Download CHS 115-125

The Thorax
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The trachea or w · d · ·
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111 plpe, IS a tu ular outgrowth of the gut. lt b gin
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as a u rom the floor of the h
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P arynx an grows ta1lwa rd into the rib
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cage. e.re It fork~,. a~d e~ch fork (called a primary bronchus) keep
~n growtng and d1~1d~ng hke a branching tree, re ulting in the formation of two lungs
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. tnstde the rib cage· As the emb ryomc
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out rom t~e ta1l end of the trachea, the celom wrap around them.
<:overed With a film of celomic lining, the lungs expand around either
stde of the heart and press against the upper abdominal viscera.
The a?dominal and. thoracic parts of the celom are broadly confluent Wit? each other m embryonic life (Fig. 8-1 ). ln amphibians and
most reptiles, they remain confluent in the adult, which thus has only
two celomic cavities-the pericardia! cavity and a single, big celomic
space that envelops the abdominal viscera (and sends left and right
extensions up on either side of the heart to wrap around each lung).
In mammals, however, the thorax and abdomen are separated in the
adult by a muscular partition called the diaphragm. An adult mammal thus has at least four celomic cavities-one around the heart, one
around each lung, and one more in the abdomen. (Human males also
have two more in the scrotum, surrounding the testes.)
Like most mammalian peculiarities, the diaphragm is associated
with a high metabolic rate. It permits a more rapid and efficient
turnover of respiratory gasses. Primitive air-breathing fish, like modern lungfish or frogs, had to rely on swallowing movements to gulp
air into their lungs. Early reptiles evolved an improved method of
breathing by suction, known as thoracic respiration. The breakthrough that made thoracic respiration possible was the appearance
of a true rib cage, produced by stretching some of the thoracic ribs
down to touch the sternum. The ribs that extended to the sternum
had a joint at each end, and they could rotate around the axis defined
by the two joints. When a typical reptile inhales, its ribs swing toward
its head, with each rib rotating around its vertebral and sternal ends.
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Fig. 8-1 Subd iviSIOn of the
embryonic celom. A Schematic cross section through
the heart of a five-week embryo. The pleura l and
pericard ia! cavities are
broad ly open into each other
and the abdominal celom.
B. Similar section at seven
weeks: the pleuropericard ial
fo lds have fused dorsal to the
heart, separating pericardia!
and pleura l cavities. A small
open ing into the abdominal
celom persists on the right;
this would normally close during the sixth week. C. Diagram
of the six celomic spaces in a
male adult. H. heart; L, lung;
T, testis.
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THE THORAX AN 0
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pericard ia! cav ity
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serous pericardium
Pleura: parietal
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THETHORA '
This movement, which is usually ompared to lifting the handle on a
bucket (Fig. 8-2), increase the side-to--i de diam ter of the th rax and
thus increa es thoracic volume. Air ru he into the lungs t fi ll thi
added volume. The inhaled air is pu hed out again by dropping the
costal "bucket handles" back toward the vertebral column again.
Hug your rib cage, inhale deeply, and o b erve where and h \ V 6ur
thorax expands to draw air in.
The pumplike action of thoracic r spiration wa an improv ment
over gulping air like a fi h. But it wa not very efficient by itself
because part of the thoracic volume was fill ed on inhalation by th
stomach, liver, and other abdomin al viscera lithering up into the
thorax. You can get some idea of how thi affects re piratory volume
by contracting your abdominal muscle harply (' ' ucking in your
gut") when you inhale, thus shoving your stomach and liver up into
your rib cage. Some device was needed to make the e vi era ray put
so that the volume added by lifting the ribs would be wholly filled by
inrushing air.
In most modern reptiles, this problem h a been handled by introducing a sheet of elastic connective tis ue stretched across the
abdominal end of the rib cage. When the ribs swing outward in
inhalation, this partition becomes taut, preventing the abdominal
viscera from moving up into the thorax. Thus, all the volume added
to the thorax by the outward swing of the ribs contributes to the
expansion of the lungs instead of being partly wasted in sliding the
stomach and liver back and forth.
