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SECTION VII
Respiratory Physiology
RESPIRATORY MUSCLES
Movement of the
diaphragm
accounts for 75% of the change
in intrathoracic volume during quiet inspiration. Attached
around the bottom of the thoracic cage, this muscle arches
over the liver and moves downward like a piston when it con-
tracts. The distance it moves ranges from 1.5 cm to as much as
7 cm with deep inspiration (Figure 35–9).
The diaphragm has three parts: the costal portion, made up
of muscle fibers that are attached to the ribs around the bot-
tom of the thoracic cage; the crural portion, made up of fibers
that are attached to the ligaments along the vertebrae; and the
central tendon, into which the costal and the crural fibers
insert. The central tendon is also the inferior part of the peri-
cardium. The crural fibers pass on either side of the esopha-
gus and can compress it when they contract. The costal and
crural portions are innervated by different parts of the
phrenic nerve and can contract separately. For example, dur-
ing vomiting and eructation, intra-abdominal pressure is
increased by contraction of the costal fibers but the crural
fibers remain relaxed, allowing material to pass from the
stomach into the esophagus.
The other important
inspiratory muscles
are the
external
intercostal muscles,
which run obliquely downward and for-
ward from rib to rib. The ribs pivot as if hinged at the back, so
that when the external intercostals contract they elevate the
lower ribs. This pushes the sternum outward and increases
the anteroposterior diameter of the chest. The transverse
diameter also increases, but to a lesser degree. Either the dia-
phragm or the external intercostal muscles alone can main-
tain adequate ventilation at rest. Transection of the spinal
cord above the third cervical segment is fatal without artificial
respiration, but transection below the fifth cervical segment is
not, because it leaves the phrenic nerves that innervate the
diaphragm intact; the phrenic nerves arise from cervical seg-
ments 3–5. Conversely, in patients with bilateral phrenic
nerve palsy but intact innervation of their intercostal muscles,
respiration is somewhat labored but adequate to maintain life.
The scalene and sternocleidomastoid muscles in the neck are
accessory inspiratory muscles that help to elevate the thoracic
cage during deep labored respiration.
A decrease in intrathoracic volume and forced expiration
result when the
expiratory muscles
contract. The internal
intercostals have this action because they pass obliquely
downward and posteriorly from rib to rib and therefore pull
the rib cage downward when they contract. Contractions of
the muscles of the anterior abdominal wall also aid expiration
by pulling the rib cage downward and inward and by increas-
ing the intra-abdominal pressure, which pushes the dia-
phragm upward.GLOTTIS
The abductor muscles in the larynx contract early in inspira-
tion, pulling the vocal cords apart and opening the glottis.
During swallowing or gagging, a reflex contraction of the ad-
ductor muscles closes the glottis and prevents aspiration of
food, fluid, or vomitus into the lungs. In unconscious or anes-
thetized patients, glottic closure may be incomplete and vom-
itus may enter the trachea, causing an inflammatory reaction
in the lung
(aspiration pneumonia).
The laryngeal muscles are supplied by the vagus nerves.
When the abductors are paralyzed, there is inspiratory stridor.
When the adductors are paralyzed, food and fluid enter the
trachea, causing aspiration pneumonia and edema. Bilateral
cervical vagotomy in animals causes the slow development of
fatal pulmonary congestion and edema. The edema is due at
least in part to aspiration, although some edema develops
even if a tracheostomy is performed before the vagotomy.BRONCHIAL TONE
In general, the smooth muscle in the bronchial walls aids res-
piration. The bronchi dilate during inspiration and constrict
during expiration. Dilation is produced by sympathetic dis-
charge and constriction by parasympathetic discharge. Stimu-
lation of sensory receptors in the airways by irritants and
chemicals such as sulfur dioxide produces reflex bronchocon-
striction that is mediated via cholinergic pathways. Cool air
also causes bronchoconstriction, and so does exercise, possi-
bly because the increased respiration associated with it cools
the airways. In addition, the bronchial muscles protect the
bronchi during coughing. There is a circadian rhythm in
bronchial tone, with maximal constriction at about 6:00
AM
and maximal dilation at about 6:00
PM. Many chemical sub-
stances including VIP, substance P, adenosine, and many cy-
tokines and inflammatory modulators can affect bronchial
tone, although their full roles in the physiologic regulation of
bronchial tone is still unsettled.
FIGURE 35–8
Volume of gas expired by a normal adult man
during a forced expiration, demonstrating the FEV
1
and the total
vital capacity (VC).
(Reproduced, with permission, from Crapo RO: Pulmonary-
function testing. N Engl J Med 1994;331:25. Copyright © 1994, Massachusetts Medical
Society.)
6 5 4 3 2 10 12345678910VC
Volume (L)FEV^1Time (s)