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The Heart and the Circulatory System

by Roger E. Phillips, Jr.

Human heart, frontal view
(Carolina Biological Supply Company.)

Imagine that you are living in the year 1535, and that you don't feel well. You have had some problems with fatigue, feeling a little more tired than usual when you walked to the market and back. You tell this to your physician, and he sends you to another physician down the street, telling you there may be some problem with your circulation. When you get to the new physician, he tells you to take off your shirt and lie down on the bench. After a quick look in your mouth, he says your vital blood is probably O.K. But he's concerned that maybe your nutritive blood is not being made fast enough. Then he starts to feel around on your abdomen. He mentions that your liver is slightly enlarged and suggests that maybe you have not been eating enough green leafy vegetables or protein. Wait a minute! You have come in with problems with your circulation, and this guy is talking about your liver and the type of foods you have been eating! What is going on here? Where did this fellow learn to practice medicine anyway?

Confusion over the nature of the heart, the blood, and the role of the blood in the body had existed for centuries. Pliny the Elder, a Roman writer who lived from AD 23-79, and author of a 37-volume treatise entitled Natural History, wrote "The arteries have no sensation, for they even are without blood, nor do they all contain the breath of life; and when they are cut only the part of the body concerned is paralyzed...the veins spread underneath the whole skin, finally ending in very thin threads, and they narrow down into such an extremely minute size that the blood cannot pass through them nor can anything else but the moisture passing out from the blood in innumerable small drops which is called sweat."

A century later Galen, a Greek physician who lived in the second century AD., spent his lifetime in observation of the human body and its functioning. Galen believed and taught his students that there were two distinct types of blood. 'Nutritive blood' was thought to be made by the liver and carried through veins to the organs, where it was consumed. 'Vital blood' was thought to be made by the heart and pumped through arteries to carry the "vital spirits." Galen believed that the heart acted not to pump blood, but to suck it in from the veins. Galen also believed that blood flowed through the septum of the heart from one ventricle to the other through a system of tiny pores. He did not know that the blood left each ventricle through arteries.

Physicians, as well as citizens, of many cultures had their own beliefs concerning the nature of the heart and circulatory system. While the Greeks believed that the heart was the seat of the spirit, the Egyptians believed the heart was the center of the emotions and the intellect. The Chinese believed the heart was the center for happiness. Even our modern society continues to put emotions under the control of the heart, speaking of having a broken heart when a loved one leaves, or stealing one's heart around Valentine's Day. These beliefs continued to be taught and taken as law until an English physician named William Harvey challenged them in the late 1620's.

William Harvey was born in 1578 in Folkstone, England. The eldest of seven sons, Harvey received a Bachelor of Arts degree from Cambridge in 1597. He then studied medicine at the University of Padua, receiving his doctorate in 1602. By all measures, Harvey was successful. After he finished his studies at Padua, he returned to England and set up practice. He then married Elizabeth Brown, daughter of the court physician to Queen Elizabeth I and King James I. This put in him in position to be noticed by the aristocracy, and Harvey quickly moved up the ladder. Eventually, he became court physician to both King James I and King Charles I.

While acting as court physician, Harvey was able to conduct his research in human biology and physiology. Harvey focused much of his research on the mechanics of blood flow in the human body. Most physicians of the time felt that the lungs were responsible for moving the blood around throughout the body. Harvey questioned these beliefs and his questions directed his life-long scientific investigations.

Opened heart showing anatomy and blood flow (Carolina Biological Supply)

Harvey's experiments involved both direct dissection and physiological experiments on animals. His observations of dissected hearts showed that the valves in the heart allowed blood to flow in only one direction. Direct observation of the heartbeat of living animals showed that the ventricles contracted together, dispelling Galen's theory that blood was forced from one ventricle to the other.Dissection of the septum of the heart showed that it contained arteries and veins, not perforations. When Harvey removed the beating heart from a living animal, it continued to beat, thus acting as a pump, not a sucking organ. Harvey also used mathematical data to prove that the blood was not being consumed. Removal of the blood from human cadavers showed that the heart could hold roughly two ounces of blood. By calculating the number of heartbeats in a day and multiplying this by two ounces, he showed that the amount of blood pump far exceeded the amount that the body could possibly make. He based this figure on how much food and liquids a person could consume. To Harvey, this showed that the teaching by Galen that the blood was being consumed by the organs of the body was false. Blood had to be flowing through a 'closed circuit' instead. Even though he lacked a microscope, Harvey theorized that the arteries and veins were connected to each other by capillaries, which would later be discovered by Marcello Malpighi some years after Harvey's death.

