Circulatory System

The circulatory system is a network that carries blood throughout an animal's body. Described as an internal transport system or a distribution system, the circulatory system maintains a constant flow of blood throughout the body, carrying nutrients and oxygen to the body's tissues and taking away its waste products. It also helps regulate the body's temperature; carries substances, like antibodies and white cells that protect the body from disease; and transports chemicals, such as hormones, that help the body regulate its activities.

All animal cells live in a liquid environment, taking in oxygen and nutrients and creating waste products. All, therefore, require a way to gain access to what they need to grow, to perform their specialized task, to reproduce, and to dispose of their waste products. For any animal larger than a single-celled organism—which meets its needs by passing substances through holes in its membrane (called diffusion)—a complex circulatory system is required in order to make sure that every single cell gets what it needs.

OPEN CIRCULATORY SYSTEM

There are two main types of circulatory systems—an open system and a closed system. Most invertebrates (animals without a backbone) have an open circulatory system. It consists of a fairly simple network of tubes and hollow spaces. In this system, a heart (or a series of hearts) pumps blood out of the vessels (a duct for circulating blood) and into the sinuses or open spaces of an animal's body. Mollusks, like clams, and arthropods, such as crayfish and grasshoppers, have an open circulatory system. In such a system blood flows slowly and under low pressure into the open spaces in these organisms' body cavities. This blood also bathes their cells, allowing them to obtain food and oxygen while eliminating waste. Open circulatory systems pump a bloodlike fluid called "hemolymph" that closely resembles seawater. However, one of the disadvantages of this type of system is that it cannot respond quickly to change, nor can it supply large amounts of oxygen.

CLOSED CIRCULATORY SYSTEM

All vertebrates (animals with a backbone) and some invertebrates, like earthworms, have a closed circulatory system. This means that the blood never leaves their vessels. For vertebrates, blood is pumped by the heart throughout the body via a network of closed vessels that become finer and finer as they get farther away from the heart. Closed systems are more efficient than open ones, since it is easier for them to respond to sudden changes and to alter the distribution of blood.

Although there are invertebrates with closed systems, it is by far the predominant characteristic of vertebrates—whether fish, amphibian, reptile, bird, or mammal. The basic components of the vertebrate circulatory system are the heart, arteries, capillaries, veins, and the blood itself. In the human body, the heart is a hollow, muscular pump that forces blood to move throughout the body. The human heart consists of two pumps that lie side by side. The left and stronger pump receives oxygen-fresh blood from the lungs and pumps it under great pressure to the cells throughout the body. The weaker right side receives "used" blood from the cells and sends it to the lungs to have carbon dioxide removed and to be freshened with oxygen. The heart muscle beats at its own automatic rhythm and pumps in a certain correct sequence. Arteries are blood vessels that carry blood away from the heart. They are specially designed, with thick, strong walls, since the blood they transport is under high pressure. Puncturing an artery can cause blood to spurt and travel through the air. Arteries also serve to smooth out the flow of blood by absorbing much of the rhythmic shock of the pumping heart. Arteries get smaller farther from the heart and are called arterioles.

As oxygen-rich blood continues the one-way trip from the heart and to the cells, the capillaries receive blood from the arterioles and pass it directly to the surrounding tissues and cells. Capillaries are finer than human hair (they are one cell thick) and have very thin walls where the critical exchange of nutrients, oxygen, and waste takes place. Capillaries merge to form venules or tiny veins, which in turn, merge to create large veins that will carry the blood back to the heart. Large veins are thinner than arteries (since the blood is now under much lower pressure) and are the primary blood vessels for the one-way, return trip of blood to the heart. Large veins also have one-way valves that prevent deoxygenated blood (depleted of oxygen and full of carbon dioxide) from flowing backwards. These veins empty into the parts of the heart called the vena cava, which in turn, empty directly into the heart's right atrium and back on to the lungs to start the cycle all over.

The route that blood travels in humans (as well as in birds and other mammals) is called "double circulation," which is in contrast to animals whose blood circulates in a single loop from the heart, around the body, and back again. In a double circulation system blood flows alternately through the lungs and throughout the body in a figure-eight pattern. After completing each loop, blood returns to a different side of the heart. In the human body, an entire double-loop cycle takes less than one minute. While the actual path that blood follows will vary among different groups of vertebrates, all are based on a similar system whether single or double loop. For example, fish have single loop circulation and a four-chambered heart, while amphibians and most reptiles have a three-chambered heart.

