502
U N I T 5
Circulatory Function
discussions of the manifestations of shock. Approximately
10% of the total blood volume can be lost or shifted
without changing cardiac output or arterial pressure. The
average blood donor loses approximately 500 mL or 10%
of his or her blood without experiencing adverse effects.
6
However, as increasing amounts of blood (15% to 30%
or 750 to 1500 mL) are removed, compensatory mecha-
nisms are triggered.
50,51
The most immediate of these
compensatory mechanisms are the sympathetic-mediated
responses designed to maintain cardiac output and blood
pressure (Fig. 20-8). Shortly after the onset of hemor-
rhage or the loss of fluid volume, tachycardia, increased
cardiac contractility, vasoconstriction, and other signs of
sympathetic and adrenal medullary activity appear.
The sympathetic vasoconstrictor response also mobi-
lizes blood that has been stored in the venous side of
the circulation as a means of increasing venous return
to the heart. There is considerable capacity for blood
storage in the large veins of the abdomen, and approxi-
mately 350 mL of blood that can be mobilized in shock
is stored in the liver.
6
Sympathetic stimulation does not
initially cause constriction of the cerebral and coronary
vessels, and blood flow to the heart and brain is main-
tained at essentially normal levels as long as the mean
arterial pressure remains above 70 mm Hg.
6
Without
(and, at times, despite) compensatory mechanisms to
maintain cardiac output and blood pressure, the loss of
vascular volume results in a rapid progression from the
initial to the progressive, and finally to the irreversible
stages of shock (more than 40% volume loss).
51
Compensatory mechanisms designed to restore blood
volume (i.e., draw volume into the intravascular space)
include absorption of fluid from the interstitial spaces,
conservation of sodium and water by the kidneys, and
thirst. Extracellular fluid is distributed between the inter-
stitial spaces and the vascular compartment. When there
is a loss of vascular volume, capillary pressures decrease
and water is drawn into the vascular compartment from
the interstitial spaces. The maintenance of vascular vol-
ume is further enhanced by renally mediated humoral
mechanisms that conserve fluid. A decrease in renal
blood flow and glomerular filtration rate results in acti-
vation of the renin-angiotensin-aldosterone mechanism,
which produces an increase in sodium reabsorption by
Normal
• Adequate oxygenation
• Normal circulating volume
• No obstruction to flow
Pump
failure
Loss of intravascular
volume
Mechanical
obstruction
Massive
vasodilation
Obstructive
Distributive
Impeded or
obstructed flow
Decreased volume
Hypovolemic
Cardiogenic
FIGURE 20-7.
Types of shock.
