Essentials of Anatomy and Physiology

2. Secondary bronchi: to the lobes of each lung (three
   right, two left)


3. Bronchioles—no cartilage in their walls.

Pleural Membranes—serous membranes of
the thoracic cavity

1. Parietal pleura lines the chest wall.


2. Visceral pleura covers the lungs.


3. Serous fluid between the two layers prevents fric-
   tion and keeps the membranes together during
   breathing.

Lungs—on either side of the heart in the
chest cavity; extend from the diaphragm
below up to the level of the clavicles

1. The rib cage protects the lungs from mechanical
   injury.


2. Hilus—indentation on the medial side: primary
   bronchus and pulmonary artery and veins enter
   (also bronchial vessels).

Alveoli—the sites of gas exchange in the
lungs

1. Made of alveolar type I cells, simple squamous
   epithelium; thin to permit diffusion of gases.


2. Surrounded by pulmonary capillaries, which are
   also made of simple squamous epithelium (see Fig.
   15–4).


3. Elastic connective tissue between alveoli is impor-
   tant for normal exhalation.


4. A thin layer of tissue fluid lines each alveolus; essen-
   tial to permit diffusion of gases (see Fig. 15–5).


5. Alveolar type II cells produce pulmonary surfactant
   that mixes with the tissue fluid lining to decrease
   surface tension to permit inflation of the alveoli.


6. Alveolar macrophages phagocytize foreign mate-
   rial.

Mechanism of Breathing

1. Ventilation is the movement of air into and out of
   the lungs: inhalation and exhalation.


2. Respiratory centers are in the medulla and pons.


3. Respiratory muscles are the diaphragm and exter-
   nal and internal intercostal muscles (see Fig. 15–6).

• Atmospheric pressure is air pressure: 760 mmHg
  at sea level.


• Intrapleural pressure is within the potential pleu-
  ral space; always slightly below atmospheric
  pressure (“negative”).


• Intrapulmonic pressure is within the bronchial
  tree and alveoli; fluctuates during breathing.

Inhalation (inspiration)

1. Motor impulses from medulla travel along phrenic
   nerves to diaphragm, which contracts and moves
   down. Impulses are sent along intercostal nerves to
   external intercostal muscles, which pull ribs up and
   out.


2. The chest cavity is expanded and expands the pari-
   etal pleura.


3. The visceral pleura adheres to the parietal pleura
   and is also expanded and in turn expands the lungs.


4. Intrapulmonic pressure decreases, and air rushes
   into the lungs.

Exhalation (expiration)

1. Motor impulses from the medulla decrease, and the
   diaphragm and external intercostal muscles relax.


2. The chest cavity becomes smaller and compresses
   the lungs.


3. The elastic lungs recoil and further compress the
   alveoli.


4. Intrapulmonic pressure increases, and air is forced
   out of the lungs. Normal exhalation is passive.


5. Forced exhalation: contraction of the internal
   intercostal muscles pulls the ribs down and in; con-
   traction of the abdominal muscles forces the
   diaphragm upward.

Pulmonary Volumes (see Fig. 15–7)

1. Tidal volume—the amount of air in one normal
   inhalation and exhalation.


2. Minute respiratory volume—the amount of air
   inhaled and exhaled in 1 minute.


3. Inspiratory reserve—the amount of air beyond
   tidal in a maximal inhalation.


4. Expiratory reserve—the amount of air beyond tidal
   in the most forceful exhalation.


5. Vital capacity—the sum of tidal volume, inspira-
   tory and expiratory reserves.


6. Residual volume—the amount of air that remains
   in the lungs after the most forceful exhalation; pro-
   vides for continuous exchange of gases.


7. Alveolar ventilation—air that reaches the alveoli
   for gas exchange; depends on normal thoracic and
   lung compliance.

• Anatomic dead space—air still in the respiratory
  passages at the end of inhalation (is normal).

The Respiratory System 363