Human Physiology, 14th edition (2016)

Cardiac Output, Blood Flow, and Blood Pressure 477

Baroreceptor Reflex

In order for blood pressure to be maintained within limits, spe-
cialized receptors for pressure are needed. These baroreceptors
are stretch receptors located in the aortic arch and in the carotid
sinuses. The baroreceptors are tonically (constantly) active, produc-
ing a baseline frequency of action potentials in their sensory neu-
rons. When blood pressure is increased the walls of the aortic and
carotid sinuses stretch, and this produces an increased frequency
of action potentials along their sensory nerve fibers ( fig. 14.26 ). A
fall in blood pressure below the normal range, by contrast, causes
a decreased frequency of action potentials in these sensory fibers.
Sensory nerve activity from the baroreceptors ascends via
the vagus (X) and glossopharyngeal (IX) nerves to the medulla
oblongata, which directs the autonomic system to respond
appropriately. The vasomotor control center in the medulla
regulates the degree of vasoconstriction/vasodilation, and hence
helps to regulate total peripheral resistance. The cardiac control
center in the medulla regulates the cardiac rate ( fig. 14.27 ).

Clinical Investigation CLUES

Mark trained for running marathons and, because he

didn’t drink properly, became dehydrated.

• How would dehydration influence blood volume,
  stroke volume, and cardiac output?


• Through what means would dehydration affect
  Mark’s blood pressure, and in what way?

Figure 14.26 The effect of blood pressure on the

baroreceptor response. This is a recording of the action

potential frequency in sensory nerve fibers from baroreceptors

in the carotid sinus and aortic arch. As the blood pressure

increases, the baroreceptors become increasingly stretched.

This results in a higher frequency of action potentials transmitted

to the cardiac and vasomotor control centers in the medulla

oblongata.

180

160

140

Mean arterial pressure (mmHg)

120

100

Action

potentials

80

60

40

Time

Resting

potential

Arch of aorta

Sensory neuron

Sympathetic neuron

Parasympathetic neuron

AV node

Common carotid artery

Carotid sinus

Cerebrum

Hypothalamus

Medulla
oblongata

Spinal cord

Sympathetic

cardiac nerve

Sympathetic chain

SA node

Parasympathetic

vagus nerve

Sensory

fibers

Figure 14.27 Structures involved

in the baroreceptor reflex. Sensory

stimuli from baroreceptors in the carotid

sinus and the aortic arch, acting via control

centers in the medulla oblongata, affect the

activity of sympathetic and parasympathetic

nerve fibers in the heart.