Page 219 - Color_Atlas_of_Physiology_5th_Ed._-_A._Despopoulos_2003

Arterial Blood Pressure By attenuating the pressure signal, only the mean BP
is recorded.
The term blood pressure (BP) per se refers to Although the mean BP falls slightly as the
the arterial BP in the systemic circulation. The blood travels from the aorta to the arteries, the
maximum BP occurs in the aorta during the P s in the greater arteries (e.g., femoral artery) is
systolic ejection phase; this is the systolic pres- usually higher than in the aorta (A1 v. A2 ) be-
sure (P s); the minimum aortic pressure is cause their compliance is lower than that of
reached during the isovolumic contraction the aorta (see pulse wave velocity, p. 190).
phase (while the aortic valves are closed) and Direct invasive BP measurements show that
is referred to as the diastolic pressure (P d) the BP curve in arteries distal to the heart is not
(! A1 and p. 191, phase I in A2). The systolic– synchronous with that of the aorta due to the
diastolic pressure difference (P s–P d) represents time delay required for passage of the pulse
the blood pressure amplitude, also called pulse wave (3–10 m/s; ! p. 190); its shape is also
Cardiovascular System dV/dP, ! p. 188). When C decreases at a con- The BP is routinely measured externally (at the level
pressure (PP), and is a function of the stroke
different (! A1/A2).
volume (SV) and arterial compliance (C =
of the heart) according to the Riva-Rocci method by
stant SV, the systolic pressure P s will rise more
sphygmomanometer (! B). An inflatable cuff is
sharply than the diastolic pressure P d, i.e., the
snugly wrapped around the arm and a stethoscope is
PP will increase (common in the elderly; de-
placed over the brachial artery at the crook of the
scribed below). The same holds true when the
flated to a pressure higher than the expected P s (the
radial pulse disappears). The air in the cuff is then
If the total peripheral resistance (TPR, ! p. 188) in-
8 SV increases at a constant C. elbow. While reading the manometer, the cuff is in-
slowly released (2–4 mmHg/s). The first sounds syn-
creases while the SV ejection time remains constant, chronous with the pulse (Korotkoff sounds) indicate
then P s and the P d will increase by the same amount that the cuff pressure has fallen below the P s. This
(no change in PP). However, increases in the TPR nor- value is read from the manometer. These sounds first
mally lead to retardation of SV ejection and a become increasingly louder, then more quiet and
decrease in the ratio of arterial volume rise to periph- muffled and eventually disappear when the cuff pres-
eral drainage during the ejection phase. Conse- sure falls below the P d (second reading).
quently, P s rises less sharply than P d and PP Reasons for false BP readings. When re-measur-
decreases. ing the blood pressure, the cuff pressure must be
Normal range. In individuals up to 45 years of completely released for 1 to 2 min. Otherwise venous
age, P d normally range from 60 to 90 mmHg pooling can mimic elevated P d. The cuff of the sphyg-
and P s from 100 to 140 mmHg at rest (while sit- momanometer should be 20% broader than the
ting or reclining). A P s of up to 150 mmHg is diameter of the patient’s upper arm. Falsely high P d
readings can also occur if the cuff is too loose or too
considered to be normal in 45 to 60-year-old small relative to the arm diameter (e.g., in obese or
adults, and a P s of up to 160 mmHg is normal in very muscular patients) or if measurement has to be
individuals over 60 (! C). Optimal BP regula- made at the thigh.
tion (! p. 212) is essential for proper tissue
perfusion. The blood pressure in the pulmonary artery is
much lower than the aortic pressure
Abnormally low BP (hypotension) can lead to shock (! p. 186). The pulmonary vessels have rela-
(! p. 218), anoxia (! p. 130) and tissue destruction. tively thin walls and their environment (air-
Chronically elevated BP (hypertension; ! p. 216) filled lung tissue) is highly compliant. In-
also causes damage because important vessels (es- creased cardiac output from the right ventricle
pecially those of the heart, brain, kidneys and retina)
are injured. therefore leads to expansion and thus to
decreased resistance of the pulmonary vessels
The mean BP (= the average measured over (! D). This prevents excessive rises in pulmo-
time) is the decisive factor of peripheral perfu- nary artery pressure during physical exertion
sion (! p. 188). when cardiac output rises. The pulmonary ves-
sels also function to buffer short-term fluctua-
The mean BP can be determined by continuous BP
206 measurement using an arterial catheter, etc. (! A). tions in blood volume (! p. 204).
Despopoulos, Color Atlas of Physiology © 2003 Thieme
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