THE HEART
Your heart is the most complex of all the organs in your body. While its primary function is to circulate blood through itself and the rest of the body, the heart also acts as an endocrine gland, producing hormones that allow it to regulate the flow of blood. Moreover, the heart is governed by an electrical system that dictates how and when to pump blood in the first place. Clearly, this is not a simple organ.
The heart is a hollow, fist-sized organ located roughly in the center of your chest, between your lungs and surrounded by your ribcage. It is composed almost entirely of cardiac muscle. Cardiac muscle is a special type of muscle tissue that possesses the characteristics of both of the two other main types of muscle that humans have. Like skeletal muscle, which generally controls voluntary movement (walking in a straight line, for example), cardiac muscle is striated (grooved). Like smooth muscle, which is associated with various internal organs, however, cardiac muscle is also considered an involuntary tissue, because it functions automatically, without direction from the central nervous system. Because it is both striated and involuntary, cardiac muscle is incredibly durable and self-regulating—uniquely qualified to keep pumping hour after hour, day after day, year after year.
Figure 3.1. The Heart
The heart is encased in a thin yet tough sac called the pericardium. Filled with fluid, the pericardium’s job is to protect the rest of the heart from trauma and friction from surrounding organs and structures. The heart itself is made up of three layers of muscle tissue: the epicardium on the outside wall, the endocardium on the inside of the heart cavity, and the thickest layer of heart muscle, the myocardium, between them.
The heart has four chambers: two atria (left and right) on top, and two ventricles (left and right) on the bottom. Generally speaking, the atria receive blood and the ventricles discharge it. The heart is further subdivided: The left side of your heart is separated from the right side of your heart by a muscle wall called the septum. Each side uses a special valve to help channel blood between its respective atrium and ventricle—the tricuspid valve regulates the flow of blood between the right atrium and right ventricle and the mitral valve regulates the flow of blood between the left atrium and left ventricle. In addition, there are two other valves that connect each of the two ventricles to their corresponding arteries—the pulmonary valve connects the right ventricle to the pulmonary arteries, and the aortic valve connects the left ventricle to the aorta.
When your heart beats, the “lub-DUB” sound you hear is created by the closing of these two sets of valves. During the first and longer part of the heartbeat, or diastole, the atria and ventricles first relax, filling with blood. After receiving a signal from the SA node, the atria contract and push blood more forcefully into the ventricles through the tricuspid and mitral valves. When the ventricles are completely filled, these valves close to prevent the blood from flowing backwards, creating the “lub” sound. During the second part of the heartbeat, or systole, the ventricles contract, sending blood out of the heart and into the main arteries through the pulmonary and aortic valves; when these valves snap shut, we get the “DUB” sound.
Your Heart Is Also an Endocrine Gland
Most people know that the heart’s main purpose is to pump blood throughout your body. But did you know that the heart can also produce hormones? Over the last fifty years, a growing body of research has shown that the heart acts as an endocrine gland, secreting at least three hormones: atrial natriuretic peptide (ANP), brain natriuretic peptide (BNP), and c-type natriuretic peptide (CNP). Among other functions, these hormones serve to lower blood pressure by maintaining electrolyte balance, dilating (widening) blood vessels, and reducing the volume of blood through them. Although the full significance of these hormones is still unclear, initial evidence indicates that they might have enormous implications for both treating and potentially preventing heart disease.
The discovery of these three hormones has revolutionized the way scientists look at the heart. No longer is the heart considered to be a simple pump; it is, in fact, a complex, powerful, self-regulating machine with many different capacities, not all of which are totally understood.
Figure 3.2. Blood Circulation
This image is a stylized representation of the ways blood circulates in the body. It is important to note that in reality, vein and artery systems overlap and interact.
Your Heart’s Circulatory Function
Each side of the heart performs a distinct circulatory task. The right side is responsible for pulmonary circulation, or the circulation of blood to the lungs: Deoxygenated blood from all areas of the body pours into the right atrium through the vena cavae and is channeled into the right ventricle, which pumps it to the lungs in order to pick up oxygen and drop off carbon dioxide. The left side is responsible for systemic circulation, or the circulation of blood to the rest of the body: After the blood receives oxygen from the lungs, it returns to the left atrium of the heart via the aorta and is channeled through to the left ventricle, which pumps this new oxygen-rich blood to the rest of the body. Two of the most important destinations for this blood are the kidneys, which filter out waste products like urea and salts and excrete them through urine, and the small intestine, where the blood receives vital sugars and other nutrients released by the digestive processes.
Pulmonary and systemic circulation occur simultaneously; with every beat of the heart, blood is pushed forward on its journey through the body. At the same time, a third type of circulation is also taking place. The heart is a voracious producer and consumer of energy; in order to ensure that this hard-working muscle has the oxygen and nutrients it needs to keep pumping blood, our bodies have developed a separate circulatory circuit whose sole purpose is to feed the heart and cart away its waste products. In this process, called coronary circulation, special coronary (heart) arteries that stem from the aorta push oxygen-rich blood into the heart muscle (myocardium) itself. When the heart has used up the oxygen and other nutrients, the depleted blood is then rerouted by special coronary veins to the right atrium of the heart; from there it reenters the pulmonary circuit to retrieve fresh oxygen.