Harvey did not let the beliefs of Galen concerning the role of natural, vital, and animal spirits and their effects on physiology affect his objectivity. Instead, Harvey asked simple, pointed questions, the types of questions that even today are the hallmark of good scientific research. Harvey asked such questions as why did both the lungs and the heart move if only the lungs were responsible for causing circulation of blood? Why should, as Galen suggested, structurally similar parts of the heart have very different functions? Why did 'nutritive' blood appear so similar to 'vital' blood? These, and other, questions gave Harvey his focus.

Harvey's lecture notes show that he believed in the role of the heart in circulation of blood through a closed system as early as 1615. Yet he waited 13 years, until 1628, to publish his findings in his work Exercitatio anatomica de motu cordis et sanguinis in animalibus or On the Movement of the Heart and Blood in Animals. Why did he wait so long? Galenism, or the study and practice of medicine as originally taught by Galen, was almost sacred at the time Harvey lived. No one dared to challenge the teachings of Galen. Like most physicians of his day, William Harvey, was trained in the ways of Galen. Conformation was not only the norm, but was also the key to success. To rebel against the teachings of Galen could quickly end the career of any physician. Perhaps this is why he waited.

Harvey's hesitation proved well-founded. After his work was published, many physicians and scientists rejected him and his findings. Using different assumptions of the amount of blood contained in the heart, scientists argued that the blood could indeed be consumed. Controversy raged for a full twenty years after publication of "On the Movement of the Heart and Blood in Animals." Yet, with time, more and more physicians and researchers accepted Harvey's hypotheses.

Like all good research, Harvey's work raised more questions than it answered. For example, if blood was not consumed by organs, how did different parts of the body obtain nourishment? If the liver did not make blood from food, where did blood originate? These questions, and others like them, directed the research of many investigations for many years to come. Medical practice in Harvey's time, however, changed little. Even though the mechanics of blood flow were understood now, the understanding of the causes of many diseases were still bathed in the mystery of spirits. In fact, the practices of bleeding, lancing, and leeching increased in the years following Harvey's work. On the positive side, medicine did make some advances, for it was during the seventeenth century that administering medicine through intravenous injections came into practice.

William Harvey's classic work became the foundation for all modern research on the heart and cardiovascular medicine. It has been said that Harvey's proof "of the continuous circulation of the blood within a contained system was the seventeenth century's most significant achievement in physiology and medicine." Further, his work is considered to be one of the most important contributions in the history of medicine. Without the understanding of the circulatory system made possible by Harvey's pioneering work, the medical miracles that we think are commonplace would be impossible. Let's take a few moments to discuss the hearts and circulatory systems found in a variety of animals.

The Types of Circulatory Systems

Remember that the circulation of the blood serves to move blood to a site or sites where it can be oxygenated, and where wastes can be disposed. Circulation then serves to bring newly oxygenated blood to the tissues of the body. As oxygen and other chemicals diffuse out of the blood cells and into the fluid surrounding the cells of the body's tissues, waste produces diffuse into the blood cells to be carried away. Blood circulates through organs such as the liver and kidneys where wastes are removed, and back to the lungs for a fresh dose of oxygen. And then the process repeats itself. This process of circulation is necessary for continued life of the cells, tissues and even of the whole organisms. Before we talk about the heart, we should give a brief background of the two broad types of circulation found in animals. We will also discuss the progressive complexity of the heart as one moves up the evolutionary ladder.

Many invertebrates do not have a circulatory system at all. Their cells are close enough to their environment for oxygen, other gases, nutrients, and waste products to simply diffuse out of and into their cells. In animals with multiple layers of cells, especially land animals, this will not work, as their cells are too far from the external environment for simple osmosis and diffusion to function quickly enough in exchanging cellular wastes and needed material with the environment.

