Cardiovascular Physiology
441
Figure 12–13
Purkinje cell action potentials have a depolarizing
pacemaker potential, like node cells (though the slope is much
more gradual), and a rapid upstroke and broad plateau, like
cardiac muscle cells.
approximately 1/3 of the distance from diastolic pressure to
systolic pressure. At a heart rate in which equal time is spent in
systole and diastole, the mean arterial blood pressure would be
approximately halfway between those two pressures.
Figure 12–40
Venous blood leaving that tissue would be lower
in oxygen and nutrients (like glucose) and higher in metabolic
wastes (like carbon dioxide).
Figure 12–42
Injecting a liter of crystalloid to replace the lost
blood would initially restore the volume (and thus the capillary
hydrostatic pressure), but it would dilute the plasma proteins
remaining in the bloodstream. As a result, the main force
opposing capillary fi ltration (
π
c
) would be reduced, causing an
increase in net fi ltration of fl uid from the capillaries into the
interstitial fl uid space. A plasma injection, however, restores the
plasma volume as well as the plasma proteins. Thus, the Starling
forces remain in balance, and more of the injected volume
remains within the vasculature.
Figure 12–47
Ingestion of fl uids supports the net fi ltration of
fl uid at capillaries by transiently elevating vascular pressure (and
thus
P
c
) and reducing the concentration of plasma proteins
(and thus
π
c
). Although refl ex mechanisms described in the
next section and in Chapter 14 minimize and eventually reverse
changes in blood pressure and plasma osmolarity, you could
expect a transient rise in interstitial fl uid formation and lymph
fl ow after ingesting extra fl
uids.
Figure 12–53
There is a transient reduction in pressure at the
baroreceptors when you fi rst stand up. This occurs because
gravity has a signifi cant impact on blood fl ow. While lying
down, the effect of gravity is minimal because baroreceptors
and the rest of the vasculature are basically level with the
heart. Upon standing, gravity resists the return of blood from
below the heart (where the majority of the vascular volume
exists). This transiently reduces cardiac output and, thus,
blood pressure. Section E of this chapter provides a detailed
description of this phenomenon, and explains how the body
compensates for the effects of gravity.
Table 12–6
The hematocrit is the fraction of the total blood
volume that is made up of erythrocytes. Thus, the normal
hematocrit in this case was 2300/5000
×
100 = 46 percent.
Immediately after the hemorrhage it was 1840/4000
×
100
= 46 percent, and 18 hours later it was 1840/1490
×
100 =
37 percent. The hemorrhage itself did not change hematocrit
because erythrocytes and plasma were lost in equal proportions.
However, over the next 18 hours there was a net shift of
interstitial fl uid into the blood plasma, due to a reduction in
P
c
. Because this occurs faster than does the production of new
red blood cells, this “autotransfusion” resulted in a dilution
of the remaining erythrocytes in the bloodstream. In the days
and weeks that follow, increased erythropoietin will stimulate
the replacement of the lost erythrocytes, and the lost ECF fl
uid
volume will be replaced by ingestion and decreased urine output.
0
0
0.15
0.30
Time (s)
–50
–100
Membrane potential (mV)
Figure 12–16
A reduction in current through voltage-gated K
+
channels delays the repolarization of ventricular muscle cell
action potentials. Thus, the T wave (ventricular repolarization)
of the ECG wave is delayed relative to the QRS waves
(ventricular depolarization). This fact gives the name to the
condition, “Long Q-T Syndrome.”
Figure 12–21
The patient most likely has a damaged semilunar
valve that is stenotic and insuffi
cient. A “whistling” murmur
generally results from blood moving forward through a stenotic
valve, whereas a lower-pitched “gurgling” murmur occurs
when blood leaks backward through a valve that does not close
properly. Systole and ejection occur between the two normal
heart sounds, whereas diastole and fi lling occur after the second
heart sound. Thus, a whistle between the heart sounds indicates
a stenotic semilunar valve, and the gurgle following the
second heart sound would arise from an insuffi cient semilunar
valve. It is most likely that a single valve is both stenotic and
insuffi cient in this case. Diagnosis could be confi rmed either by
determining where on the chest wall the sounds were loudest or
by diagnostic imaging techniques.
Figure 12–25
Ejection fraction (EF) = Stroke volume (SV)/end-
diastolic volume (EDV); end-systolic volume (ESV) = EDV – SV.
Based on the graph, under control conditions the SV is 75 mL
and during sympathetic stimulation it is 110 ml. Thus, Control
ESV = 140 – 75 = 65 mL and EF = 75/140 = 53.6 percent;
sympathetic ESV = 140 – 110 = 30 mL and EF = 110/140 =
78.6 percent.
Figure 12–31
At resting heart rate, the time spent in diastole
is twice as long as that spent in systole (i.e., 1/3 of the total
cycle is spent near systolic pressures) and the mean pressure is
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