The Medical College - Page 70
153.
D.
Whereas the pressure to eject blood
must
increase, the volume of blood the ventricle
ejects
is less. Therefore, the ventricular work
depends
on the exact nature of the two curves. Because
the
ventricle cannot maintain the required pressure
at
low volumes, the total volume of blood ejected
is
less and the end systolic volume is
increased.
154.
A.
In this case, the ventricle is incapable of
gener-
ating the same pressure at low volumes.
There-
fore, the volume of blood ejected is less (i.e.,
the
stroke volume is smaller), and the end
systolic
volume is correspondingly larger. Note that
this
question is a play on words. If the stroke
volume
decreases with a constant end diastolic
volume,
the volume left in the ventricle after
contraction
will be more. The ventricular work is
reduced
because the pressure remains constant while
the
volume of blood ejected falls so that the
product
of the two decreases.
155.
C.
Ventricular Volume
Mean
Aor
tic
Pressure
Ventricular Volume
Mean
Aor
tic
Pressure
Ventricular Volume
Mean
Aor
tic
Pressure
The key to this question is to remember
which
two cases are being compared. The increase
in
end diastolic volume allows the failing
ventricle
to increase the stroke volume as compared to
the
uncompensated case. With a constant pressure,
an
increase in stroke volume causes an increase
in
the work (i.e., pressure
· stroke
volume). Because
the aortic pressure is constant and the active
pres-
sure curve is unchanged in the two cases, the
vol-
ume remaining in the ventricle at the point
at
which the ventricle stops ejecting blood
will
remain constant.
156.
C.
The key to this question, as in question 155,
is
that one must remember what conditions
are
being compared. Increasing the end diastolic
vol-
ume sufficiently could theoretically allow a
fail-
ing ventricle to eject a larger stroke volume
than
is the case in normals. However, the
compensa-
tion for a failing ventricle usually is not
sufficient
for this to occur under physiological
conditions.
Because the stroke volume is indeterminate
rela-
tive to the normal condition, the ventricular
work
is also indeterminate given a constant mean
aortic
pressure. Because by definition a failing
ventricle
cannot maintain the same pressure at low
vol-
umes, the end systolic volume must increase
in
comparison to that of normal.
157.
B.
The definitions of isotonic, hypotonic,
and
hypertonic need to be recalled. Remember that
the
structure placed in a medium is surrounded by
a
semipermeable membrane; the structure is
perme-
able to small particles (e.g., certain
inorganic ions
and water), but not to large particles (fat and
pro-
tein molecules). An isotonic environment
exerts
the same osmotic pull as the medium does on
the
other side of a semipermeable membrane. It
con-
sequently has to possess the same
concentration
of particles and, therefore, the net gain or
loss of
water during osmosis is zero. If the red
blood
cells were placed in a hypotonic solution,
hemoly-
sis would occur because a hypotonic
solution
would exert a lesser osmotic pull than the
medium
Ventricular Volume
Mean
Aor
tic
Pressure
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