b. Located in the middle of the thorax, between the two pleural cavities, is the
mediastinum ("I stand between"). The mediastinum is a tissue- and organ-filled space.
Within it, the heart (of the blood circulatory system) is located at the same level as the
7-36. LUNG STRUCTURE
The two lungs occupy their respective sides of the thoracic cavity.
a. The left lung tends to be smaller. This makes room for the extension of the
heart into the left side of the thorax.
b. In general, the right lung is divided into three major lobes. The left lung is in
two major lobes.
c. Due to the branching pattern of the respiratory tree (and associated NAVL),
each lung consists of broncho pulmonary segments--10 in the right lung and 8 in the left
7-37. PLEURAL CAVITIES
Surrounding each lung individually is a serous cavity, called the pleural cavity.
The minute quantity of serous fluid in the cavity serves as a lubricant. This serves to
minimize friction for the expansion and contraction of the lungs during breathing.
a. Each lung is intimately covered with a serous membrane, the visceral pleura.
b. The outer wall of the pleural cavity is lined with another serous membrane
known as the parietal pleura. Areas of the parietal pleura are variously named
according to their location.
(1) The mediastinal pleura forms the lateral wall of the mediastinum.
(2) The diaphragmatic pleura covers the superior surface of the diaphragm.
(3) The costal pleura lines the inner surface of the rib cage.
(4) The cupolar pleura is a dome-like extension into the root of the neck. It
contains the apex of the lung.
c. When each lung is in its smaller volume, its corresponding diaphragmatic
pleura lies close to the lower costal pleura. The slit-like cavity between them is called
the costophrenic sinus. Fluids of each pleural cavity tend to collect in this sinus, since it
is the lowest area for each. When the diaphragm contracts and flattens out, each
costophrenic sinus opens up and the inferior portion of the expanding lung occupies this