Fig. 8-2 Rib movements in breathing. When we
1n ale. our ribs swing out and sideways (B) like
he hand le on a bucket (A). They also rotate
arou d an axis determ ined by each rib 's two
.o'n s wi h the vertebrae (C), wh ich causes the
thorax o expand ventrally as well (dotted lines).
c
B
A
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THE THORAX AND ITS VI SCERA
In mammals, this arrangement is still fu_rther improved by making
the partition out of voluntary muscle. Th1s muscular dome, the diaphragm, forms the floor of the human thorax (Fig. 8-3). When it
contracts, it flattens out, increasing the volume of the thorax and
shoving the abdominal viscera directly below it down toward the
pelvis. Mammals can therefore breathe without moving their nb
merely by contracting and relaxing the diaphragm. Thi kind of
breathing is called abdominal respiration. The acquisition of a dia.
phragm in mammals probably explains why mammals lack rib ir
their abdominal body wall ; the abdomen must be able to bulge our
freely to accommodate the viscera that are shoved tailward when the
diaphragm contracts.
Abdominal breathing is characteristic of human babies. Babie,·
ribs are mostly cartilaginous; and they lie in a more horizontal plane,
so the "bucket handles" are already fixed in the lifted position and
cannot be lifted much to expand the thorax. In adults, the ribs have
the bony rigidity and the orientation needed to transmit and with·
Ag. 8-3 Midline section of diaphragm (black)
seen from the left side, showing the vertebral
levels at which it is pierced by the inferior vena
cava (IVC), esophagus (ES), and aorta (AO). The
heart (H) is shown as a dotted outline.
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THE THORAX
stand the stresses of thoracic breathing, so both abdominal and thoracic breathing are normally combined in each inh alation.
• Anatomy of the Human Diaphragm
The diaphragm is a sheet of hypaxial muscle, derived mostl y from
cervical body segments. Its muscle fibers can be tho ught of as the
missing parts of the incomplete layer of body-wall muscles in the
neck, torn away and carried tailward by the expanding lungs as they
grew down from the pharynx. In the adult, these o riginally cervical
fibers are attached all around the caudal margin of the rib cage (including the lower end of the sternum) and to the top three lumbar
vertebrae. From this ring-shaped origin, the diaphragm 's muscle
fibers converge on a tendinous patch lying just below (ca udal to) the
heart. The diaphragm is therefore dome-shaped, with the heart sitting
atop the apex of the dome (Fig. 8-3). The dome's tendinous apex is
called the central tendon of the diaphragm. It is roughly V-shaped,
with the legs of the V extending dorsally around either side of the
vertebral column. This tendinous part of the diaphragm is derived
from the septum transversum (Fig. 9-4 ), a mass of unsegmented
mesoderm that started out as a partition between the amniotic cavity
and the yolk sac back when the heart was sitting out in front of the
embryonic head. The heart remains attached to the central tendon
indirectly via ligaments binding the fibrous pericardium to the apex of
the V, so the heart rides up and down on the diaphragm as we
breathe.
When the diaphragm is at rest, the apex of the central tendon lies in
front of vertebra T.8 or T.9; in deep inhalation, it may fall as low as
T.ll, carrying the heart with it. Because the diaphragm's ventral
attachments are at sternal level (about T.9) and its dorsal attachments
are to the lowest ribs and upper lumbar vertebrae, the dorsal surface
of the diaphragm slopes steeply tail ward (Fig. 8-3 ).
