THE MARVELS OF THE
Circulatory System

IMAGINE a home with a plumbing system so sophisticated that the fluid flowing through it can safely carry food, water, oxygen, and waste products. More than that, these pipes have the means to repair themselves and can proliferate with changing demands of the home. What engineering brilliance!

Yet, your body's "plumbing" does even more. Besides helping to regulate your body temperature, it carries a bewildering array of hormones, or chemical messengers, and potent defenses against diseases. The whole network is also soft and pliable, allowing it to absorb shocks and to flex with your body members. No human engineer could design such a system, yet that is what the Creator did when he formed the veins, arteries, and capillaries of the human body.

The System's Main Components

The human circulatory system is really two systems that work together. One is the cardiovascular system, which includes the heart, the blood, and all the blood vessels. The other is the lymphatic system—a web of vessels that transport excess fluid, called lymph, from the body's tissues back into the bloodstream. If the blood vessels of just one adult were laid end to end, they would stretch out for 60,000 miles [100,000 kilometers] and could encircle the earth two and a half times! This extensive system carries life-giving blood, which makes up about 8 percent of the body's weight, to billions of cells.

The powerhouse behind the cardiovascular system is, of course, the heart. About the size of your fist, it pumps at least 2,500 gallons [9,500 liters] of blood throughout your body daily—roughly the equivalent of raising a one-ton weight to a height of 40 feet [some 10 m] every 24 hours!

A Tour of the Cardiovascular System

What path does the blood take? Let us begin with the oxygen-depleted blood arriving at the heart through the two large veins—the superior (top) and inferior (bottom) venae cavae. (See diagram.) These veins empty into the first chamber of the heart, the right atrium. The right atrium then squeezes the blood into a more muscular chamber, the right ventricle. From here the blood goes to the lungs through the pulmonary trunk and the two pulmonary arteries—the only arteries carrying oxygen-depleted blood. This is normally done by veins.

While in the lungs, blood releases carbon dioxide and absorbs oxygen. It then flows down into the heart's left atrium through the four pulmonary veins—the only veins carrying oxygen-rich blood. The left atrium empties into the heart's most powerful chamber, the left ventricle, which pumps oxygenated blood out through the aorta and into the body. The two atria contract together followed by the two ventricles, the dual sequence constituting a heartbeat. Four internal valves ensure a one-way flow of blood through the heart.

Because it has to pump blood to the extremities of the body, the more muscular left ventricle has about six times the force of the right ventricle. The resulting pressure could easily cause aneurysms (bulges or dilations in arterial walls) or even potentially deadly strokes in the brain were it not for an ingenious mechanism for absorbing the pressure surges.

Elastic Arteries

Your body's largest artery, the aorta, and its main branches constitute the "elastic arteries." Their lumen, or internal space, is large, allowing blood to flow easily. They also have thick, muscular walls enmeshed with concentric sheets of elastin, a rubberlike protein. When the left ventricle pumps blood into these arteries, they expand or swell, absorbing the high pressure and propelling the blood toward the next group of arteries, the muscular, or distributing, arteries, which also have elastin in their walls. Thanks to this remarkable design, blood pressure is steady by the time it reaches the delicate capillaries.*

The distributing arteries range in diameter from about half an inch down to 0.01 inch [1 cm-0.3 mm]. By dilating or constricting as directed by special nerve fibers, these vessels help regulate blood flow, making the circulatory system extremely dynamic. In the event of trauma or alarm, for instance, pressure sensors in arterial linings signal the brain, which, in turn, signals the appropriate arteries to restrict blood flow to less important areas such as the skin and shunt it to the vital organs. Says New Scientist magazine: "Your arteries can 'feel' the blood flowing, and respond." Is it any wonder that arteries have been described as "smart pipes"?

By the time blood leaves the smallest arteries—the arterioles—its pressure is steady at about 35 millimeters of mercury. Steady, low pressure is vital here because the arterioles merge with the smallest blood vessels of all, the capillaries.

