The upper respiratory tract includes the
nose, pharynx and larynx; the lower
respiratory tract starts at the trachea (Fig. 25a). The two lungs are
enclosed within the thoracic cage, formed from the ribs, sternum and vertebral
column, with the dome-shaped diaphragm separating the thorax from the abdomen.
The left lung has two lobes, the right three. The airways, blood vessels and lymphatics
enter each lung at the lung root or hilum, where the pulmonary nerve plexus
receives autonomic nerves from the vagus and sympathetic trunk. The vagus contains
sensory afferents from lung receptors (Chapter 29) and bronchoconstrictor
parasympathetic efferents leading to the airways; sympathetic nerves are bronchodilatory
(Chapter 7). Each lung lobe is made up of several wedge-shaped bronchopulmonary
segments supplied by their own segmental bronchus, artery and vein. The lungs
are covered by a thin membrane (visceral pleura), continuous with the parietal
pleura that lines the inside surface of the thoracic cage. The tiny space between
the pleura is filled with lubricating pleural fluid.
Airways (Fig. 25a)
The trachea divides into two
main bronchi; their walls contain U-shaped cartilage segments linked by smooth
muscle. On entering the lung, the bronchi divide repeatedly into lobar, segmental
(generations 3 and 4) and small (generations 5–11) bronchi, the smallest
having a diameter of ∼1 mm. These
all have irregular cartilaginous plates
and helical bands of smooth muscle. Bronchioles (generations 12–16) lack cartilage and are held open by surrounding
lung tissue. The smallest (terminal) bronchioles lead to respiratory bronchioles
(generations 17–19), and thence to alveolar ducts and sacs (generation
23), the walls of which form alveoli and contain only epithelial cells (Fig.
25c,d). Small pores (alveolar pores, pores of Kohn) allow
pressure equalization between alveoli. Adult human lungs contain ∼17
million branches and ∼300 million alveoli, providing an exchange
surface of ∼85 m2. The bronchial circulation supplies airways down to the terminal bronchioles; respiratory bronchioles
and below obtain nutrients from the pulmonary circulation (Chapter 16).
Epithelium and airway clearance
The airways from the trachea to the
respiratory bronchioles are lined with ciliated columnar epithelial cells.
Goblet cells and submucosal glands secrete a 10–15-µm thick, gel-like
mucus that floats on a more fluid
sol phase (Fig. 25b). Synchronous beating of the cilia moves the mucus and associated debris to the mouth (mucociliary
clearance). Factors that increase the thickness or viscosity of the mucus (e.g.
asthma, cystic fibrosis) or reduce cilia activity (e.g. smoking) impair
mucociliary clearance and lead to recurrent infections. Mucus contains
substances that protect the airways from pathogens (e.g. antitrypsins, lysozyme,
immunoglobulin A).
Epithelial cells forming the walls of
the alveoli and alveolar ducts are unciliated, and largely very thin type I alveolar
pneumocytes (alveolar cells; squamous epithelium) (Fig. 25d). These form
the gas exchange surface with the capillary endothelium (alveolar–capillary
membrane). A few type II pneumocytes secrete surfactant which
reduces the surface tension and prevents alveolar collapse (Chapter 26). Macrophages (mobile phagocytes)
in the airways ingest foreign materials
and destroy bacteria; in the alveoli, they take the place of cilia by clearing debris.
Respiratory muscles
The main respiratory muscles are inspiratory,
the most important being the diaphragm; contraction pulls down the dome,
reducing pressure in the thoracic cavity, and thus drawing air into the lungs. The
external intercostal muscles assist by elevating the ribs and increasing
the dimensions of the thoracic cavity. Quiet breathing is normally diaphragmatic;
accessory inspiratory muscles (e.g. scalene, sternomastoids)
aid inspiration if airway resistance or ventilation is high. Expiration is achieved
by passive recoil of the lungs and chest wall, but, at high ventilation rates,
this is assisted by the contraction of abdominal muscles which speed recoil
of the diaphragm by raising abdominal pressure (e.g. exercise).
Lung volumes and pressures (Fig.
25e)
The tidal volume (TV) is the
volume of air drawn into and out of the lungs during normal breathing; the resting
tidal volume is normally
∼500 mL but,
like all lung volumes, is dependent on age, sex and height. The vital capacity (VC) is the maximum
tidal volume, when an individual breathes
in and out as far as possible. The difference in volume between a resting and maximum
expiration is the expiratory reserve volume (ERV); the equivalent for inspiration
is the inspiratory reserve volume (IRV). The volume in the lungs after a
maximum inspiration is the total lung capacity (TLC), and that after a maximum
expiration is the residual volume (RV).
The functional residual capacity
(FRC) is the volume of the lungs at the end of a normal breath, when the respiratory
muscles are relaxed. It is determined by the balance between outward elastic
recoil of the chest wall and inward elastic recoil of the lungs. These
are coupled by the fluid in the small pleural space, which therefore has a negative
pressure (intrapleural pressure: –0.2 to –0.5 kPa). Perforation of the
chest therefore allows air to be sucked into the pleural space, and the chest wall
expands while the lung collapses (pneumothorax). Dis- eases that affect lung
elastic recoil alter FRC; fibrosis increases recoil and therefore
reduces FRC, whereas in emphysema, where lung structure is lost, recoil is
reduced and FRC increases.
During inspiration, the expansion
of the thoracic cavity makes the intrapleural pressure more negative, causing the
lungs and alveoli to expand, and reducing the alveolar pressure. This creates a
pressure gradient between the alveoli and the mouth, drawing air into the lungs.
During expiration, intrapleural and alveolar pressures rise, although,
except during forced expiration (e.g. coughing), the intrapleural pressure remains
negative throughout the cycle because expiration is normally passive.
The dead space refers to the
volume of the airways that does not take part in gas exchange. The anatomical
dead space includes the respiratory tract
down to the terminal bronchioles; it is normally ∼150 mL. The alveolar dead space refers to
alveoli incapable of gas exchange;
in health, it is negligible. The physiological dead space is the sum of the anatomical and alveolar dead spaces.