The aorta, inferior vena cava, and gut run longitudinally between
thorax and abdomen, so they must pass through holes in the diaphragm. The descending aorta, lying just ventral to the vertebral
column, squeezes through between the vertebral bodies and the edge
of the diaphragm. The diaphragm's edge is formed here by a tendinous arch over the aorta, the median arcuate ligament, from which
some muscle fibers of the diaphragm take origin (Fig. 8-4). T either
side of this median Ligament, the diaphragm is thickened to form a
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THE THORAX AND IT S VI SC ERA
caval opening (IVC)
Fig. 8-4 Lower surface of d iaphragm and its
attachments. IVC, inferior vena cava.
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Arcuate ligaments:
central tendon
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1. median
___ esophag eal opening
12th rib
crura
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pair of muscular pillars, the left and right crura (L., " legs"). The cru··
originate from the lumbar vertebral bodies and fan out craniad
insert into the central tendon. Because the heart rides on the centr
tendon and the inferior vena cava passes directly into the right atriur
of the heart, the caval opening for that vein lies in the tendon ju t to
the right of the midline. The third big opening in the diaphragm lie
just outside the central tendon, cradled in the notch of th e V. Through
it passes the esophagus, the thoracic part of the gut that connect th
pharynx with the stomach. The esophageal opening lies on the t l:t
side, where the stomach sits. The muscle fibers of the diaphragm·
right crus, oddly enough, divide and swing over to the left to form 3
loop around the esophagus. This loop may have some limited sphin teral action on the esophagus. Two longitudinal hypaxial rnu d e
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THETHORAX
lying alongside the vertebral column pass between the body wall and
the edge of the diaphragm and are therefore (like the aorta) bridged
by tendinous arches from which diaphragmatic fibers arise (Fig. 8-4):
the medial (not median) arcuate ligament bridging the hind-limb muscle Psoas Major and the lateral arcuate ligament bridging Quadratus
Lumborum. All these dorsal tendinous arches along the back edge of
the diaphragm are tacked down to the bodies and transverse processes of the upper lumbar vertebrae.
• The Lungs and Pleura
The trachea lies in the midline of the thorax, ventral to the esophagus
and dorsal to the ascending aorta and the three great branches from
the peak of the aortic arch. The trachea ends by dividing into the two
primary bronchi, each of which subdivides further into secondary and
tertiary bronchial branches inside each lung. The resulting "bush" is
known as the respiratory tree. Its surface area is huge, providing
roughly 100 square meters of respiratory epithelium for gas exchange. The lung is composed of the respiratory tree and the highly
vascular mesodermal tissues that surround it. As the fetal lungs form
around the proliferating bronchi, branches of the great pulmonary
vessels grow into each lung and branch along with the branches of the
respiratory tree, getting ready to flood the lung's tissues with right
ventricular blood and drain oxygenated blood back to the left atrium
in postnatal life. The arterial branches generally run in front of (ventral to) the branches of the bronchial tree. The upper (superior) pulmonary vein on each side enters the lung in front of the primary
bronchus and drains the ventral parts of the lung; the lower (inferior)
vein runs behind the bronchus and drains the more dorsal parts. As
usual, the veins are more likely than the arteries to deviate from the
standard setup; there may be one or three veins on either side. The
point where the bronchus and vessels disappear into the substance of
the lung is called the root of the lung.
Because the heart lies mostly to the left of the midline, the left lung
has less room on its side and is smaller than the right lung. This
difference between the two lungs shows up in the respiratory tree.
The right primary bronchus divides into three secondary bronchi, but
its counterpart on the left has only two secondary branches.
The celomic space surrounding each lung is called a pleural cavity.
The two pleural cavities are separated by the mass of structures in the
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THE T HOR A ,'
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lung substance around its branches, forming little sub-lung called
lobes. The divisions between them are visible grossly beca use the
visceral pleura dips in between them. There are, of course, three lobes
in the right lung and two in the left, corresponding to the number of
secondary bronchi in each. Although each tertiary bronchus al o
supplies its own separate subdivision of a lobe, these smaller unit or
bronchopulmonary segments are not separated by folds of visceral
pleura the way the lobes are. They therefore cannot be seen in disse tion. The usual arrangement of tertiary bronchi is shown in Figure
8-7. It is of considerable clinical importance, because infection o r
obstructions may affect only a single tertiary bronchus and hence ma y
be confined to a single bronchopulmonary segment.
