Human Physiology, 14th edition (2016)

478 Chapter 14

carotid sinus, a procedure sometimes employed by physicians

to reduce tachycardia and lower blood pressure, also evokes

this reflex. Such carotid massage should be used cautiously,

however, because the intense vagus-nerve-induced slowing of

the cardiac rate could cause loss of consciousness (as occurs in

emotional fainting). Manual massage of both carotid sinuses

The baroreceptor reflex consists of (1) the aortic arch
and carotid sinus baroreceptors as the sensors; (2) the vasomo-
tor and cardiac control centers of the medulla oblongata as the
integrating centers; and (3) parasympathetic and sympathetic
axons to the heart and blood vessels as the effectors. Acting
through the baroreceptor reflex, a fall in blood pressure evokes
an increase in sympathetic nerve activity while the activity of
the parasympathetic division decreases. As a result, there is a
compensatory increase in cardiac output and total peripheral
resistance. Conversely, a rise in blood pressure will produce a
decline in sympathetic nerve activity while the activity of the
parasympathetic division increases. As a result, a rise in blood
pressure will evoke a reduction in cardiac output and total
peripheral resistance.
The baroreceptor reflex helps maintain normal blood
pressure on a beat-to-beat basis (longer-term regulation of
blood pressure is achieved by the kidneys, through regulation
of blood volume). The reflex is somewhat more sensitive to
decreases in pressure than to increases, and is more sensitive
to sudden changes in pressure than to more gradual changes.
A good example of the importance of the baroreceptor reflex
in normal physiology is its activation whenever a person goes
from a lying to a standing position.
When a person goes from a lying to a standing position,
there is a shift of 500 to 700 ml of blood from the veins of the
thoracic cavity to veins in the lower extremities, which expand
to contain the extra volume of blood. This pooling of blood
in the lower extremities reduces the venous return and cardiac
output, but the resulting fall in blood pressure is almost imme-
diately compensated for by the baroreceptor reflex. A decrease
in baroreceptor sensory information, traveling in the glosso-
pharyngeal nerve and the vagus nerve to the medulla oblongata,
inhibits parasympathetic activity and promotes sympathetic
nerve activity. This produces an increase in cardiac rate and
vasoconstriction, which help to maintain an adequate blood
pressure upon standing ( fig. 14.28 ).
Input from baroreceptors can also mediate the opposite
response. When the blood pressure rises above an individ-
ual’s normal range, the baroreceptor reflex causes a slowing
of the cardiac rate and vasodilation. Manual massage of the

Figure 14.28 The negative feedback control of

blood pressure by the baroreceptor reflex. This reflex helps

to maintain an adequate blood pressure upon standing.

Stimuli

Negative feedback

response

Going from laying

to standing position

Venous

return

End-diastolic

volume

Stroke

volume

Cardiac

output

Sensory neurons

Sympathetic

Parasympathetic

Cardiac

rate

Cardiac

output

Vasoconstriction

of arterioles

Total peripheral

resistance

Blood pressure

Baroreceptors

Medulla oblongata

Sensor

Integrating center

Effector

Blood pressure

CLINICAL APPLICATION

Orthostatic (or postural ) hypotension is a lowering of blood

pressure upon standing that causes a person to feel dizzy

and weak, and in extreme cases can even cause fainting ( syn-

cope ). Normally, the baroreceptor reflex compensates for the

fall in blood pressure when a person stands, which causes

about 700 mL of blood to pool in the lower limbs. However,

if a person has low blood pressure because of dehydration,

medications (such as beta-adrenergic receptor blockers), or

any other cause—including postprandial hypotension among

the elderly, where the pressure falls after eating—orthostatic

hypotension can result.

Clinical Investigation CLUES

Mark experienced dizziness upon standing after he engaged

in prolonged exercise without adequate drinking.

• What caused the dizziness upon standing, and how
  is this normally prevented?


• What condition did Mark have when he got dizzy,
  and what caused it?