C

capillary beds The meshlike network of arteri-
oles and venules, the body’s tiniest BLOODvessels,
where OXYGEN–CARBON DIOXIDE EXCHANGE takes
place. Arterial pressure forces blood into the capil-
lary beds. Erythrocytes (red blood cells) carry oxy-
gen and other NUTRIENTS to the capillary beds,
where these molecules pass through to cells. Cor-
respondingly, the cells pass metabolic waste such
as CARBON DIOXIDE and lactic acid through to
the blood. The arterioles and the venules inter-
twine in the capillary beds, becoming indistin-
guishable. Capillary beds are present throughout
the body.
For further discussion of the capillary beds
within the context of cardiovascular structure and
function, please see the overview section “The
Cardiovascular System.”
See also ARTERY; ERYTHROCYTE; LUNGS; VEIN.

cardiac arrest Cessation of the heart’s contrac-
tions. Cardiac arrest may occur as the result of
arrhythmias (irregularities in the heartbeat),
MYOCARDIAL INFARCTION(clot that blocks the flow of
BLOODthrough the CORONARY ARTERIESto the HEART
MUSCLE), HYPOXIA(such as in drowning), ELECTRO-
CUTION, COCAINEuse, or blunt trauma to the chest.
Without immediate resuscitative efforts to restore
heartbeat, OXYGENATION, and circulation, death
occurs within minutes. About 350,000 Americans
die of cardiac arrest each year.
Although cardiac arrest can follow HEART
ATTACK, they are not the same event. Cardiac arrest
typically occurs suddenly and with few warning
indications. People at highest risk for cardiac arrest
are those who have disorders of the heart’s electri-
cal system such as LONG QT SYNDROME(LQTS), SICK
SINUS SYNDROME, BUNDLE BRANCH BLOCK, WOLFF-

PARKINSON-WHITE SYNDROME, and ventricular

arrhythmias.

More than half of cardiac arrests occur in people

who do not know they have CARDIOVASCULAR DIS-

EASE. Most cardiac arrests take place at home with

no one witnessing the event. By the time someone

finds the person who has had cardiac arrest, often it

is too late for resuscitation to succeed. Health

experts believe prompt resuscitation could save 70

to 80 percent of people who experience cardiac

arrest. CARDIOPULMONARY RESUSCITATION(CPR) is most

effective within four minutes of the onset of cardiac

arrest. Automated external defibrillators (AEDs),

small computerized devices that automatically read

the heart’s rhythm and can administer a jolt of elec-

tricity to shock the heart into a functional rhythm,

have become more common in public locations and

at workplaces. AEDs have saved numerous lives of

people who have had cardiac arrests.

See also ARRHYTHMIA; AUTOMATED EXTERNAL DEFIB-

RILLATOR (AED); CARDIOPULMONARY RESUSCITATION

(CPR); SUDDEN CARDIAC DEATH.

cardiac capacity The ability of the HEART to

increase CARDIAC OUTPUT to meet the body’s

increased needs for oxygen during physical activity

or exercise. Cardiac capacity is a combination of the

heart’s physical condition and the body’s AEROBIC

FITNESSlevel. Damage to the heart, such as may

occur with MYOCARDIAL INFARCTION and ISCHEMIC

HEART DISEASE(IHD), reduces cardiac capacity, as do

conditions that weaken the heart MUSCLEsuch as

CARDIOMYOPATHYand HEART FAILURE. Cardiac capacity

also diminishes as a normal dimension of aging.

Aerobic conditioning through consistent, moderate

to intense, AEROBIC EXERCISEimproves cardiac capac-

ity and helps sustain cardiovascular health.

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