The various parts of the parietal pleura are named for the part of
the thoracic walls to which they adhere-diaphragmatic pleura over
the upper (thoracic) surface of the diaphragm, costal pleura over the
inside of the rib cage, and mediastinal pleura lying against the mediastinum. The parietal pleura also bulges out a little through the superior
thoracic outlet, forming a dome known as the cupola of the pleura.
This bulging dome is protected by the clavicle in front and by the
head of the first rib in back and is upported by the apex of the lung
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THE THORAX AND ITS VI SCER A
Fig. 8-7 Lungs, lobes. and segments. U, upper
lobe; M, middle lobe; L, lower lobe. Bronchopulmonary segments: 1, apical ; 2, apicoposterior;
3, an terior; 4. lateral ; 5, medial ; 6. posterior;
7, superior: 8, anterior basal; 9, latera l basal ;
10. medial basal ; 11 , posterior basal ; 12, inferior
lingular; 13, superior lingular
LATERAL
ANTERIOR
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oblique fissure
horizontal
fissure
- . . - - - - MEDIAL
oblique
fissure
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RIGHT LUNG
LEFT LUNG
pressing against it from underneath. Because the lung cannot exp nd
upward past this pleural cupola during inhalation, it must expand
downward. It does this by sliding into and filling the potential space
between the costal and diaphragmatic pleura when the diaphragm
descends. This potential space between the lower ribs and the sloping
dorsal face of the diaphragm is called the pleural recess. The function
of the pleura is to lubricate the movements of the lungs as they slide
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THE THORAX
up and down inside the rigid rib cage with each cycle of breathing.
When infection or scarring leaves the inside of the pleural sac less
slippery, there may be chafing between visceral and parietal pleura,
whereupon breathing becomes painful. This condition (or any other
inflammation of the pleura) is known as pleurisy.
• Mechanics of Breathing
During quiet breathing, the diaphragm is the most important respiratory muscle in adult human beings. The role of the Intercostals is
debated. Electromyographic studies suggest that the Scaleni may be
more important than the lntercostals in lifting the ribs during inspiration, especially during deep inspiration. The Scaleni, which represent
the remnants of the lateral body-wall layers in the neck, extend from
the cervical transverse processes down to the first two ribs. The first
costal cartilage has no synovial joint with the sternum but is joined
directly to the manubrium by a synchondrosis. Therefore, when the
Scaleni contract, the first ribs and the sternum swing forward and
upward as a single unit. Anteroposterior diameter of the rib cage is
increased directly by this means and by rotation of each rib on its two
vertebral facets (Fig. 8-2). The indirect pull exerted by the Scaleni on
the other ribs (via the intercostal muscles) raises the costal "bucket
handles" and thus increases the transverse diameter of the thorax.
The Intercostals help to lift the ribs but do not draw them closer
together. If you press your left fingertips to the right side of your neck
in front, just above the first rib, you may be able to feel the Scaleni
contract when you inhale deeply.
In forced inhalation or exhalation, all the muscles attaching to or
wrapping around the rib cage can come into play. You can easily feel
the lateral muscles of your own abdominal wall contracting when you
cough; this helps push the diaphragm upward and anchors the lower
ribs so that the Internal Intercostals can draw the ribs downward. A
vigorous cough produces a sudden increase in intraabdominal pressure and so may precipitate a rupture of a loop of intestine through
one of the weaker spots in the abdominal wall. Some of the limb
muscles that originate from the rib cage also aid in forced exhalation,
as you can observe by coughing with your hand thrust into your
armpit and feeling the surrounding shoulder muscles contract.