If a person rapidly loses more than 10% or 20% of total
blood volume from a large vein, the inadequate intravascular
volume causes sequential decreases in central blood volume,
venous return, ventricular fi lling, stroke volume, cardiac
output, and thus mean arterial pressure. However, if the
blood loss comes from a large peripheral artery, the mean
arterial pressure in central arteries does not fall until cardiac
output falls secondary to decreased venous return. Of course,
if the blood loss occurs from a blown aortic aneurysm, mean
arterial pressure falls immediately.
Large hemorrhages, in which one loses 30% or more of
total blood volume, produce hypovolemic shock. Shock is a
state of peripheral circulatory failure that is characterized by
inadequate perfusion of the peripheral tissues. During shock,
the systolic arterial pressure is usually below 90 mm Hg, and
the mean arterial pressure is below 70 mm Hg. For reasons
that will become clear, by the time one records a signifi cant
fall in mean arterial pressure, other signs of shock are evident.
The fi rst signs may be narrowing of the pulse pressure and
a sensation of faintness when sitting or standing. The subject
in hypovolemic shock has cold and moist (i.e., “clammy”)
skin as well as a rapid and weak pulse. Moreover, urine
output drops to less than 25 mL/hr, even though fl uid intake
had been normal.