IN OUR sophisticated society we are fairly well informed concerning the cardiovascular system and some of its widespread problems. Nutritionists admonish us to be cautious in our selection of foods. We are told that saturated fats must generally be avoided, since studies suggest they contribute to degenerative vascular disease. Such terms as atherosclerosis, cerebral vascular accident, and coronary artery disease are now frequently included in prime-time television fare and are no longer restricted to the domain of the clinician.
Most of us know that a healthy, well-functioning heart is essential to our enjoyment of a normal, active life. Also generally appreciated is the important role of the arteries, which transport oxygen-laden blood from the left side of the heart to the various body tissues, and the veins, which conduct blood containing carbon dioxide back to the right side of the heart. From here it is pumped into the lungs, where the carbon dioxide is exchanged for oxygen. It then completes its circuit by returning to the left side of the heart
With so much emphasis being given to the heart and major blood vessels, relatively little is said about a most significant part of the vascular network, the microcirculation. It would be of little use for the heart to pump blood at the rate of five liters (approximately five quarts) per minute if there were no efficient mechanism to deliver it to each of the millions of specialized cells in the body. The major arteries are simply too large and too few in number to handle such a monumental task.
Utilizing His remarkable engineering ability, our Creator devised an astoundingly diffuse delivery system. As blood courses through the aorta, the largest artery, it is distributed into a vast, branching system of tubes of ever-decreasing dimensions. At the extreme limits, these living tubes are too small to be observed with the unaided eye (hence this division of the cardiovascular system is commonly referred to as the microcirculation).
Although individual microvessels are extremely small, they are so numerous that their combined length has been estimated at something in excess of 45,000 miles. Their total blood-carrying capacity greatly exceeds that of all other blood vessels combined.
The smallest arteries are called arterioles and range in diameter from 25 to 30 microns (a micron is equal to 1/1,000 of a millimeter or about 1/25,000 of an inch). The wall of the arteriole contains a number of muscle cells so arranged that, when they con tract, the vessel narrows (or constricts) when they relax, the vessel widens (or, dilates). By virtue of a very important principle of physics, an increase or reduction in the diameter of a blood vessel greatly alters the volume of blood that can flow through it. For example, if a blood vessel dilates to twice its original diameter, the flow of blood through it will not simply be doubled, but will in crease sixteen-fold. On the other hand, if the vessel were to narrow to half its original diameter, blood flow through it would be diminished to one sixteenth its original volume. Thus, arterioles are of great significance in the distribution of blood and in the regulation of blood pressure.
The smallest of all blood vessels are the capillaries (about four to eight microns in diameter). They are so small and so numerous that no individual body cell is farther than approximately thirty-five microns from its life-sustaining blood supply. In fact, in tissues such as heart muscle, with very high metabolic activity (internal life processes of the individual fibers), there is essentially one capillary for every muscle fiber. The major function of the capillary is to provide the ultimate contact between individual tissue cells and blood. Among the many substances that constantly pass through its wall are nutrients, oxygen, ions (e.g., potassium, sodium, calcium), and numerous essential biochemicals. Of just as critical importance is the removal of the by-products of life processes (e.g., carbon dioxide and certain acids) that cannot be allowed to accumulate around and within the cells.
The capillary is structurally well-suited for its role. The entire circulatory system is lined with flat cells arranged somewhat like tiles, forming a single layer called the endothelium. The walls of larger blood vessels contain elastic fibers, muscle cells, and supportive tissue surrounding the delicate endothelium. As vessels diminish in size, fewer of the supportive tissues are retained until, at the capillary level, essentially all that remains is the lining. Hence, the capillary has been defined as an endothelial tube. The obvious advantage of this arrangement is the fact that most substances that must pass between blood and the surrounding tissues find the thin capillary wall (one micron thick) only a slight obstacle, so ex change is enhanced. This is particularly important in the transfer of oxygen and carbon dioxide. The smallest capillaries are no larger in diameter than red blood cells (about eight microns). This forces the cells to pass through the capillary single file, in close contact with the wall. In this ideal situation oxygen leaves the hemoglobin with which it was combined in the lungs, crosses the capillary endothelium, and reaches waiting tissue cells. Carbon dioxide produced in the tissues enters the blood across the same thin barrier and is carried away to the lungs.
Situated between the arterioles and capillaries are collars of muscle cells called precapillary sphincters. A sphincter is an effective floodgate. When it is constricted, blood cannot flow beyond it. When it is dilated, blood flows freely through the capillary bed, which it oversees. This device is remarkable for the apparent simplicity of its operation. When a tissue area becomes more active, the concentration of metabolic by-products increases. These substances cause the precapillary sphincter muscle to relax so that more blood flows into the tissue area to support its increased activity. For obvious reasons, this process is called autoregulation (auto = self). What could be more logical than to provide a localized tissue the means for adjusting its own blood supply, according to its need?
Have You Heard of Venules?
Once the blood has passed through the vast capillary network, it is collected by an equally vast system of microscopic veins, the venules (twenty to fifty microns in diameter). Although they contain some muscle, the smallest venules are also thin-walled (two microns thick) and are, to some extent, involved in the exchange function already described for the capillaries. Venules and small veins comprise a vast reservoir that, along with the larger veins, contains approximately 75 per cent of the total blood volume. In exercise, the extensive venous reservoir becomes smaller as individual venules and veins constrict. This effectively redistributes the blood to allow filling of the now-dilated vascular network in the active muscle. Without this ability to adjust to the body's changing demands, increased physical activity could not be sustained for long periods of time.
Perhaps the most important point to be made in all of this is the fact that all cardiovascular function is directed toward the one purpose of providing optimal blood flow through vessels so small they cannot be seen without a microscope, yet so important that life would be impossible without them. The next time you are told to think about your heart, do so. Having done that, pause a moment longer to consider where the real action is. Think about those marvelous microvessels!
Notes:
Folkow, B., and E. Neil. Circulation. New York: Oxford University Press, Inc., 1971.
Selkurt, E. Physiology, 3d ed. Boston: Little, Brown and Company, 1971.
Zweifach, B. "The Microcirculation of the Blood," Scientific American, January, 1959.
