 |
|
|
|
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.*
|
Blood cells travel through 60,000 miles of blood vessels |
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.
* Blood pressure is measured by the distance, in millimeters, it elevates a column of mercury. The upper and lower
pressures caused by the beating and relaxing of the heart are called the systolic and diastolic pressures. These vary in individuals
as a result of their age, sex, mental and physical stress, and fatigue. Blood pressure tends to be lower in women than in
men, lower in children, and higher in the elderly. Although opinions may vary slightly, a healthy young person may have a
reading of 100 to 140 millimeters of mercury systolic, and 60 to 90 millimeters diastolic. |
