A u tonom ic nervous system.
In times of physical or emotional
stress, the ANS can stimulate the heart to beat faster.
H orm ones.
For
example,
epinephrine
and
norepi-
nephrine released from the adrenal medullae during
exercise and stress increase both heart rate and con-
tractility.
Ions.
Elevated levels of sodium and potassium decrease
heart rate and contractility. A moderate increase in
calcium ions increases heart rate.
B ody temperature.
Increased body temperature, as occurs
during a fever or strenuous exercise, increases heart rate.
A ge.
Newborns have higher heart rates (120 beats/
min) than adults. The heart rate slows down gradually
during childhood, until it reaches the adult rate of
around 72 beats/min.
Gender.
Females tend to have higher resting heart rates
than males.
P h ysica lfitn ess.
Regular exercise tends to slow the resting
heart rate. Well-trained athletes have slower resting heart
rates
(40-60 beats/min)
but
have
normal resting
cardiac outputs because their hearts are enlarged,
giving them larger stroke volumes.
What Can Go Wrong with Blood
Flow Through the Heart?
In many organs and tissues, including the heart, the blood
vessels branch so that blood has many alternate routes
to reach the cells within. These
branches,
called
anastomoses
,
help maintain
sufficient blood
supply to an organ when one of
the branches is blocked. However,
when blood flow in the coronary
arteries
gets
partially
blocked
(
myocardial ischemia
) or totally blocked (
myocardial
infarction
), the heart muscle gets starved for oxygen and
may weaken or die. Such obstructions may occur due to
blood clots, coronary artery disease, or narrowing of the
arteries.
Atherosclerosis
, for example, is an inflamma-
tory disease condition in which fatty materials (mostly
cholesterol and triglycerides) build up in the walls of the
blood vessels, forming
lesions called
atherosclerotic
plaques
. These plaques narrow the diameter of the vessel
and roughen the inner surface, which can attract platelets
and lead to the formation of a thrombus (
Figure 11.9
).
CONCEPT CHECK
1.
What
is the function of the atria?
2.
Blood
returning to the heart from the body
enters which of the chambers?
3.
What
does the atrioventricular node do?
4.
What
is the significance of the QRS complex?
5.
What
happens during ventricular systole?
6.
How
does preload influence cardiac output?
anastomoses
(a-nas-to-MO-ses)
Anatomical connec-
tions between tubular
structures, especially
blood vessels.
N orm al and o b stru c te d a rte rie s • Figure 11.9
Photomicrographs of transverse sections of a. a normal artery and
b.
an artery partially obstructed by an atherosclerotic plaque.
U M 20x
a.
Normal
artery
A M 20x
Partially
obstructed
space through
which blood
flows
Atherosclerotic
plaque
b. Obstructed
artery
The Heart Pumps Blood Through Blood Vessels to All Tissues
319
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