In the first of these, blood circulates from the
t to the
s and back to
the
. In the second, blood circulates from the
to the rest of the body and
back to the
.
18.
The primary motive force for driving the blood along the arteries is the
,
which consists of four separate
s.
19.
Why is the wall of the left ventricle especially thick?
The amount of blood forced out of each ventricle in one contraction is called the
s
ev
e. The amount of blood pumped out of the ventricles in a minute is
called the c
co
t.
20.
Structures ensuring that fluids pass through them in only one direction are called
s.
The contraction of the atrial walls forces the blood from the atria through the
valves and into the
s. When the ventricles contract, the openings
between the atria and the ventricles are sealed by the
of the
valves.
When the ventricles relax, the openings between the great arteries and the
ventricles are sealed by the cuplike
s of the
valves.
21.
The action of the heart is accelerated by the
thetic portion of the nervous
system. It is slowed down by the
thetic portion.
The sinoatrial (SA) node is often called the pacemaker of the heart because it
.Impulses begin in the
node, pass
to the
node, and then descend through the
l bundles to stimulate the
m
m of the ventricular walls to contract.
22.
Nutrient blood is supplied to the walls of the heart by the right and left
y
arteries.
The openings leading into the coronary arteries are located in the base of the
ascending
, just behind the cusps of the
valve. When this valve is
open, its cusps cover the openings of the
y
s. When the valve is
closed, the backpressure within the aorta fills the
s with blood. The
coronary arteries then distribute the blood to all of the tissues of the relaxed
.
MD0007
10-26
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