary circulation functions similarly to your systemic circulation but is limited to the lungs, where oxygen exchange occurs at a cellular level.
    The arteries subdivide into smaller vessels called arterioles. The arteries and arterioles have flexible muscular walls that can dilate (widen) and contract, with a critical impact on directing blood ?ow. Blood ?ows more easily to areas where there is less resistance, so arter- ies that widen increase the circulation to that area, while a constricted artery reduces blood ?ow. Branching off from the arterioles are the smallest vessels, the capillaries. Most capillary walls are only one cell thick. Specialized capillaries in different types of body tissue allow the passage of different types of molecules through their walls. In the lungs, for example, molecules of carbon dioxide (a waste product) pass into the tissue to be breathed out, while molecules of oxygen pass into the blood cells. In your intestinal system, nutrients from digested food pass through the capillary walls into the blood.

    The Veins

    At the level of individual cells throughout your body, the capillaries receive spent blood from body tissue that has a lower level of oxygen. The capillaries ?ow into larger vessels called venules, which converge and form still larger veins. The pressure in veins is signi?cantly lower than the pressure in arteries, and the walls are thinner, which is why blood samples are typically taken from a vein. As with arteries, the walls of veins can expand or contract. Any tensing of your muscles squeezes the veins, helping to counteract gravity and keep blood ?owing toward your heart. Larger veins also have a system of one-way valves that keep the returning blood ?owing the right way.
    Venous blood from the body enters the heart via two major vessels: the superior vena cava, bringing blood from the upper part of the body, and the inferior vena cava, returning blood from the lower part. These large veins enter the right atrium, where the blood is sent into the pul- monary circulation for oxygen pickup.

    Blood

    Blood is the ?uid vehicle by which oxygen, enzymes (proteins that pro- mote body processes), and other life-sustaining nutrients are brought to body cells in order to maintain an optimal environment for growth. Blood is composed of specialized blood cells—red blood cells, white blood cells, and platelets—and of plasma, the ?uid in which the blood cells are suspended.
    The vast majority of blood cells are red blood cells, also called ery- throcytes or red corpuscles, which do the work of oxygen transport. An individual red blood cell is saucer-shaped to maximize its surface area for ef?cient oxygen exchange. Chemically, a red blood cell contains large quantities of hemoglobin, an iron-rich protein that is the body’s oxygen transport carrier molecule. As red blood cells travel through the lungs, where oxygen levels are high, the hemoglobin readily combines with oxygen. When the blood cells reach body tissues where oxygen levels are relatively low, the hemoglobin just as effectively releases oxy- gen. The red blood cells also pick up the waste product carbon dioxide and carry it back to the lungs, where it is released and then exhaled out of the body. Red blood cells are formed in the bone marrow at the rate of about 8 million a second, or many billion in a single day. They live from 3 to 4 months.
    White blood cells, or leukocytes, play a critical role in protecting the body against infection. One type of white blood cell, called a lympho- cyte, identi?es invading microorganisms or other harmful substances in the body and triggers the body’s immune response. The number of white blood cells increases when your body is ?ghting infection. Also suspended in the plasma are cell fragments called platelets, which initi- ate a blood-clotting response when you are injured or a blood vessel is damaged. White blood cells and platelets make up about 1 to 2 percent of blood volume.
    About 55 percent of the blood volume is plasma, a yellowish, watery substance that contains proteins, glucose (sugar), cholesterol, and other components. Proteins in the plasma perform varied roles such as carrying nutrients, contributing to the clotting factor, and acting as infection-?ghting antibodies in an immune response.

  • Your Heart’s Performance

    Both the rate at which your heart beats and the volume of blood your heart moves in a single beat determine how ef?ciently your heart pumps blood. Cardiologists calculate cardiac output to measure your heart’s
    ef?ciency. Cardiac output is, quite simply, the amount of blood your heart pumps through your circulatory system in one minute. It is calcu- lated by multiplying how much blood the left ventricle squeezes out in a single contraction (stroke volume) by the number of times the heart contracts in a minute (heart rate).
    Most typically, when your body needs more blood (for instance, when you are running up stairs) the heart increases its output by beat- ing faster. If your heart beats at a fast rate for very long, the muscle begins to tire and the resting phase of the heartbeat becomes too short for the chambers to ?ll adequately. If you are physically ?t, your heart muscle is stronger and can pump more blood with each contraction. That is, your stroke volume is higher, so your heart can deliver adequate blood to your body without tiring as quickly. A physically ?t person may actually have a low resting heart rate, because he or she has strength- ened the heart muscle so that it can pump more blood, delivering adequate oxygen to the body with fewer strokes. When a ?t person exercises, he or she may have the same heart rate as someone who is less ?t, but the ?t person is able to do more work, such as run longer with- out tiring.
    A healthy resting heart rate is usually between 50 and 75 beats per minute. When you exercise, your heart rate may increase to as much as
    165 beats or more. Age plays a role in determining your maximum heart rate; the maximum number of beats per minute can be very roughly predicted by the formula 220 minus your age. A number of other factors can cause your heart rate to increase, including stress, some medica- tions, caffeine, alcohol, and tobacco. When a healthy person sleeps, his or her heart rate may dip to as low as 40 beats per minute. As you age, your heart rate may decrease somewhat.
    Stroke volume in most people is about 3 ounces. That means that the ventricles pump out about half the blood they contain. A good athlete may be able to increase his or her stroke volume by 5 percent or more. A diminishing stroke volume is one of the ?rst signs of a fail- ing heart.
    A pregnant woman’s body demands more blood ?ow and oxygen for the developing placenta. Stroke volume increases early in preg- nancy, and later the heart rate increases to maintain a cardiac output 40 to 50 percent above normal. These changes reverse after the baby is delivered.