Figure 4-1
Figure 4-2
Until William Harvey, an English doctor who lived in the 17th century, it was thought that blood did not circulate in the body, and that, therefore, the heart was not a pump. There was a strong church bias against anyone who disputed Galen's (first century a.d. - Greek, I think) theory that blood originated in the liver, and filled the body, much like a container of wine. Anyone disputing those views was considered to be a heretic. Harvey postulated, and eventually proved, that blood circulates through the body as a result of the heart's pumping action. What prompted him to start his investigation into this issue is interesting, at least to me. He was about to start an autopsy on a man just brought down from the gallows. Just before he made his first incision, he discovered that the man was still alive - arisen from the dead, as it were. His pondering how a man declared dead could come back to life turned out to be the beginning of his investigation of blood circulation. This is a brief synopsis of Harvey's story, and some of the details may have been confused, but are essentially correct.
At any rate, a rehash of anatomy history isn't the goal here, so let's get to the point.
The heart is a fist-sized organ located in the chest cavity between the lungs, on top of the diaphragm (Figure 4-1), and behind the sternum and anterior ribs. It is slightly angled, so that its somewhat pointed apex sticks out from behind the sternum on the left side. (This seems kind of backwards, since I have always thought of an apex as the top, but the heart's apex is at its bottom (maybe because it is pointed). The heart is a four-chambered pump constructed of cardiac muscle (smooth, not striated like skeletal muscle).
The heart is covered by a fibrous layer called the pericardium, or pericardial sac. Its major job is to anchor the heart in place. At the bottom of the pericardium is a viscous layer that functions to reduce friction between the beating heart, and the outer wall of the pericardial sac.
The pericardium is subject to a couple of diseases: 1) pericarditis, where the sac becomes infected and inflamed, which can result in roughening of the viscous lining, which can lead to pain or fluid build-up, which can compress the heart and limit its expansion and the strength of the ensuing heart beats; and 2) cardiac tamponade, which is, in essence, a 'heart plug' caused by excess fluid. It is treated by stabbing into the heart to drain the fluid.
Figure 4-2 shows the four chambers of the heart: the right and left atria, and the right and left ventricles. Circulation of the blood the heart follows this path: dirty (deoxygenated) blood flows from the body through the superior vena cava (from above the heart) and the inferior vena cava (from below the heart). This 'dirty' blood travels into the right atria, and then into the right ventricle, from whence it is pumped through the Pulmonary trunk into the lungs to be reoxygenated. From the lungs, reoxygenated blood returns through the pulmonary veins to the heart's left atria. The reoxygenated blood is then pumped into the left ventricle, and then out through the aorta to the rest of the body . The left ventricle is the largest and strongest of the four chambers. The thicker and stronger muscle is necessary for the strong propulsion of "clean" blood out to the rest of the body. The preceding description of the flow of blood through the heart is necessarily serial, although in reality, the chambers are pumped in pairs - atria, then ventricles.
Figure 4-2
The AV valves are, obviously, between the atria and ventricles: the right AV valve, or cuspid; and the left AV valve, or bicuspid (sometimes called the mitral valve). The purpose of these valves is to keep blood from flowing backward into the atria when the ventricles contract. When the ventricles fill, they force the valves to close to prevent backflow. Cords, called Chordae tendinae, are anchored to the papillary muscles on the floor of the ventricles and are attached to the underside of the AV valves. They prevent the pressure from filling ventricles from causing the valves from collapsing back up into the atria, which would facilitate backflow. The cordae tendinae are, in effect, 'guy wires' that keep the valves flat (and closed). At the appropriate time, the papillary muscles contract to pull the guy wires taut. These muscles don't have any effect on the valves except to keep them from collapsing up into the atria.
The semilunar valves prevent backflow from the ventricles into the aorta or pulmonary vein.
A word about blood pressure (BP) is appropriate now that the pumping action through the heart's chambers has been described. When we go to a doctor's office, the first thing that usually happens is that a nurse scurries out and checks your blood pressure - expressed as xxx/yy, where xxx = systolic pressure, and yy = diastolic pressure. Systolic pressure is measured as arterial pressure generated as blood is forced by left ventricular contraction out through the aorta and into the rest of the body. Diastolic pressure is the pressure remaining as the left ventricle relaxes. Average pressure for a young adult is about 120/80. Blood pressures typically vary all day long depending on things like activity level and psychological stress, so a resting pulse is taken. Some people feel enough stress from a visit to a doctor's office that BP needs to be checked two or more times to get a true resting BP.
The conducting system of the heart keeps it beating independent of the nervous system. There are certain cardiac muscle cells that maintain the heartbeat. The components of the heart's conducting system are the sinoatrial (SA) node, the atrioventricular (AV) node, and the atrioventricular bundles (the right and left bundle branches), and the Purkinje fibers.
The SA node, as shown in Figure 4-3, sets a general heart rate (HR) of about 70-80 beats per minute. This rate is variable, and is often much lower in very aerobically fit individuals; however lower or higher HRs may indicate a problem. The signals go quickly from the SA node to the atria to effect simultaneous contractions of both atria. The signals are stopped from moving directly to the ventricles by nonconducting tissue. The signals are held until the atria complete their contraction, and are then picked up by the AV node and shipped through a series of conducting fibers (AV bundles and Purkinje fibers) into the walls of the ventricles (begin-ning at the heart's apex to ensure a complete pump), and finally resulting in contraction of the ventricles and ejection of blood through the aorta and out to the rest of the body.
