Articles on Medical Diseases and Conditions

Month: July 2009

  • 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.

  • Coronary Circulation

    Because your heart must operate continuously to supply the rest of your body with blood, it works harder and requires a richer blood supply of its own than any other muscle in your body. It cannot extract oxygen and nutrients from the blood that moves through it, so it maintains its own dedicated circulatory system of arteries and veins. This coronary circulation begins with two coronary arteries that branch off of the aorta just above the aortic valve (on the left side). These arteries extend over the surface of the heart and branch into smaller vessels that pene- trate the heart muscles to provide oxygen. After the muscles of the heart have been nourished, the blood travels through coronary veins into the coronary sinus and then the right atrium. At this point, it ?ows in with the oxygen-depleted blood from the rest of the body.
    The left coronary artery supplies blood to most of the powerful left ventricle. The circum?ex coronary artery is really a branch of the left coronary artery. It wraps around the back of the heart and has several smaller branches. The right coronary artery supplies part of the left ventricle and most of the right ventricle. Interestingly, the con?gura- tion and even the sizes of the coronary arteries differ signi?cantly from person to person.
    The coronary arteries deliver oxygen-rich blood to the cardiac mus- cle cells according to the demand at the moment. If you are exerting yourself physically, your heart beats faster and more vigorously, and your coronary arteries expand to allow greater blood ?ow.

  • The Heart’s Electrical Conduction System

    The electrical activity that stimulates and paces the heartbeat is critical. In order to deliver an appropriate blood supply to body tissues, the heart must beat at an adequate rate, and the timing and sequence of muscular contractions must be precisely coordinated.
    Your heart’s natural pacemaker is the sinoatrial (SA) node, a microscopic group of specialized electrical cells located at the top of the right atrium. Each heartbeat originates in the SA node when it ?res off an electrical impulse. This impulse travels via specialized pathways to the cells in the muscle tissues of the heart wall. The impulse ?rst stim- ulates the upper chambers, the atria, to contract and squeeze blood out into the ventricles.
    Then the impulse moves to another area of electrical cells called the atrioventricular (AV) node, located over the ventricles. This node acts as a relay station, allowing for a brief interval during which the atria empty completely before releasing the impulse along branching pathways that travel to the two ventricles to stimulate ventricular contraction. The ventricles similarly contract and empty, and blood is pumped into the pulmonary artery and the aorta.
    The SA node speeds up when your body needs more blood. It also slows down during rest or in response to some medications. The mes- sage to increase or decrease the rate of impulses is controlled by the autonomic nervous system—the part of the nervous system that con- trols unconscious, automatic body functions including heart rate, blood pressure, and breathing. Autonomic nervous system activity regulates the release of the hormones epinephrine and norepinephrine, which act as accelerators for the heart’s electrical impulses during times of stress or exercise.

    3Your heart’s electrical activity can be followed and recorded on paper as an electrocardiogram (ECG, see pages 122–125). The initial impulse from the SA node is seen as a wave on the ECG, followed by a more static interval. The ECG recording shows spikes as the impulse travels from the AV node through the ventricular pathways and is again fol- lowed by a static interval that is a segment of recovery.

  • The Heartbeat

    The continuous function of your heart is probably easiest to understand if you break it down into a single unit of pumping action, the heartbeat. A healthy heart beats between 50 and 75 times per minute, so a single heartbeat may occur in less than a second. It involves two distinct phases, the systole and the diastole. The systole is the pumping phase and the diastole is the resting phase.
    The systole actually occurs in two sequential pumping actions: the atrial systole and the ventricular systole. The lub-dub that is heard through a stethoscope is the sound of the heart valves closing during the heartbeat’s pumping cycle. The ?rst heart sound, the “lub,” coincides with the closing of the mitral and tricuspid valves. The second heart sound, the “dub,” occurs with the closing of the aortic and pulmonary valves.

    A single heartbeat moves a quantity of blood through the heart in two phases: a resting, or dilating (diastolic), phase and a pumping, or squeezing (systolic), phase. During the diastolic phase, the heart relaxes and fills, as oxygen-depleted blood flows into the right atrium from the body, and oxygen-rich blood flows from the lungs into the left atrium. The ventricles fill partially. Then, during the systolic phase (right), an electrical impulse causes the heart to contract. First, the atria contract and completely fill the ventricles with blood. Then the ven- tricles contract, pumping blood out of the heart.

