573
CHAPTER FIFTEEN
Cardiovascular System
PRACTICE
27
Which nerves supply parasympathetic f
bers to the heart? Which
nerves supply sympathetic f
bers?
28
How do parasympathetic and sympathetic impulses help control
heart rate?
29
How do changes in body temperature aF
ect heart rate?
15.4
BLOOD VESSELS
The blood vessels are organs of the cardiovascular system.
They form a closed circuit of tubes that carries blood from
the heart to the body cells and back again. These vessels
may ±lutter occasionally, this condition is more
likely to be due to damage to the myocardium
(±ig. 15E).
A heart chamber
flutters
when it contracts
regularly, but very rapidly, such as 250–350
times per minute. Although normal hearts
Any inter±erence or block in cardiac impulse
conduction may cause arrhythmia, the type vary-
ing with the location and extent o± the block.
Such arrhythmias arise because certain cardiac
tissues other than the SA node can ±unction as
pacemakers.
The SA node usually initiates seventy to
eighty heartbeats per minute, called a sinus
rhythm. I± the SA node is damaged, impulses orig-
inating in the AV node may travel upward into the
atrial myocardium and downward into the ven-
tricular walls, stimulating them to contract. Under
the in²
uence o± the AV node acting as a
second-
ary pacemaker,
the heart may continue to pump
blood, but at a rate o± ±orty to sixty beats per min-
ute, called a nodal rhythm. Similarly, the Purkinje
±ibers can initiate cardiac impulses, contracting
the heart f
±teen to ±orty times per minute.
An
artificial pacemaker
can treat a disorder
o± the cardiac conduction system. This device
includes an electrical pulse generator and a lead
wire that communicates with a portion o± the
myocardium. The pulse generator contains a per-
manent battery that provides energy and a micro-
processor that can sense the cardiac rhythm and
signal the heart to alter its contraction rate.
An artif
cial pacemaker is surgically implanted
beneath the patient’s skin in the shoulder. An
external programmer adjusts its ±unctions ±rom
the outside. The f
rst pacemakers, made in 1958,
were crude. Today, thanks to telecommunications
advances, a physician can check a patient’s pace-
maker over the phone. A device called a pace-
maker-cardioverter-def
brillator can correct both
abnormal heart rhythm and cardiac arrest.
FIGURE 15E
³Atrial³²
utter is an abnormally rapid rate o± atrial depolarization.
FIGURE 15D
Bradycardia is a slow heartbeat.
Excess potassium ions (hyperkalemia) alter the usual polarized state
o± the cardiac muscle f
bers, decreasing the rate and ±orce o± contrac-
tions. High potassium ion concentration may block conduction o±
cardiac impulses, and heart action may suddenly stop (cardiac arrest).
Conversely, i± the potassium concentration drops below normal
(hypokalemia), the heart may develop a potentially li±e-threatening
abnormal rhythm (arrhythmia).
Excess calcium ions (hypercalcemia) increases heart action, risk-
ing that the heart will undergo a prolonged contraction. Conversely,
low calcium ion concentration (hypocalcemia) depresses heart action
because these ions help initiate muscle contraction.
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