WILLIAM HARVEY DISCOVERS BLOOD CIRCULATION

The English physician William Harvey (1578–1657) discovered blood circulation in 1628. He demonstrated, contrary to widely-held beliefs that

WILLIAM HARVEY

English physician and biologist William Harvey (1578–1657) was the first to describe the circulation of the blood through the heart and the blood vessels. As a great experimenter, Harvey was able to prove that blood does not ebb and flow like waves in the body as people believed, but instead travels in a closed, or one-way circle. Many consider Harvey to be the founder of modern physiology (which studies how the different processes in living things work).

William Harvey was born in Folkestone, England, the son of well-to-do parents. After earning his degree from Cambridge in 1597, he did what anyone who wanted the best medical education available did: he went to Padua, Italy. At that time, Padua was able to boast that it had many great scholars, including the young astronomer, Galileo Galilei (1564–1642), on its faculty. Harvey stayed in Padua until he obtained his degree in 1602, and then he moved back to England. Given his father's connections, he soon had important patients, and later became physician to the king. However, Harvey was more interested in experimentation than in his day-to-day medical practice, as evidenced by his claim to have dissected eighty different species of animals by 1616. Harvey always dissected with a purpose, and most often, he focused on learning more about the heart and its vessels. Because of his extensive hands-on experience, Harvey already knew that heart muscle was basically a pump that acted by contracting, which pushed the blood out. He also learned that the valves that separated the heart's two upper and two lower chambers were one-way valves, and that veins (a branching system of vessels through which blood returns to the heart) even had one-way valves. All of this was leading Harvey far away from what most medical men of his time believed.

Traditional medicine was based on the writings of Galen (a.d. 129–c.199) who lived some 1,400 years before Harvey and whose human physiology was based on his dissections of animals. Galen taught that the liver was the important organ for the blood and that the blood that formed there ebbed and flowed throughout the body like the back-and-forth of the oceans' tides. Before Harvey decided to refute Galen, who he knew to be wrong, he performed even more experiments on animal and human hearts. He monitored the beating heart and made rough calculations of the volume of blood leaving the heart per beat and then compared that to total blood volume. It seemed impossible to him that blood could be broken down and reformed, as Galen said, fast enough to account for the amount of blood in the human body. The only explanation was that it must be the same blood moving in circles throughout the body. Finally, in 1628, Harvey published his slim, seventy-two-page book in Holland, in which he argued that blood was in motion all the time and that it circulated. The translated title of his landmark work is On the Motions of the Heart and Blood.

Since Harvey's book was a direct assault on Galen and all those who supported his ancient ideas, it is not surprising that Harvey found himself being ridiculed and condemned. His medical practice suffered for a time, but Harvey refused to be drawn into a defensive debate and instead, let the facts speak for themselves. Throughout this, he retained the king's favor. By his old age, the theory of the circulation of the blood was accepted and he had become a highly respected and even revered man of science. Interestingly, the one part of Harvey's theory that he could not prove was the crucial statement that blood moved from the arteries (a branching system of blood vessels that carry blood away from the heart) to the veins. He was unable to point out any visible connections between the two. However, he knew from his dissections that both vessels divided down to smaller and finer vessels, again and again, and so he simply assumed that they were too small to see with the naked eye. He was, of course, correct and was proven so about twenty-five years later when his countryman, Marcello Malpighi (1628–1694), used his new microscope and discovered extra-fine blood vessels called capillaries that connected the smallest arteries to the smallest veins.

blood flowed from the heart in a continuous, one-way cycle. The current understanding of the circulatory system was born of Harvey's work. It is now known that the circulatory system performs many vital functions in respiration (delivering oxygen to the cells and removing carbon dioxide); in nutrition (carrying nutrients to the cells and liver); and in waste removal (transporting poisons like salts and ammonia to the liver for disposal). The circulatory system also helps the body fight disease by acting as a means of transport for the lymphatic system, which is part of the body's immune system. This system filters harmful substances out of the bloodstream and carries white blood cells that destroy harmful bacteria, viruses, and other invaders.

[See alsoBlood; Heart; Lymphatic System; Respiratory System ]