Human Physiology, 14th edition (2016)

Respiratory Physiology 555

bodies send sensory information to the medulla in the vagus

nerve (X); the carotid bodies stimulate sensory fibers in the

glossopharyngeal nerve (IX). The neural and sensory control

of ventilation is summarized in figure 16.26.

Effects of Blood P CO

2

and pH

on Ventilation

Chemoreceptor input to the brain stem modifies the rate and

depth of breathing so that, under normal conditions, arterial

P^ CO 2 , pH, and P^ O 2 remain relatively constant. If hypoventilation

(inadequate ventilation) occurs, P^ CO 2 , quickly rises and pH falls.

The fall in pH occurs because carbon dioxide can combine with

water to form carbonic acid, which, as a weak acid, can release

H^1 into the solution. This is shown in these equations:

C O^2  1 H^2 O → H^2 C O^3

H^2 C O^3 → H 1 1 HC O^32

The oxygen content of the blood decreases much more

slowly because of the large “reservoir” of oxygen attached to

hemoglobin. During hyperventilation, conversely, blood P^ CO 2

quickly falls and pH rises because of the excessive elimina-

tion of carbonic acid. The oxygen content of blood, on the

other hand, is not significantly increased by hyperventilation

(because hemoglobin in arterial blood is 97% saturated with

oxygen even during normal ventilation).

around the aortic arch, and the carotid bodies, located in each
common carotid artery at the point where it branches into the
internal and external carotid arteries ( fig.  16.25 ). The aortic
and carotid bodies should not be confused with the aortic and
carotid sinuses (chapter 14, section 14.6) that are located within
these arteries. The aortic and carotid sinuses contain receptors
that monitor the blood pressure.
The peripheral chemoreceptors control breathing indi-
rectly via sensory nerve fibers to the medulla. The aortic

Figure 16.25 Sensory input from the aortic and
carotid bodies. The peripheral chemoreceptors (aortic and
carotid bodies) regulate the brain stem respiratory centers by
means of sensory nerve stimulation.

Carotid body

Aorta

Sensory nerve fibers

(in glossopharyngeal nerve)

Sensory nerve fibers

(in vagus nerve)

Aortic bodies

Heart

Common

carotid artery

Carotid sinus

Figure 16.26 The regulation of ventilation by the

central nervous system. The feedback effects of pulmonary

stretch receptors and “irritant” receptors on the control of

breathing are not shown in this flowchart.

Motoneurons to

respiratory muscles

Pneumotaxic

center

Apneustic

center

Chemoreceptors

Aortic and carotid

bodies

Chemoreceptors

Medulla oblongata

Automatic breathing

Cerebral cortex

Pons

Medulla oblongata

Spinal cord

Voluntary breathing