There’s a reason artificial hearts have been, at best, a grand experiment (although 3-D bioprinting offers intriguing possibilities, as we’ll explore in Part III). Simply put, the human heart is a magnificent biological machine, at once exceedingly complex and elegantly simple.
If all the blood vessels in the human body, big and small, were laid end to end, the resulting tube would be an incredible 12,400 miles long. Every diagnostic and therapeutic tool in the field of cardiology is dedicated to preserving the ability of the heart to function efficiently as the main element of a simple system, which effectively pumps blood through this long tube. This is the heart’s only purpose; it has no other role.
So let’s take a quick journey along this 12,400-mile pathway to see exactly how your heart works.
The heart is a hollow, muscular, four-chambered organ that is about the size of an adult fist and weighs less than one pound. It lies slightly to the left of the middle of the chest and is protected by the breastbone and the ribs. The major pumping chamber of the heart, the left ventricle, is a thick-walled compartment that delivers blood into one end of the long vascular tube with sufficient pressure to pump the fluid along its entire length. After the oxygen and nutrients carried in the blood have been removed by the various organs and tissues located along the length of the arterial system, the blood is returned at extremely low pressure, via the venous system, to the right atrium, the lower collecting chamber on the right side of the heart.
It next goes through the tricuspid valve to the relatively thin-walled right ventricle, which then gently ejects the blood still under relatively low pressure across the pulmonic valve into the lungs. The 300 million air sacs located in the lungs enrich the blood with oxygen obtained from the air we breathe. Simultaneously, carbon dioxide gas is removed from the blood and exhaled into the air. The oxygen-enriched blood then moves, still under relatively low pressure, into the lower collecting chamber of the left side of the heart, the left atrium.
It next goes through the well-known mitral valve (the one that is frequently affected in patients with rheumatic heart disease) into the workhorse of the heart, the lower left chamber, called the left ventricle. The internal pressure in the left ventricle increases dramatically, and then, approximately once each second, this extremely strong and efficient chamber ejects the blood forcefully out of the heart through the aortic valve into the aorta, the body’s main artery.
The right and left coronary arteries branch off the aorta at its very beginning, and they then divide first into a few major arteries, next into many medium-sized arteries, and finally into thousands of smaller and smaller arteries, which transport the blood to every part of the body.
Each of the smallest blood vessels (called arterioles) gives rise to a network of even smaller blood-carrying tubules known as capillaries, which reach out to nourish the tiny tissue cells themselves. The thin walls of the capillaries permit oxygen and nutrients to be absorbed by the cells and tissues and, in turn, receive waste products, which are first carried back to the heart by the venous system and then transported to the lungs and kidneys by the arterial system for proper disposal.
The four heart valves (aortic, pulmonic, mitral, and tricuspid) normally allow blood to flow in only one direction, without backflow or leakage. (When leakage does occur as a result of injury or disease—for example, due to rheumatic heart disease or infection—the efficiency of the heart as a pump becomes impaired and permanent damage may result.) The blood simply circulates like water in a fountain with a closed circuit. The water is pumped out the top of the fountain, flows to the bottom, and then is pumped under high pressure to spray once again out the top. It all sounds a bit complicated but is really quite simple.
And what an incredible pump it is! No mechanical pump of equal capacity has been devised by humans with similar efficiency, longevity, and reliability. Consider the following facts: The normal heart beats between 87,000 and 144,000 times per day. During the average lifetime, the heart will beat over 2.5 billion times, moving well over 50 million gallons of blood. The two thousand gallons of blood pumped daily are fully recirculated every ten to fifteen seconds.
The heart is like all other organs—it too must receive fresh oxygenated blood filled with life-giving nutrients if it is to survive and continue its vigorous pumping activities. The coronary arteries comprise the superb personal circulatory system, which nourishes this miraculous pump. (The word “coronary” is derived from the Latin word corona, meaning “crown,” as in a king’s crown, which pictorially describes the arrangement of these arteries on the surface of the heart.)