Amazing Heart Facts
Your heart is almost entirely composed of muscle; the amount of work that it performs in a single hour is enough to lift a small car weighing approximately 3,000 pounds one foot off the ground.
The heart beats an average of 103,000 times per day every day of your life. That equates to approximately 37.6 million times a year and 26 billion times over a seventy-year-long lifespan.
The pressure created in your heart during a single heartbeat is enough to propel blood a distance of nearly thirty feet.
The average volume of blood pumped per heartbeat when you are at rest is 2.5 ounces. Over the course of 24 hours, your heart thus moves nearly 2,000 gallons (approximately 20,000 pounds) of blood. That’s 62,000 gallons each month and 744,000 gallons each year!
The heart is the most efficient consumer of oxygen in the body, extracting 70 to 75 percent of available oxygen from blood while other organs (such as the liver) can only obtain about 20 to 45 percent. Because the heart is so good at extracting oxygen from the blood, however, the coronary arteries tend to be fairly narrow, admitting the smallest volume of blood that the heart needs to continue working. And because the coronary arteries are so narrow, blockages to them, as in atherosclerosis, can be particularly dangerous. It is vitally important that the heart receive a continuous supply of oxygen and nutrients; when deprived of these substances, as you saw in Chapter 1, your heart—and you—will quickly die.
But how does the heart know when to beat? What keeps the heart from stopping?
Your Heart’s Electrical System
Although most people understand that the heart’s basic purpose is to circulate blood around the body, few people know that this circulatory function is made possible by the heart’s internal electrical system, or cardiac conduction system. The heart’s electrical system creates signals that
tell the heart when and how to beat; without these electrical impulses, blood wouldn’t be able to circulate at all. Thus the heart is not only a pump, but also the electrical generator that allows it to perform its pumping action in the first place. Producing signals that create their own measurable magnetic field, the heart is quite literally a source of power.
There are three parts to the heart’s electrical system: the sinoatrial (SA) node, located in the right atrium near the entrance of the superior vena cava; the atrioventricular (AV) node, located on the floor of the right atrium near the right ventricle; and the His-Purkinje system, which consists of bunches of fibers dispersed along the ventricle walls. All three components are made up of cardiac pacemaker cells, which are specialized heart cells that are uniquely capable of generating their own electrical signals. Cardiac pacemaker cells operate automatically and independently from the body’s central nervous system. Although your actual heart rate is always being adjusted by the autonomic nervous system (ANS) to meet the body’s particular energy and oxygen requirements, even without any such input from the ANS, your heart’s pacemaker cells can maintain a steady beat. As long as these sophisticated cells are still alive, your heart will continue to beat—even if your brain has died, and even (for a time) if your heart is taken out of your body!
Here’s how the electrical system regulates the heartbeat, or cardiac cycle. The SA node acts as a natural pacemaker, regulating the rhythm of the heart. During diastole (see page 73), the SA node generates and sends out an electrical signal telling the atria to contract, pushing blood through the tricuspid and mitral valves and into the ventricles. The original impulse continues to travel along an electrical pathway, eventually arriving at the AV node. Once there, the signal slows briefly, allowing the right and left ventricles to fill with blood. When this is accomplished, the signal continues on to the His-Purkinje system, where it initiates systole by dispersing throughout the ventricles and stimulating them to contract, pushing blood through the pulmonary and aortic valves and out to the aorta and pulmonary artery. After the ventricles have expelled their blood, they relax, allowing new blood to enter. And then the entire cardiac cycle begins all over again, ensuring that your heart beats 60 to 100 times each minute, every minute, 24 hours a day.
For every heartbeat, therefore, there is first an electrical signal. The specific number of signals—or the number of times the heart beats each minute—depends on the amount of oxygen your cells demand. When your body requires more oxygen, say, because you’re going for a run or dealing with a stressful emotional situation—sophisticated biofeedback mechanisms within your nervous system sense this need and direct the SA node to fire more frequently, causing your heart to beat faster and deliver more oxygen to your cells.
Whether your body is at work or at rest, your heart’s electrical system is constantly producing signals that allow your heart to keep functioning. If the electrical signal is interrupted in any way, the result is an arrhythmia (see pages 11 to 14). The most common way to evaluate the proper functioning of your heart’s electrical system remains the electrocardiogram (EKG), which measures the strength, frequency, and path of your heart’s electrical signals as they travel from the top to the bottom of your heart. Increasingly, other tests are also being used, including the magnetocardiograph, which measures the strength of the magnetic fields created by your heart’s electrical signals.
Now that you have a more general idea of how the heart’s electrical system works, and why it is essential to proper circulation, let’s delve deeper, exploring the way this system works at the cellular level.
How The Cells of Your Heart Muscle Work
Your heart muscle is made up of special cells called cardiomyocytes (literally “heart muscle cells”). As you have read, these cells have two essential functions. One is electrical: to conduct electrical signals initiated by the SA node. The other is mechanical: to contract and relax, helping to push blood through the cardiovascular system. Essentially, the electrical function supports the mechanical function—the electrical signals conducted by your pacemaker cells make it possible to perform the more fundamental task of pumping blood.