In higher animals, there are two primary types of circulatory systems -- open and closed. Arthropods and most mollusks have an open circulatory system. In this type of system, there is neither a true heart or capillaries as are found in humans. Instead of a heart there are blood vessels that act as pumps to force the blood along. Instead of capillaries, blood vessels join directly with open sinuses. "Blood," actually a combination of blood and interstitial fluid called 'hemolymph', is forced from the blood vessels into large sinuses, where it actually baths the internal organs. Other vessels receive blood forced from these sinuses and conduct it back to the pumping vessels. It helps to imagine a bucket with two hoses coming out of it, these hoses connected to a squeeze bulb. As the bulb is squeezed, it forces the water along to the bucket. One hose will be shooting water into the bucket, the other is sucking water out of the bucket. Needless to say, this is a very inefficient system. Insects can get by with this type system because they have numerous openings in their bodies (spiracles) that allow the "blood" to come into contact with air.

The closed circulatory system of some mollusks and all higher invertebrates and the vertebrates is a much more efficient system. Here blood is pumped through a closed system of arteries, veins, and capillaries. Capillaries surround the organs, making sure that all cells have an equal opportunity for nourishment and removal of their waste products. However, even closed circulatory systems differ as we move further up the evolutionary tree.

One of the simplest types of closed circulatory systems is found in annelids such as the earthworm. Earthworms have two main blood vessels -- a dorsal and a ventral vessel --which carry blood towards the head or the tail, respectively. Blood is moved along the dorsal vessel by waves of contraction in the wall of the vessel. These contractible waves are called 'peristalsis.' In the anterior region of the worm, there are five pairs of vessels, which we loosely term "hearts," that connect the dorsal and the ventral vessels. These connecting vessels function as rudimentary hearts and force the blood into the ventral vessel. Since the outer covering (the epidermis) of the earthworm is so thin and is constantly moist, there is ample opportunity for exchange of gases, making this relatively inefficient system possible. There are also special organs in the earthworm for the removal of nitrogenous wastes. Still, blood can flow backward and the system is only slightly more efficient than the open system of insects.

As we come to the vertebrates, we begin to find real efficiencies with the closed system. Fish possess one of the simplest types of true heart. A fish's heart is a two-chambered organ composed of one atrium and one ventricle. The heart has muscular walls and a valve between its chambers. Blood is pumped from the heart to the gills, where it receives oxygen and gets rid of carbon dioxide. Blood then moves on to the organs of the body, where nutrients, gases, and wastes are exchanged. However, there is no division of the circulation between the respiratory organs and the rest of the body. That is, the blood travels in a circuit which takes blood from heart to gills to organs and back to the heart to start its circuitous journey again.

Frogs have a three-chambered heart, consisting of two atria and a single ventricle. Blood leaving the ventricle passes into a forked aorta, where the blood has an equal opportunity to travel through a circuit of vessels leading to the lungs or a circuit leading to the other organs. Blood returning to the heart from the lungs passes into one atrium, while blood returning from the rest of the body passes into the other. Both atria empty into the single ventricle. While this makes sure that some blood always passes to the lungs and then back to the heart, the mixing of oxygenated and deoxygenated blood in the single ventricle means the organs are not getting blood saturated with oxygen. Still, for a cold-blooded creature like the frog, the system works well.

Humans and all other mammals, as well as birds, have a four-chambered heart with two atria and two ventricles. Deoxygenated and oxygenated blood are not mixed. The four chambers ensure efficient and rapid movement of highly oxygenated blood to the organs of the body. This has helped in thermal regulation and in rapid, sustained muscle movements.

We have learned much about the heart and circulatory system since Harvey's pioneering work. Scientific research has replaced mystical spirits as the basis for medical practice. In the next part of this chapter, thanks to the work of William Harvey, we will discuss our human heart and circulation, some of the medical problems that can occur, and how advances in modern medical care allow treatment of some of these problems.

The Anatomy of the Heart

Heart Activities

Heart Resources

Heart Glossary

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