    The diastole, the ?rst and longer resting phase, occurs as blood col- lects in the two (right and left) atria. In the right atrium, depleted blood enters from the body, and in the left atrium, oxygen-rich blood ?ows in from the lungs.
    Systole begins when an electrical signal from the heart’s pacemaker cells stimulates the atria to contract and empty. The tricuspid and mitral valves open and blood ?ows into the two ventricles.
    When the ventricles are full, the electrical impulse passes into an area just above the ventricles and triggers the ventricular systole, the third and ?nal step. All four valves are in action: the tricuspid and mitral valves close to prevent back?ow from the ventricles to the atria, and the pulmonary and aortic valves are pushed open as blood surges out. On the right side of the heart, oxygen-poor blood travels from the right ventricle into the pulmonary artery on its way to the lungs to acquire oxygen. On the left side of the heart, oxygen-enriched blood ?ows from the left ventricle through the aorta and into the general and coronary circulation.
    After the blood has left the ventricles, they relax, and the pulmonary and aortic valves close. As the ventricles relax, the pressure in the ven- tricles lowers, allowing the tricuspid and mitral valves to open, and the cycle begins again.
    Throughout this cycle, the two adjacent pumps move exactly the same amounts of blood; the volume of blood that enters and leaves the right chambers is the same as the volume that passes through the left chambers. Any change in the amount of blood entering the right side of your heart—in response to exertion, stress, or temperature changes, for example—causes a corresponding change in the amount of blood passing through the left side. Your brain is constantly monitoring the conditions that might require a change in blood supply and adjusting your heart’s function accordingly.

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  • The Heart Valves

    The blood ?ow through the heart needs to be one-way and carefully regulated. Four one-way valves between the chambers ensure that the blood moves through the heart and lungs in sequence and never dams up or back-?ows. All the heart valves are constructed of overlapping ?aps (lea?ets or cusps) that open and close to control blood ?ow. The valves differ by structure and function.
    The pulmonary and aortic valves between a ventricle and the great artery are called the semilunar valves because of their crescent-shaped lea?ets. The tricuspid and mitral valves between the right and left atria and a ventricle are also called atrioventricular valves. The lea?ets of the two atrioventricular valves are attached to the ventricular walls by ?brous cords. When the ventricle contracts and the valve closes, the cords secure the lea?ets in place so they are not blown backward by the force of the contraction.
    The tricuspid valve, on the right side of the heart, is named for its three leaflets, or cusps. Returning, oxygen-depleted blood flows through this valve into the right ventricle.
    As the blood ?ows out of the right ventricle and into the pulmonary circulation, it passes through the pulmonary valve. The pulmonary valve’s three lea?ets open as the right ventricle contracts and close again as it relaxes.
    As the oxygen-enriched blood passes back into the left atrium, it passes through the mitral valve (named for its shape, which resembles a type of bishop’s hat called a miter). This valve has just two highly mobile cusps that can close rapidly when the powerful left ventricle contracts. This valve is attached by cords to muscles within the ventricle.
    Finally, as the blood ?ows out of the left ventricle and into the aorta, the three-part aortic valve opens against the walls of the aorta. When the blood has passed into the aorta, the valve falls shut.

  • Diagnosing a bladder problem in Multiple Sclerosis

    The most helpful information for a doctor or other health professional to assist in diagnosing your problems is a brief history of any bladder symptoms you may have, for example:

    • What is your major concern about your bladder/urination?
    • How often do you urinate during the day/night?
    • Do you leak when you laugh, or cough, or do you have an accident? How often? In what circumstances?
    • Do you find it hard to begin urinating? Do you feel that you empty your bladder?
    • Do you wear pads or protection? If so how often?
    • When and how often have you had kidney infections?
    • Do you have pain on urinating or blood in your urine?
    • Have you had any formal investigations before, or are you taking any medications?

    If responses to these questions suggest the existence of bladder problems, then it is likely that you will asked to take some tests.

    Tests
    Increasingly there are different tests being used to determine more accurately what the exact problem is. Your GP will probably only undertake
    tests for urinary tract infections, and it will be your neurologist who may refer you to specialists, e.g. a urologist, for other tests, if necessary. The two most significant tests assess:

    • urinary tract infection, and
    • control of urinary flow.

    Tests for urinary tract infection. Doctors are recognizing that urinary tract infections are an increasing problem for people with MS and often associated with retention of urine in the bladder. However, it is important that you ask your doctor to undertake such tests regularly. If your doctor suspects that an infection is present, a ‘mid-flow’ sample of your urine is normally requested and, after the specimen has been
    ‘cultured’ to identify the particular bacteria present, you will be prescribed the most appropriate antibiotic.
    Tests for urinary flow. More and more sophisticated tests, known as
    ‘urodynamic’ tests, are being developed to measure ‘urinary flow’. A more recent test investigates this flow and the amount of urine remaining in the bladder after urination by taking a non-invasive ultrasound picture of your bladder. Of particular importance is the measurement of the amount of urine remaining in the bladder after you have urinated – it is this residue that can give rise to infection. This overall test, called an ‘ultrasound cystodynogram’ (USCD), is gradually replacing one that measures the rate of flow or urine by the introduction of a
    ‘catheter’ (a thin tube) through your urethra (the opening in your body from where urination occurs) to your bladder. The remaining urine then flows out and can be measured. To obtain additional information, further ultrasound pictures might be taken of your kidneys. Very occasionally, a far more intrusive investigation – ‘cystometry’ – is performed, usually only in very rare cases indeed, to allow the examination of the inside of your bladder (almost as a final resort after all other methods have been tried with no success), and when surgery is being considered. Surgery is rarely, however, considered for urinary problems in MS, for it is often associated with a range of side effects and difficulties.