How do these functions take place on the cellular level? Think of your heart as a car, with each of its cells acting like a tiny rechargeable battery. In order for a battery to function properly, it needs a constant supply of electricity. This the heart muscle cell gets from four minerals—calcium, magnesium, sodium, and potassium. Each of these minerals carries a specific electrical charge; for this reason, they are also known as electrolytes or ions. These electrolytes exist in a state of flux within your body. Because your body is essentially homeostatic—that is, it regulates itself in order to maintain a certain physiological equilibrium—it uses a system of checks and balances to keep its electrolytes at constant, optimal levels. Thus, the presence of magnesium helps to counterbalance the presence of calcium (and vice versa), and the presence of potassium helps regulate the presence of sodium (and vice versa).
What is Defibrillation?
Anyone who has ever seen a television show set in a hospital is probably familiar with the sequence of events that unfolds when a patient goes into cardiac arrest. Doctors rush in and apply two large paddles to the patient’s bare chest, delivering a shock that dramatically jolts the patient back to life.
In reality, the process of defibrillation is a bit more complicated. Although on television it may look like the defibrillator uses an electrical shock to restart the heart, the opposite is true—the electrical shock actually stops the heart! By stunning the heart, the defibrillator effectively resets the heart’s batteries, allowing the SA node to initiate a new electrical signal from a resting position.
Moreover, because they serve to reset the heart, defibrillators can be used not only to treat cardiac arrest, but also cases of severe or chronic arrhythmia, in which electrical disturbances have caused an abnormal heartbeat. Consequently, there are several different kinds of defibrillators available for use besides the manual external defibrillator you’ve seen on television. There are also automatic external defibrillators (AEDs), which are simpler units that use advanced computing to analyze the heart rhythms of the patient automatically before delivering the shock, and can be used by the public with little to no prior emergency care experience. For patients with chronic life-threatening arrhythmias, a special type of defibrillator called an implantable cardioverter-defibrillator (ICD) can be surgically inserted into the heart, where it can monitor the heartbeat more closely and deliver shocks as needed.
When a cardiomyocyte is at rest, calcium and sodium ions can be found on the outside of the cell, with magnesium and potassium ions on the inside. Because of the differences between the specific concentrations of these electrolytes, an electrical imbalance is built up between the inside and the outside of the cell, resulting in a net electrical potential that is slightly negative. When the cardiomyocyte receives a signal from the SA node, however, the electrical imbalance is reversed: The electrolytes switch positions, with the calcium and sodium rushing into the cell and the magnesium and potassium pouring out of it. As this switch occurs, the electrical potential of the cell changes from negative to positive, creating a tiny electrical current that essentially discharges the battery and sends an electric signal down the cellular conduits of your heart.
The Calcium Connection
Each of your cardiomyocytes is a rectangular cell encased by a thin membrane. This membrane is highly permeable, studded with special channels through which only specific electrolytes can pass—sodium channels for sodium ions, potassium channels for potassium ions, and so on. As you read earlier, an electrical signal, or action potential, is generated when ions on the outside of the cell (calcium and sodium) pass through their respective channels and switch places with ions on the inside of the cell (potassium and magnesium), creating an electrical imbalance.
But how is this action potential (electrical signal) is converted into action (muscle contraction)? The
electrolyte that is the key to this process is calcium. When a cardiomyocyte receives a signal, the calcium ions are the first to respond, commanding the calcium channels in the cell membrane to open up. With the doors to the cell flung wide, calcium ions flood the interior of the cell, initiating the process by which the electrical signal is conducted, and, with ATP, enabling the chemical reactions that allow the muscle fibers in the cardiomyocyte to band together and contract. As soon as the muscle fibers contract, however, magnesium re-enters the cell, forcing the calcium back out. Once the calcium ions leave the cell, the muscle fibers are no longer able to perform the chemical reactions that let the cardiomyocyte contract; as a result, the cell relaxes.
While you might think that having lots of calcium in your blood stream would be useful, the opposite is true. The more calcium ions enter the interior of your cardiomyocytes, the more frequently and forcefully your heart will contract, increasing your blood pressure and thus your risk of hypertension. In fact, special drugs called calcium channel blockers are often prescribed by doctors in order to treat hypertension or angina. By obstructing calcium’s access to the interior of your heart cells, these drugs slow the rate at which your heart contracts, effectively lowering your blood pressure. But there are better ways to prevent this surplus of calcium. As you’ll read in Chapter 4, magnesium regulates calcium naturally; taken in adequate doses, magnesium can actually act as a calcium channel blocker, limiting the amount of calcium that enters your cells, and thus preventing hypertension.
Clearly, your heart depends on electrolytes in order to generate and conduct electrical signals. Whenever your electrolyte levels are disrupted due to either dehydration or overhydration, the functioning of your heart’s electrical system is compromised, leading to arrhythmias and, occasionally, cardiac arrest. Accordingly, it is essential that you have a steady supply of these important minerals; without them, your heart’s batteries simply can’t charge.
Magnificent Magnesium Page 9