The coronary arteries are the very first that branch off the aorta as it receives blood from the left ventricle. The all-important left coronary artery nourishes most of the left ventricle, and the right coronary artery carries blood to the walls of the right ventricle as well as to the back and lower surfaces of the heart.
Tiny terminal branches called collateral arteries connect the circulation between the two sides. These connecting arteries are called into play when the circulation to one side of the heart or the other is diminished. When this occurs, fresh blood can still reach the region of the heart deprived of its usual blood flow through this physiological “back door.” This backup system tends to enlarge for the several weeks after a coronary artery is obstructed, and it can even grow large enough to prevent another heart attack if a second coronary artery later becomes blocked.
The right coronary artery divides into smaller arteries at a relatively distant location along its path; the left coronary artery, however, quickly splits into two major branches, the left anterior descending and the left circumflex arteries. “Triple vessel disease” describes patients afflicted with severe narrowing or blockage of three coronary arteries, usually the left anterior descending, the left circumflex, and the right coronary arteries. When a patient requires five or six grafts during a coronary artery bypass operation, the grafts will usually include replacements for these three vessels as well as two or three of their branches.
The heart is a model of efficiency and uniformity; its regularity is due to tiny electric impulses that are self-generated on an average of once every second by the sinoatrial node, the natural electrical “battery” pacemaker of the heart. This tiny “battery” automatically generates electric currents, which are conducted along a system of “wires” within the heart muscle itself. If the sinoatrial node fails to operate properly, its function is taken up by one of a series of backup “batteries” scattered throughout the heart. They usually are activated only when the heart’s primary pacemaker fails or if a malfunction occurs in its electrical conduction system.
The pumping chambers are sparked in a carefully timed sequence by the currents generated in the sinoatrial node; the net result is a contraction or squeezing down of these chambers in an orderly fashion, at the appropriate time. The blood is forced to systematically flow from the upper chambers (the atria) to the lower chambers (the ventricles) and then into the general circulation—specifically, to the lungs from the right ventricle and to the rest of the body from the left ventricle. This harmonious efficiency would be destroyed if a malfunction were to occur in one or more of the three major regions described above: (1) in the sinoatrial node (the “battery”), (2) in the conducting “wires,” or (3) in the “pump” (heart muscle) itself.
Oxygenated blood is pumped from the left ventricle into the coronary arteries, which carry the blood over the heart’s surface to its most distant regions and then penetrate the heart muscle itself. These indispensable supply lines provide nutrition for all parts of this incredible pumping organ, which normally operates with harmonious efficiency. However, if the blood flow through to the arteries becomes inadequate due to atherosclerotic injury or narrowing, the heart’s perfect timing can be affected, likely because of resulting damage to one of its component parts (battery, wires, pump), thereby diminishing or destroying the heart’s ability to nourish the various organs of the body, including itself.
Nature has been kind to us in many ways. For example, we have two eyes, two kidneys, and two adrenal glands. If we lose one, we always have the other to take up the function of its lost partner, and it usually does so quite successfully. It’s nature’s grand plan for built-in redundancy. If system one fails, then switch to its identical twin system. It’s a concept embraced by mechanical engineering, architecture, aeronautics … any system that has unacceptable consequences if the primary system fails.
Even though we have only one heart, nature has again thoughtfully provided a backup feature, an enormous safety factor. If a significant portion of the heart is lost due to damage or injury, temporary or permanent, the remaining portion of the heart will usually take on the load and carry on quite nicely, albeit somewhat less efficiently. A heart that has been permanently damaged due to a heart attack will frequently last as long and be almost as efficient as a healthy heart, unless the damage is extremely large or critically placed. In fact, one can usually get along quite well for many years (or even for a normal life span) if only 5 or 10 percent of the heart has been damaged.
If the degree of damage is very severe, physical incapacity may result, and frequently the life span will be shortened. Therefore, we do everything we can to preserve the heart, to prevent damage, and, if damage does occur, to limit the size of the injury. The contemporary approach to treatment of an acute heart attack is aggressively dedicated to the prevention of complications and to saving as much heart muscle as is possible.