There is a condition called heart block, which is when the AV node is somehow damaged, and its signals don't get into the ventricle walls. The ventricles will then beat at their own rate, which is slower than that of the atria, and disrupts normal circulation. This condition is typically rectified by the insertion of an artificial pacemaker.
As long as we are in the heart's electrical system, we should mention a diagnostic aid used as part of determining the condition of a person's heart - the electrocardiogram (ECG, previously called an EKG).
Anyone who has had an ECG or has seen a TV medical show, like ER, has seen those squiggly lines (Figure 4-4) on what looks like a computer monitor, and would probably be interested in knowing what they are, so here is the answer to your prayers. An ECG picks up the electrical waves generated in the heart's conduction system. Of principal interest is the PQRST segment of the wave. The P wave shows the con traction of the atria. The QRS waves show depolarization (like cocking a gun for the next shot), and ventricular contraction (R). The ST segment is important in that it shows a period of recovery and refilling of the atria (from the vena cavae and pulmonary veins) - 'reloading' for the next atrial contraction. Figure 4-4 shows a normal healthy heart rhythm. A gob (plethora) of heart problems can be discovered by analyzing an ECG readout (e.g., arrhythmias, heart blocks, irregular beats, premature contractions, and more).
The heart's own blood supply comes from the left and right coronary arteries which originate at the base of the aorta. These two arteries (some people only have one) branch into many vessels that feed the entire heart. Cardiac veins carry 'dirty' blood to the right atrium and ultimately to the lungs for reoxygenation and return to the heart through the coronary veins - back to the left atrium for ejection into the aorta, where the cycle continues.
Figure 4-4
From the Cooper Institute for Aerobics Research, in Dallas, TX comes a report that fat people gain benefit from working out, even if no weight is actually lost. Exercise can still extend your life, but the Institute quickly added that remaining obese is not healthy, exercising (i.e.; getting fit) and losing weight is still a better option than just exercising but not losing weight. According to this particular study (of 25,000 men) fitness had more to do with mortality rate than body weight.
R,R., Dallas, TX
Here is a household tip that you may find useful. Hanging pest strips may be dangerous, some of them contain a known carcinogen that is constantly released into the air.
H.H., SF, CA
There is a new fake-fat substance which has recently approved by the FDA. It is called "olestra," and tastes and acts ver much like fat, but is calorie-free. This product has been around for 25 years and something like 200 million dollars has been spent on its development and promotion, but it has taken until now to get FDA approval.
Approval seems to have been based on the fact that no conclusive evidence has been shown that olestra causes any health problems, despite some reported problems; one of which may be important. That is that olestra prevents absorption of some nutrients - specifically, the fat-soluble vitamins (A, D, E, and K). So the developer, Procter & Gamble, stuffed those vitamins into olestra to compensate; but, carotenoids, like Beta-caratine, are also rushed through the system without being absorbed - with unknown but probable risks. Carotenoids are thought to be important to health, but only now are being studied in an effort to determine exactly what they do.
Also, a number of GI problems have been reported, including "anal leakage," a none too pleasant sounding happening. These pleasantries are basically being ignored, apparently because there have been no proven medically harmful effects (like a few deaths).
I don't think I want to touch this stuff with a 10-foot pole. It may not cause any medical problems, but will likely add to the growing obesity of the U.S. population. We have had an abundance of fat-free and low-fat products for years, but people have only gotten fatter over that same time. Specu-lation is that fat-free or low-fat somehow equates to calorie-free in many minds; but it doesn't, and sometimes is even higher in calories, while being lower in fat. Olestra will most likely add to this problem, and with a big unknown attached. My choice is to avoid it when-ever possible.
M.A., Sunnyvale, CA
It was once reported in this Newsletter that 'walking' may be one of the best exercises. I just ran across some support for that theory in the December '95 issue of the Penn State Sports Medicine Newsletter. It mentioned that walking can improve cardiovascular fitness. It is possible to reach your target heartrate zone, but you must use "fast fitness walking" - strolling around the block won't help much, but you have to start somewhere. Other well-known walking attributes were mentioned, like little risk of injury, weight control, and increased bone density. To in-crease intensity, and benefits, walk hills, or on an incline treadmill. It was also su-ggested that cross-training (e.g., walk and run, or weight train, or swim - mix activities) is particularly useful - different muscles and movements are used.
A.L., Los Altos, CA
Of interest to arthritis suf-ferers is some information on "capsaicin" creams (like Zostrix, Capzasin-P, or Capsin) and its effectiveness. Capsaisin is partly a counter-irritant, partly as a pain-impulse suppressant. Not many studies have been done on these products, so their long-term effects are unknown. Some people are helped by it, but some find that it irritates their skin, and its use should be stopped if that should happen. If you choose to try it, you may have to ask your pharmacist for it because it often is not prominently displayed even though these are OTC (over-the-counter) products.
L.A.M., Mtn Vw, CA
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