Red Cells in Single File

Eight to ten micrometers (millionths of a meter) in diameter, capillaries are so fine that red blood cells pass through in single file. Although capillary walls are just a single layer of cells thick, they transfer nutrients (carried in the plasma, or the fluid part of the blood) and oxygen (transported by red cells) to adjacent tissues. At the same time, carbon dioxide and other wastes diffuse from the tissues back into the capillaries for disposal. By means of a tiny nooselike muscle called a sphincter, capillaries can also regulate the blood flowing through them according to the needs of the surrounding tissue.

From Venules to Veins to the Heart

When blood leaves the capillaries, it enters tiny veins called venules. Between 8 and 100 micrometers in diameter, venules join to form veins that return blood to the heart. When blood reaches the veins, it has lost nearly all its pressure, so venous walls are thinner than arterial walls. They also have less elastin. However, their lumen is larger, resulting in the veins' holding fully 65 percent of your body's blood.

To compensate for their low blood pressure, veins have an ingenious way of getting blood back to the heart. First, they are equipped with special cuplike valves that prevent gravity from draining the blood away from the heart. Second, they employ your body's skeletal muscles. How so? When your muscles flex, say in your legs as you walk, they compress nearby veins. This, in turn, forces blood through the one-way valves toward the heart. Finally, pressures in the abdomen and chest cavity, altered by breathing, help the veins empty their contents into the right atrium of the heart.

The cardiovascular system is so efficient that even when a person is at rest, it returns about 5 quarts of blood to the heart every minute! Walking increases this to about 8 quarts, and a fit marathon runner might have 37 quarts [35 liters] of blood coursing through his heart every minute—a sevenfold increase over the resting volume!

In some instances venous valves may leak because of a genetic predisposition or because a person develops obesity, becomes pregnant, or stands for long periods of time. When these valves fail, blood forms pools below them, causing the veins to distend and become what is known as varicose veins. Similarly, straining, such as to deliver a baby or to effect a bowel movement, increases pressure on the abdominal cavity, which impedes the return of blood from the veins of the anus and the large intestine. When this happens, varicose veins called hemorrhoids may result.

How the Heart Beats

The Lymphatic System

When capillaries deliver nutrients to the tissues and retrieve wastes, they pick up slightly less fluid than they deliver. Important blood proteins leak out into the tissues. Thus, the need for the body's lymphatic system. It collects all the excess fluid, called lymph, and returns it to the bloodstream by way of a large vein at the root of the neck and another in the chest.

As with arteries and veins, there are several orders of lymphatic vessels. The smallest, the lymph capillaries, occur in beds of blood capillaries. Highly permeable, these tiny vessels absorb excess fluid and channel it to larger lymphatic collecting vessels that carry lymph to the lymph trunks. These unite to form lymph ducts, which, in turn, empty into the veins.

Lymph flows only one way—toward the heart. Hence, lymphatic vessels do not form a circuit as the cardiovascular system does. Weak muscle action in the lymph vessels, aided by the pulsation of nearby arteries and the movement of limbs, helps to propel lymph fluid through the system. Any blockage of lymphatic vessels causes fluid to accumulate in the affected region, creating a swelling called an edema.

Lymphatic vessels also provide routes for disease organisms. Hence, our Creator empowered the lymphatic system with potent defenses, the lymphoid organs: the lymph nodes—scattered along the lymphatic collecting vessels—the spleen, the thymus, the tonsils, the appendix, and the lymphoid follicles (Peyer's patches) in the small intestine. These organs help to produce and house lymphocytes, the primary cells of the immune system. A healthy lymphatic system, therefore, contributes to a healthy body.

Here our journey around the circulatory system ends. Yet, even this brief tour has revealed an engineering wonder of astounding complexity and efficiency. What is more, it goes about its endless tasks quietly, without your conscious awareness—unless it gets sick. So look after your circulatory system, and it, in turn, will look after you.