proximal portion of the ascending aorta (the part closest to the heart).
Tis part of the aorta is located within the pericardium. Te tear then
proceeded distally but was contained within the aortic wall, causing the
back pain. An uncontained tear in the aortic arch or descending aorta
would lead to bleeding in the thoracic and/or abdominal cavities, which
was not observed in this case.
In the face of reduced SV, blood return-
ing to the heart backs up, leading to a rise in venous pressure. Tis is a
key sign of tamponade.
Check Your Understanding 1.
Te sympathetic nervous system inner-
vates blood vessels. Te sympathetic nerves innervate the tunica media.
Te eﬀector cells in the tunica media are smooth muscle cells.
When vascular smooth muscle contracts, the diameter of the blood
vessel becomes smaller. Tis is called vasoconstriction.
play a major role in dampening the pulsatile pressure of heart contrac-
tions. Dilation or constriction of arterioles determines blood ﬂow to in-
dividual capillary beds. Muscular arteries have the thickest tunica media
relative to their lumen size.
If you were doing calf raises, your capillary
bed would be in the condition depicted in part (a). Te true capillaries
would be ﬂushed with blood to ensure that the working calf muscles
could receive the needed nutrients and dispose of their metabolic wastes.
Valves prevent blood from ﬂowing backwards in veins. Tey are
formed from folds of the tunica intima.
In the systemic circuit, veins
contain more blood than arteries (see Figure 19.5).
Te three factors
that determine resistance are blood viscosity, vessel length, and vessel
diameter. Vessel diameter is physiologically most important.
of ﬂow will decrease 81-fold from its original ﬂow (3
When you ﬁrst stand up, mean arterial pressure (MAP) temporarily
decreases and this is sensed by aortic and carotid baroreceptors. Medul-
lary cardiac and vasomotor center reﬂexes increase sympathetic and
decrease parasympathetic outﬂow to the heart. Heart rate and contrac-
tility increase, increasing cardiac output, and therefore MAP. Further,
sympathetic constriction of arterioles increases peripheral resistance, also
increasing MAP. (In addition, increased constriction of veins increases
venous return, which increases end diastolic volume, increasing stroke
volume, and therefore cardiac output and MAP.) See also Figure 19.9
Te kidneys help maintain MAP by inﬂuencing blood
volume. In renal artery obstruction, the blood pressure in the kidney is
lower than in the rest of the body (because it is downstream of the ob-
struction). Low renal blood pressure triggers both direct and indirect re-
nal mechanisms to increase blood pressure by increasing blood volume.
Tis can cause hypertension (called “secondary hypertension” because
it is secondary to a deﬁned cause—in this case the renal artery obstruc-
In a bicycle race, autoregulation by intrinsic metabolic controls
causes arteriolar smooth muscle in your legs to relax, dilating the vessels
and supplying more O
and nutrients to the exercising muscles.
trinsic mechanisms, primarily the sympathetic nervous system, prevent
blood pressure from plummeting by constricting arterioles elsewhere
(such as gut, kidneys). In addition, cardiac output increases, which also
helps maintain MAP.
(a) An increase in interstitial ﬂuid osmotic
) would tend to pull more ﬂuid out of capillaries (causing
localized swelling, or edema). (b) An increase of OP
to 10 mm Hg would
increase the outward pressure on both the arteriole and venule ends of
the capillary. Te NFP at the venous end would become 1 mm Hg (27
26 mm Hg). (c) Fluid would ﬂow out of the venous end of the
capillary rather than in.
Bob is in vascular shock due to anaphylaxis,
a systemic allergic reaction to his medication. His blood pressure is low
because of widespread vasodilation triggered by the massive release of
histamine. Bob’s rapid heart rate is a result of the baroreceptor reﬂex trig-
gered by his low blood pressure. Tis activates the sympathetic nervous
creases friction caused by movement of the layers against one another.
Te papillary muscles and chordae tendineae keep the AV valve ﬂaps
from everting into the atria as the ventricles contract.
Te right side
of the heart acts as the pulmonary pump, whereas the le± acts as the
(a) ²rue. Te le± ventricle wall is thicker than the
right. (b) ²rue. Te le± ventricle pumps blood at much higher pressure
than the right ventricle because the le± ventricle supplies the whole
body, whereas the right ventricle supplies only the lungs. (c) False. Each
ventricle pumps the same amount of blood with each beat. If this were
not true, blood would back up in either the systemic or pulmonary cir-
culation (because the two ventricles are in series).
Te branches of the
right coronary artery are the right marginal artery and the posterior in-
(a) Te refractory period is almost as long as the
contraction in cardiac muscle. (b) Te source of Ca
for the contraction
is only SR in skeletal muscle. (c) Te AP exhibits a plateau phase in car-
diac muscle. (d) Both skeletal muscle and cardiac muscle have troponin.
(e) Only skeletal muscle has triads.
Cardiac muscle cannot go into
tetany because the absolute refractory period is almost as long as the
Te subendocardial conducting network excites ven-
tricular muscle ﬁbers. Te depolarization wave travels upward from the
apex toward the atria, and from endocardium to epicardium.
QRS wave occurs during ventricular depolarization. (b) Te ² wave of
the ECG occurs during ventricular repolarization. (c) Te P-R interval
of the ECG occurs during atrial depolarization and the conduction of
the action potential through the rest of the intrinsic conduction system.
Te second heart sound is associated with the closing of the semilu-
Te murmur of mitral insuﬃciency occurs during ven-
tricular systole (because this is when the valve should be closed, and the
murmur is due to blood leaking through the incompletely closed valve
into the atrium).
Te periods when all four valves are closed are the
isovolumetric contraction phase and the isovolumetric relaxation phase.
Exercise activates the sympathetic nervous system. Sympathetic
nervous system activity increases heart rate. It also directly increases
ventricular contractility, thereby increasing Josh’s stroke volume.
the heart is beating very rapidly, the amount of time for ventricular ﬁll-
ing between contractions is decreased. Tis decreases the end diastolic
volume, decreases the stroke volume, and therefore decreases the car-
Te foramen ovale and the ductus arteriosus both allow
blood to bypass the fetal lungs.
Elderly athletes may be hampered by
sclerosis and thickening of heart valve ﬂaps, a decline in cardiac reserve,
ﬁbrosis of cardiac muscle, and atherosclerosis.
Review Questions 1.
Case Study 1.
Te weak and thready pulse indicates a drop in stroke
volume (SV). Te pulse is felt as blood is ejected from the heart during
ventricular contraction (systole). Te weak and thready pulse suggests
that less blood is being ejected during each contraction (a lower SV).
An increase in heart rate leads to an increase in cardiac output (recall
SV). Mr. Ayers’s CO is abnormally low (as shown by his
decreasing blood pressure). Tis is probably due to a decrease in SV. Te
increase in HR is an attempt to compensate for the decrease in SV in or-
der to maintain CO as close to normal as possible.
In cardiac tampon-
ade, the ﬂuid around the heart compresses the heart and prevents it from
fully expanding as it relaxes (diastole). As a result of this restriction, less
blood will ﬂow into the heart (ventricular ﬁlling). With less blood ﬂowing
into the ventricles, the degree of stretch of the heart muscle (preload) will
also be reduced. Tese events lead to a reduction in SV.
are produced by the closing of heart valves during a normal cardiac cycle.
When EDV is reduced, there is both reduced SV and reduced force of
contraction, leading to slower, quieter valve closure.
Te enlarged me-
diastinum and pericardial eﬀusions suggest that the bleeding is restricted
within these compartments. Te tear would most likely be located in the