569
CHAPTER FIFTEEN
Cardiovascular System
(b)
(c)
(d)
(e)
(f)
Millivolts
0
–.5
.5
1.0
Milliseconds
0
200
400
600
Millivolts
0
–.5
.5
1.0
Milliseconds
0
200
400
600
Millivolts
0
–.5
.5
1.0
Milliseconds
0
200
400
600
P
Millivolts
0
–.5
.5
1.0
Milliseconds
0
200
400
600
S
Q
R
QRS complex
(h)
(g)
Millivolts
0
–.5
.5
1.0
Milliseconds
0
200
400
600
Millivolts
0
–.5
.5
1.0
Milliseconds
0
200
400
600
T
Millivolts
0
–.5
.5
1.0
Milliseconds
0
200
400
600
(a)
FIGURE 15.21
ECG pattern. (
a
) A normal ECG. In this set of drawings
(
b–h
), the yellow areas of the hearts indicate where depolarization is
occurring, and the green areas indicate where tissues are repolarizing;
the portion of the ECG pattern produced at each step is shown by the
continuation of the line on the graph paper.
increase in the impulses slows the heart rate, and a decrease
in the impulses releases the parasympathetic “brake” and
increases heart rate.
Sympathetic f
bers reach the heart by means oF the
accel-
erator nerves,
whose branches join the SA and AV nodes
as well as other areas oF the atrial and ventricular myocar-
dium. The endings oF these f
bers secrete norepinephrine in
response to nerve impulses. Norepinephrine increases the
rate and Force oF myocardial contractions.
ReFlexes called
baroreceptor reflexes
arising From the
cardiac control center
oF the medulla oblongata maintain
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