  • Bladder control

    This is one of the most difficult issues to deal with in MS, despite being a very common symptom. Research has suggested that between 80–90% of people with MS have urinary problems of some kind, although they vary widely in type and seriousness. More expertise and resources are now being devoted to dealing with it.
    If particular nerves in the spinal cord are damaged by Multiple Sclerosis, then urinary control will be affected. There are several kinds of urinary control in people with MS that might then be affected:

    • They may urinate involuntarily – either just dribbling a little, or sometimes even more (a problem of ‘incontinence’).
    • They may wish to urinate immediately (a problem of ‘urgency’).
    • People may wish to urinate more often than before (a problem of frequency). When people have frequency at night, i.e. needing to urinate several times during the night, it is called ‘nocturia’.
    • They may fail to empty their bladder (a problem of ‘voiding’).
    • They may find it difficult to begin to, or to continue to urinate (a problem of ‘hesitancy’).

    The major bladder problems in Multiple Sclerosis can be summarized as either:

    • a failure to store
    • a failure to empty, or
    • a combination of both.

    In general the more serious the MS, the more serious your urinary symptoms are likely to be. About 65% of people with urinary problems have difficulties with urgency, or frequency and incontinence resulting from urgency. About 25% have difficulties in relation to urine retention and bladder emptying, and the remaining 10% may have both sets of problems.
    Whilst many of the common urinary problems above that people with MS experience are indeed a result of damage to the nervous system caused by the disease, others may be caused by ‘urinary tract infections’. Urinary tract infections are not caused directly by the MS itself, but are more likely in people with MS because of some of its functional effects – for example through infections from a failure to empty the bladder. Thus it is very important that you are regularly tested for urinary infections. This is particularly important if the bladder problems you have are significant.

  • The Heart Chambers

    The heart is constructed of four chambers: the right atrium and the right ventricle, and the left atrium and the left ventricle. These four chambers function as two side-by-side pumps, each of which sends blood through a completely different system of circulation. The right side of the heart pumps blood through the less forceful pulmonary circulation of the lungs, where oxygen-depleted blood is replenished in lung tissue with oxygen from the air we breathe. The left side of the heart pumps blood into the rest of your blood vessels, allowing  nourishing, oxygen-rich blood to leave the heart and travel throughout the rest of the body.
    The atria receive blood from the body on the right side and from the lungs on the left side. The two ventricles are the pumping chambers that expel the blood. The two pumps operate in synchronized fashion, the two atria and then the two ventricles contracting and relaxing simultaneously. The two sides of the heart are separated by a thick mus- cular wall called the septum, which prevents blood from passing directly from one side of the heart to the other. To understand the mechanics of pulmonary (lung) circulation, you can trace the path of about half a cup of blood—the amount pumped in a given heartbeat through the right side of the heart. Blood enters the right atrium of the heart through two large veins: the superior vena cava, which collects blood from your head and upper body, and the inferior vena cava, which collects blood from your legs and abdomen. At this point, the red blood cells in the blood return- ing to the right atrium have delivered oxygen and nutrients to other body tissues. The depleted blood has a low oxygen content. The right atrium contracts and sends the blood through a one-way valve into the right ventri- cle, which in turn contracts and pushes the blood out through the pulmonary artery into the lungs. As the blood circulates through the lungs, it unloads carbon dioxide, a waste prod- uct of cellular function that it has carried from body tissues. The red blood cells then pick up fresh oxygen, and the blood is enriched, or oxygenated.

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    Similarly, on the upper left side of the heart, circulation to the rest of your body starts with the left atrium. Bright red, oxygen-rich blood enters the chamber via the pulmonary vein, from the lungs. The walls of the left atrium contract and push the blood through a one-way valve into the left ventricle. Then the left atrium relaxes while the powerful left ventricle contracts with the considerable force required to propel the blood into the aorta—the major artery at the top of the heart that directs the blood throughout the body. The left ventricle is the main pump and the strongest muscle tissue in the heart.

  • The Structure and Function of the Heart

    In the heart, blood makes a ?gure-eight passage. It enters on the right side through two major veins, moves through the two right chambers, loops back through the lungs to pick up oxygen, then passes back into the left chambers and out through the aorta. The blood ?ow is pro- pelled through the heart and body as the heart’s muscles contract. Flow is directed by the opening and closing of one-way heart valves between the chambers of the heart and the great vessels, or veins and arteries.