Ganong's Review of Medical Physiology, 23rd Edition

CHAPTER 37Regulation of Respiration 637

increase in O 2 extraction from each unit of blood (see Figure
36–3). Because this increase is accompanied by a 30-fold or
greater increase in blood flow, it permits the metabolic rate of
muscle to rise as much as 100-fold during exercise.

EXERCISE TOLERANCE & FATIGUE

What determines the maximum amount of exercise that can
be performed by an individual? Obviously, exercise tolerance
has a time as well as an intensity dimension. For example, a fit
young man can produce a power output on a bicycle of about
700 watts for 1 min, 300 watts for 5 min, and 200 watts for 40
min. It used to be argued that the limiting factors in exercise
performance were the rate at which O 2 could be delivered to
the tissues or the rate at which O 2 could enter the body in the
lungs. These factors play a role, but it is clear that other factors
also contribute and that exercise stops when the sensation of
fatigue progresses to the sensation of exhaustion. Fatigue is
produced in part by bombardment of the brain by neural im-
pulses from muscles, and the decline in blood pH produced by
lactic acidosis also makes one feel tired, as do the rise in body
temperature, dyspnea, and, perhaps, the uncomfortable sensa-
tions produced by activation of the J receptors in the lungs.

CHAPTER SUMMARY

■ Breathing is under both voluntary control (located in the cere-

bral cortex) and automatic control (driven by pacemaker cells in

the medulla). There is a reciprocal innervation to expiratory and

inspiratory muscles in that motor neurons supplying expiratory

muscles are inactive when motor neurons supplying inspiratory

muscles are active, and vice versa.

■ The pre-Bötzinger complex on either side of the medulla con-

tains synaptically coupled pacemaker cells that allow for rhyth-

mic generation of breathing. The spontaneous activity of these

neurons can be altered by neurons in the pneumotaxic center,

although the full regulatory function of these neurons on nor-

mal breathing is not understood.

■ Breathing patterns are sensitive to chemicals in the blood

through activation of respiratory chemoreceptors. There are

chemoreceptors in the carotid and aortic bodies and in collec-

tions of cells in the medulla. These chemoreceptors respond to

changes in PO 2 and PCO 2 as well as H+ to regulate breathing.

■ Receptors in the airway are additionally innervated by slowly

adapting and rapidly adapting myelinated vagal fibers. Slowly

adapting receptors can be activated by lung inflation. Rapidly

adapting receptors, or irritant receptors, can be activated by chem-

icals such as histamine and result in cough or even hyperpnea.

■ Receptors in the airway are also innervated by unmyelinated va-

gal fibers (C fibers) that are typically found next to pulmonary

vessels. They are stimulated by hyperinflation (or exogenous

substances including capsaicin) and lead to the pulmonary

chemoreflex. The physiologic role for this response is not fully

understood.

MULTIPLE-CHOICE QUESTIONS

For all questions, select the single best answer unless otherwise directed.

1. The main respiratory control neurons
   A) send out regular bursts of impulses to expiratory muscles
   during quiet respiration.
   B) are unaffected by stimulation of pain receptors.
   C) are located in the pons.
   D) send out regular bursts of impulses to inspiratory muscles
   during quiet respiration.
   E) are unaffected by impulses from the cerebral cortex.


2. Intravenous lactic acid increases ventilation. The receptors
   responsible for this effect are located in the
   A) medulla oblongata.
   B) carotid bodies.
   C) lung parenchyma.
   D) aortic baroreceptors.
   E) trachea and large bronchi.


3. Spontaneous respiration ceases after
   A) transection of the brain stem above the pons.
   B) transection of the brain stem at the caudal end of the
   medulla.
   C) bilateral vagotomy.
   D) bilateral vagotomy combined with transection of the brain
   stem at the superior border of the pons.
   E) transection of the spinal cord at the level of the first thoracic
   segment.


4. The following physiologic events that occur in vivo are listed in
   random order: (1) decreased CSF pH; (2) increased arterial
   PCO 2 ; (3) increased CSF PCO 2 ; (4) stimulation of medullary
   chemoreceptors; (5) increased alveolar PCO2.
   What is the usual sequence in which they occur when they
   affect respiration?
   A) 1, 2, 3, 4, 5
   B) 4, 1, 3, 2, 5
   C) 3, 4, 5, 1, 2
   D) 5, 2, 3, 1, 4
   E) 5, 3, 2, 4, 1


5. The following events that occur in the carotid bodies when they
   are exposed to hypoxia are listed in random order: (1) depolar-
   ization of type I glomus cells; (2) excitation of afferent nerve
   endings; (3) reduced conductance of hypoxia-sensitive K+ chan-
   nels in type I glomus cells; (4) Ca2+ entry into type I glomus
   cells; (5) decreased K+ efflux.
   What is the usual sequence in which they occur on exposure
   to hypoxia?
   A) 1, 3, 4, 5, 2
   B) 1, 4, 2, 5, 3
   C) 3, 4, 5, 1, 2
   D) 3, 1, 4, 5, 2
   E) 3, 5, 1, 4, 2


6. Stimulation of the central (proximal) end of a cut vagus nerve
   would be expected to
   A) increase heart rate.
   B) stimulate inspiration.
   C) inhibit coughing.
   D) raise blood pressure.
   E) cause apnea.