Encyclopedia of Health and Medicine

20 The Cardiovascular System
icant risk of uncontrolled bleeding because of the
unstable nature of the AVM, and sometimes the
risk of attempting treatment is greater than the risk
of leaving the AVM untreated. Treatment that successfully
removes or seals the AVM ends the threat
of hemorrhage.
See also ANEURYSM; ARTERY; BIRTH DEFECTS; CON-
GENITAL ANOMALY; VEIN.
artery A flexible, muscular BLOOD vessel that carries
blood from the HEART and oxygenated blood to
tissues throughout the body. The wall of an artery
has three layers:
• adventitia, the outermost layer, which is primarily
connective tissue that gives the artery its
FLEXIBILITY
• media, the middle layer, which is mostly
smooth MUSCLE tissue that gives the artery the
ability to contract and relax
• intima, the inner layer, which is epithelial tissue
that provides a smooth surface to facilitate
the flow of blood
The adventitia is more prominent in larger
arteries such as the AORTA and the carotid arteries,
encasing the artery in a weblike fashion without
clear direction to its fibers. In smaller arteries, the
media often dominates the artery’s structure. The
muscle fibers of the media encircle the artery,
helping strengthen and stabilize the artery’s walls.
The delicate intima contains two structural levels,
the basement or foundation membrane and the
subepithelial layer, both of which run lengthwise.
Each may be only a cell’s thickness in small arteries,
indistinguishable without magnification.
The intima’s two-level structure gives the artery
its ability to carry blood cells without having them
stick to its inner walls. However, it also makes the
artery vulnerable to ATHEROSCLEROSIS, which develops
between the intima’s two levels. The tiniest of
the body’s arteries, about the thickness of a hair,
are arterioles. The body’s largest artery is the
aorta, which carries blood from the heart to the
network of arteries that then carry the blood
throughout the body.
Fibrous sheaths enclose most of the body’s
arteries, usually along with the companion VEIN
and NERVE. These sheaths often parallel skeletal
structures for protection and stability, or run deep
within the body. Arteries also receive blood themselves
from other arteries, which deliver oxygen
and other nutrients to the layers of the artery, and
contain nerves that deliver the signals to constrict
or dilate. The walls of the arteries constrict and
dilate in wavelike contractions that coordinate
with the heartbeat to help push blood through the
body. These pulsations are detectable as the PULSE
at points where the artery is near the surface of
the skin, such as at the wrist and the groin.
For further discussion of the artery within the
context of cardiovascular structure and function,
please see the overview section “The Cardiovascular
System.”
See also ARTERIOVENOUS MALFORMATION (AVM);
ATHEROSCLEROSIS; ARTERIOSLCEROSIS; CAROTID BRUIT;
CAROTID STENOSIS; CORONARY ARTERY DISEASE (CAD).
aspirin therapy A form of ANTICOAGULATION THER-
APY to help prevent BLOOD clots from developing,
which doctors prescribe as a prophylactic measure
for HEART ATTACK and STROKE. Aspirin has a moderate
anticoagulation effect. It interferes with
PLATELET AGGREGATION, the first step in the clotting
process. Aspirin blocks the body’s production of
PROSTAGLANDINS, chemicals the platelets need to
help them aggregate (clump together). Cardiologists
generally recommend aspirin therapy for:
• men between the ages of 40 and 75
• women who are beyond MENOPAUSE
• men and women under age 40 who have
HYPERTENSION, DIABETES, or OBESITY
• men and women under age 40 who smoke cigarettes
People who do not have increased risk for CAR-
DIOVASCULAR DISEASE (CVD)—are under age 40 and
have no predisposing health conditions or lifestyle
factors—likely do not receive enough benefit from
aspirin therapy to offset the potential risks. The primary
risks of aspirin therapy are gastrointestinal
upset and excessive bleeding. Aspirin may cause
GASTROINTESTINAL BLEEDING in people who have PEP-
TIC ULCER DISEASE, and extended bleeding during
dental procedures and surgeries or with wounds
such as lacerations. Doctors recommend a DOSE of