Page 121 - Color_Atlas_of_Physiology_5th_Ed._-_A._Despopoulos_2003

Mechanics of Breathing To exploit the motion of the diaphragm and
chest for ventilation, the lungs must be able to
Pressure differences between the alveoli and follow this motion without being completely
the environment are the driving “forces” for the attached to the diaphragm and chest. This is
exchange of gases that occurs during ventila- achieved with the aid of the pleura, a thin
tion. Alveolar pressure (PA = intrapulmonary fluid-covered sheet of cells that invests each
pressure; ! B) must be lower than the lung (visceral pleura), thereby separating it
barometric pressure (PB) during inspiration from the adjacent organs, which are covered
(breathing in), and higher during expiration by the pleura as well (parietal pleura).
(breathing out). If PB is defined as zero, the al- In its natural state, the lung tends to shrink
veolar pressure is negative during inspiration due to its intrinsic elasticity and alveolar sur-
and positive during expiration (! B). These face tension (! p. 118). Since the fluid in the
pressure differences are created through pleural space cannot expand, the lung sticks to
coordinated movement of the diaphragm and the inner surface of the chest, resulting in suc-
chest (thorax), resulting in an increase in lung tion (which still allows tangential movement
volume (V pulm) during inspiration and a of the two pleural sheets). Pleural pressure
decrease during expiration (! A1,2). (P pl) is then negative with respect to at-
Respiration costal muscles. Their contraction lowers (flat- pleural (P ip) or intrathoracic pressure, can be
The inspiratory muscles consist of the dia-
mospheric pressure. P pl, also called intra-
phragm, scalene muscles, and external inter-
measured during breathing (dynamically)
tens) the diaphragm and raises and expands
using an esophageal probe (! P pl). The inten-
5 the chest, thus expanding the lungs. Inspira- sity of suction (negative pressure) increases
when the chest expands during inspiration,
tion is therefore active. The external intercostal
muscles and accessory respiratory muscles are and decreases during expiration (! B). P pl usu-
activated for deep breathing. During expira- ally does not become positive unless there is
tion, the diaphragm and other inspiratory very forceful expiration requiring the use of
muscles relax, thereby raising the diaphragm expiratory muscles. The difference between
and lowering and reducing the volume of the the alveolar and the pleural pressure (PA - P pl)
chest and lungs. Since this action occurs pri- is called transpulmonary pressure (! p. 114).
marily due to the intrinsic elastic recoil of the Characterization of breathing activity. The
lungs (! p. 116), expiration is passive at rest. In terms hyperpnea and hypopnea are used to de-
deeper breathing, active mechanisms can also scribe abnormal increases or decreases in the
play a role in expiration: the internal inter- depth and rate of respiratory movements.
costal muscles contract, and the diaphragm is Tachypnea (too fast), bradypnea (too slow), and
pushed upward by abdominal pressure created apnea (cessation of breathing) describe abnor-
by the muscles of the abdominal wall. mal changes in the respiratory rate. The terms
Two adjacent ribs are bound by internal and hyperventilation and hypoventilation imply
external intercostal muscle. Counteractivity of that the volume of exhaled CO 2 is larger or
the muscles is due to variable leverage of the smaller, respectively, than the rate of CO 2 pro-
upper and lower rib (! A3). The distance sepa- duction, and the arterial partial pressure of CO 2
rating the insertion of the external intercostal (PaCO 2) decreases or rises accordingly
muscle on the upper rib (Y) from the axis of ro- (! p. 142). Dyspnea is used to describe difficult
tation of the upper rib (X) is smaller than the or labored breathing, and orthopnea occurs
distance separating the insertion of the when breathing is difficult except in an upright
muscles on the lower rib (Z!) and the axis of ro- position.
tation of the lower rib (X!). Therefore, X!–Z! is
longer and a more powerful lever than X–Y.
The chest generally rises when the external in-
tercostal muscles contract, and lowers when
the opposing internal intercostal muscles con-
108 tract.

Despopoulos, Color Atlas of Physiology © 2003 Thieme
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