Human Physiology, 14th edition (2016)

Cardiac Output, Blood Flow, and Blood Pressure 479

Measurement of Blood Pressure

The first documented measurement of blood pressure was

accomplished by Stephen Hales (1677–1761), an English

clergyman and physiologist. Hales inserted a cannula into the

artery of a horse and measured the heights to which blood

would rise in the vertical tube. The height of this blood col-

umn bounced between the systolic pressure at its highest and

the diastolic pressure at its lowest, as the heart went through

its cycle of systole and diastole. Modern clinical blood pres-

sure measurements, fortunately, are less direct. The indirect,

or auscultatory, method is based on the correlation of blood

pressure and arterial sounds first described by the Russian

physician Nicolai Korotkoff in 1905.

In the auscultatory method, an inflatable rubber bladder

within a cloth cuff is wrapped around the upper arm, and a stetho-

scope is applied over the brachial artery ( fig. 14.29 ). The artery is

normally silent before inflation of the cuff, because blood travels

smoothly through the arteries. Laminar flow occurs when all

parts of a fluid move in the same direction, parallel to the axis

of the vessel. The term laminar means “layered”—blood in the

central axial stream moves the fastest, and blood flowing closer

to the artery wall moves more slowly. If flow is perfectly laminar,

there is no transverse movement between these layers that would

produce mixing. The blood flows smoothly and doesn’t produce

vibrations of the artery wall that would cause sounds. By con-

trast, turbulent flow is when some parts of the fluid move in

different directions, churning and mixing the blood. Turbulent

flow causes vibrations of the vessel, which may produce sounds.

Before the blood pressure cuff is inflated, blood flow in the bra-

chial artery has very little turbulence and so is silent.

When the artery is pinched, however, blood flow through

the constriction becomes turbulent. This causes the artery to

produce sounds, much like the sounds produced by water flow-

ing through a kink in a garden hose. The ability of the cuff pres-

sure to constrict the artery is opposed by the blood pressure. So

the cuff pressure must be greater than the diastolic pressure to

constrict the artery during diastole. If the cuff pressure is also

simultaneously can even cause cardiac arrest in susceptible
people.

Atrial Stretch Reflexes

In addition to the baroreceptor reflex, several other reflexes
help to regulate blood pressure. The reflex control of ADH
release by osmoreceptors in the hypothalamus, and the control
of angiotensin II production and aldosterone secretion by the
juxtaglomerular apparatus of the kidneys, have been previously
discussed. Antidiuretic hormone and aldosterone increase blood
pressure by increasing blood volume, and angiotensin II stimu-
lates vasoconstriction to cause an increase in blood pressure.
Other reflexes important to blood pressure regulation are
initiated by atrial stretch receptors located in the atria of
the heart. These receptors are activated by increased venous
return to the heart and, in response (1) stimulate reflex tachy-
cardia, as a result of increased sympathetic nerve activity;
(2) inhibit ADH release, resulting in the excretion of larger
volumes of urine and a lowering of blood volume; and (3) pro-
mote increased secretion of atrial natriuretic peptide (ANP).
The ANP, as previously discussed, lowers blood volume by
increasing urinary salt and water excretion.

FITNESS APPLICATION

The Valsalva maneuver is a bearing down, as if attempt-

ing a forceful exhalation, while preventing air from escaping

through the mouth and nose. This raises intrathoracic pres-

sure, compressing thoracic veins and reducing the venous

return to the heart. It also briefly raises the aortic pressure,

causing a slowing of the heart via the baroreceptor reflex.

However, the fall in venous return decreases cardiac out-

put, so that aortic blood pressure then falls, stimulating the

baroreceptor reflex and causing the cardiac rate to increase.

When the person again breathes, the thoracic pressure falls

and the aortic pressure briefly falls (reflexively producing an

increase in cardiac rate) before the improved venous return

raises aortic pressure and (via the baroreceptor reflex) slows

the cardiac rate. Weightlifters attempting to lift a heavy load

while holding their breaths, and people straining at stools,

also perform this Valsalva maneuver, which can be danger-

ous in those with coronary heart disease.

Clinical Investigation CLUES

After Mark was diagnosed with essential hypertension,

his physician advised him not to hold his breath when

lifting heavy weights.

• What sequence of events occurs when a person lifts
  a heavy weight while holding the breath?


• How is the baroreceptor reflex involved in the these
  events?
  Figure 14.29 A pressure cuff and
  sphygmomanometer are used to measure blood
  pressure. The examiner is listening for the Korotkoff sounds.