Download Thoracic wall and pleural cavities

Thoracic wall and pleural cavities
<< lower respiratory tract
The skeletal framework of the thoracic wall is designed in such a way that it appears like a bony cage which protects the vital organs (heart and lungs) placed within this cage. The skeletal
elements of the thoracic wall consist of twelve thoracic vertebrae posteriorly, twelve pairs of
ribs, and a sternum anteriorly. The ribs make joints ( the costovertebral joints) with all the
thoracic vertebrae posteriorly and are attached to the sternum anteriorly with the help of costal
cartilages (the
cho
ndrosternal joints
).
The muscles of the thoracic wall include those that fill and support the intercostal spaces, those
that pass between the sternum and the ribs, and those that cross several ribs between costal
attachments. The muscles of the thoracic wall, together with the muscles between the vertebrae
and ribs posteriorly alter the position of the ribs and sternum and so change thoracic volume
during breathing. They also reinforce the thoracic wall.
The intercostal muscles are the flat muscles found in each intercostal space that pass between
adjacent ribs. They provide structural support for the intercostal spaces during breathing. They
can also move the ribs.
The external intercostal muscles: The eleven pairs of external intercostal muscles extend
from the inferior edges of the ribs above to the superior surfaces of the ribs below. The muscle
fibers pass obliquely anteroinferiorly. These muscles are most active during inspiration.
The internal intercostal muscles: The eleven pairs of internal intercostal muscles pass
1/4
Thoracic wall and pleural cavities
between the most inferior lateral edge of the costal grooves of the ribs above, to the superior
surface of the ribs below. The muscle fibers pass in the opposite direction to those of the
external intercostal muscles, i.e. the fibers pass posteroinferiorly. These muscles are most
active during expiration.
The Diaphragm is a dome-shaped musculotendinous structure that fills the inferior thoracic
aperture and seperate the thoracic cavity from the abdominal cavity. Its covex upper surface
forms the floor of the chest cavity while, the concave under surface of the dome forms the roof
of the abdominal cavity.
The diaphragm is composed of two portions, a muscular peripheral portion and a tendinous
central portion
. The peripheral muscular fibers (sternal, costal, and the vertebral) converge radially from their
origins towards the strong, sheet-like aponeurosis called the central tendon which is fused with
the inferior surface of the fibrous pericardium.
The diaphragm receives its motor innervation by the right and left phrenic nerves (fibers from
third, fourth, and fifth cervical spinal nerves).
Movements of the thoracic wall and Diaphragm during breathing
One of the principal functions of the thoracic wall and the diaphragm is to alter the volume of the
thorax and thereby move air in and out of the lungs.
Elevation and depression of the diaphragm significantly alter the vertical dimensions of the
thorax. Depression results when the muscle fibers of the diaphragm contract. Elevation results
when the fibers relax.
Changes in the anteroposterior and lateral dimensions result from elevation and depression of
the ribs. Because th anterior ends of the ribs are inferior to the posterior ends, when the ribs are
elevated, they move the sternum upward and forward.When the ribs are depressed, the
sternum moves downward and backward. This ' pump handle' type of movement changes the
2/4
Thoracic wall and pleural cavities
dimensions of the thoracic cavity in the anteroposterior direction. As well as the anterior ends of
the ribs being lower than the posterior ends, the middles of the shafts tend to be lower than the
two ends. When the shafts are elevated, the middles of the shafts move laterally. This
bucket handle
' movement increases the lateral dimensions of the thorax.
PLEURAL CAVITIES AND PLEURA
Two pleural cavities, one on either side of the mediastinum, surround the lungs. Superiorly, they
extend above the first rib into the root of the neck. Inferiorly, they extend to a level just above
the costal margin. The medial wall of each pleural cavity is the mediastinum. Each pleural cavity
is lined by a single layer of flat cells, mesothelium, and an associated layer of supporting
connective tissue; together they form the pleura
The pleura is a thin, double layered serosa. The parietal pleura lines the thoracic wall and the
superior surface of the diaphragm. It continues around the heart and between the lungs, forming
the lateral walls of the mediastinal enclosure and snugly enclosing the root of the lung. From the
root, the pleura extends as the
vi
sceral, or pulmonary, pleura
to cover the external lung surface, dipping into the lining and fissures. The pleurae produce the
pleural fluid that fills the slitlike
pleural cavity
between them. This lubricating serous secretion allows the lungs to glide easily over the thorax
wall during breathing movements. The pleurae also divide the thoracic cavity into three seperate
chambers - the
central mediastinum
and the two
lateral pleural compartments
, each containing a lung. this compartmentalization helps prevent one mobile organ (heart or
lung) from interfering with another. It also limits the spread of local infections.
3/4
Thoracic wall and pleural cavities
>> Mechanics of Respiration
4/4