Blood pressure is normally directly proportional to the
volume of the blood in the cardiovascular system. Thus, any
changes in the blood volume can initially alter the blood pres-
sure. For example, if a hemorrhage reduces blood volume,
blood pressure at ± rst drops. A transfusion that restores nor-
mal blood volume may reestablish normal pressure. Blood vol-
ume can also fall if the ﬂ uid balance is upset, as happens in
dehydration. Fluid replacement can reestablish normal blood
volume and pressure. Clinical Application 15.4 describes how
the unusual conditions of microgravity in outer space affect the
distribution of blood volume and control of blood pressure.
Friction between blood and the walls of the blood vessels
produces a force called
tans), which hinders blood ﬂ
ow. Blood pressure must
overcome this force if the blood is to continue flowing.
Therefore, factors that alter the peripheral resistance change
blood pressure. For example, contraction of smooth muscles
in the walls of contracting arterioles increases the peripheral
Blood pressure varies with the cardiac output. If either
the stroke volume or the heart rate increases, so does the car-
diac output, and, blood pressure initially rises. Conversely, if
the stroke volume or the heart rate decreases, the cardiac out-
put decreases, and blood pressure also initially decreases.
equals the sum of the formed elements and
plasma volumes in the vascular system. Although the blood
volume varies somewhat with age, body size, and sex, it is
usually about 5 liters for adults or 8% of body weight in kilo-
grams (1 kilogram of water equals 1 liter).
Blood volume is determined by injecting a known volume of an
indicator, such as radioactive iodine, into the blood. After thorough
mixing, a blood sample is withdrawn, and the concentration of the
indicator measured. The total blood volume is calculated using this
formula: blood volume = amount of indicator injected/concentration
of indicator in blood sample.
sound (Korotkoff’s sound) heard through the
stethoscope. The sound results from turbulence in
the narrowed artery. The pressure indicated on the
pressure gauge when this first tapping sound is
heard represents the
arterial systolic pressure
As the cuff pressure continues to drop,
increasingly louder sounds are heard. Then,
when the cuF
pressure is approximately equal to
that within the fully opened artery, the sounds
become abruptly muffled and disappear. The
pressure indicated on the pressure gauge when
this happens represents the
arterial diastolic pres-
The results of a blood pressure measurement
are reported as a fraction, such as 120/80. The
upper number indicates the systolic pressure in
mm Hg (SP), and the lower number indicates the
diastolic pressure in mm Hg (DP). ±igure 15H shows
how these pressures decrease as distance from the
left ventricle increases. The difference between
the systolic and diastolic pressures (SP-DP), called
(PP), is about 40 mm Hg.
The average pressure in the arterial system
is also of interest because it represents the force
ective throughout the cardiac cycle for driving
blood to the tissues. This force, called the
is approximated by adding the
diastolic pressure and one-third of the pulse pres-
sure (DP + 1/3PP).
Systemic blood pressure (mm Hg)
Distance from left ventricle
Blood pressure decreases as the distance from the left ventricle increases.
Systolic pressure occurs during maximal ventricular contraction. Diastolic pressure occurs when
the ventricles relax.