Flow in the Lungs - Biofluidmechanics: An Introduction to Fluid Mechanics Macrocirculation and Microcirculation - page 258

are in equilibrium ( Section 9.3 will discuss oxygen and carbon dioxide exchange in more
detail). The distance that separates air within the alveoli and blood within the capillary is
approximately 0.5

μ

m (which is the thickness of the respiratory boundary).
Moving to the gross anatomy of the lungs, each lung is composed of distinct lobes. The
right lung has three lobes (the superior lobe, the middle lobe, and the inferior lobe) that
are separated by the horizontal and the oblique fissures, respectively. The left lung has
two lobes (the superior lobe and the inferior lobe) that are separated by the oblique fissure.
The right lung is wider than the left lung because the heart is located within the left tho-
racic cavity and requires some physical space. The right lung is slightly shorter than the
left lung to accommodate the liver, which is directly inferior to the right lung. Each lung is
located within a space called the pleural cavity within the chest and is surrounded by a
membrane termed the pleura. The pleura membrane is composed of two layers, the parie-
tal pleural and the visceral pleural. The parietal pleural is the exterior layer, and it is con-
nected to the thoracic wall, the diaphragm, and the ribs. The visceral pleural is the inner
layer and covers the outer surface of each lung. Between the parietal pleural and the vis-
ceral pleural is a thin fluid layer. The fluid within this layer is termed the pleural fluid
and is critical during respiration.
In fact, the relationship among the atmospheric pressure, the interpulmonary pressure,
and the intrapleural pressure determines the direction of airflow in the lungs. Following
simple fluid mechanics laws, if the atmospheric pressure and the interpulmonary pressure
are the same, there is no net air movement in the respiratory system. If the interpulmonary
pressure drops below the atmospheric pressure, then air will flow into the lungs. When
the interpulmonary pressure rises above the atmospheric pressure, then air will flow out
of the lungs. However, there is no internal mechanism for the interpulmonary pressure to
change by itself because of some demand on the system (e.g., the lungs do not actively
change pressure by contracting or relaxing). Also, there is clearly no mechanism for the
body to alter the atmospheric pressure in relation to the interpulmonary pressure. In order
to alter interpulmonary pressure, the body makes use of Boyle's Law. Boyle's Law states
that at a constant temperature the pressure of a system is inversely related to the volume
of the system. A constant temperature is a good assumption for most biological systems,
and under normal breathing we will typically assume that the temperature is constant.
Mathematically, Boyle's Law is written as
P 1 V 1 5 P 2 V 2

ð 9 : 1 Þ

Using the principle of Boyle's Law, the interpulmonary pressure is altered by changing
the volume of the lungs. During inhalation, the rib cage moves upward and outward,
while the diaphragm moves downward. Because the parietal pleura is attached to the dia-
phragm and the rib cage, it expands at the same rate as the diaphragm and ribs. This acts
to decrease the intrapleural pressure, because the parietal pleura moves outward as the
visceral pleural remains in the same location, to approximately negative 5 mmHg (gauge
pressure). Due to the surface tension of the pleural fluid, the visceral pleural is pulled
toward the parietal pleural. Because the visceral pleura is attached to the lobes of the
lungs, the lung tissue gets pulled outward as well. Under normal conditions, this accounts
for an approximately 20% change in lung volume, which decreases the interpulmonary
pressure by 1 mmHg to 2 mmHg (gauge pressure), allowing for air to flow into the lungs.