Structure Of The Respiratory System: Lungs, Airways
And Dead Space
Lungs
The respiratory system consists of a pair of lungs within the thoracic
cage (Chapter 2). Its main function is gas exchange, but other roles
include speech, filtration of microthrombi arriving from systemic veins and
metabolic activities such as conversion of angiotensin I to angiotensin II and
removal or deactivation of serotonin, bradykinin, norepinephrine, acetylcholine
and drugs such as propranolol and chlorpromazine. The right lung is
divided by transverse and oblique fissures into three lobes:
upper, middle and lower. The left lung has an oblique fissure and
two lobes (Fig. 1a). Vessels, nerves and lymphatics enter the lungs on their
medial surfaces at the lung root or hilum. Each lobe is divided into a
number of wedge-shaped bronchopulmonary segments with their apices at
the hilum and bases at the lung surface. Each bronchopulmonary segment is
supplied by its own segmental bronchus, artery and vein and can be removed
surgically with little bleeding or air leakage from the remaining lung.
The pulmonary nerve plexus lies behind each hilum, receiving fibre
from both vagi and the second to fourth thoracic ganglia of the sympathetic
trunk. Each vagus contains sensory afferents from lungs and airways,
parasympathetic bronchoconstrictor and secretomotor efferents, and
non-cholinergic non-adrenergic nerves (NANC). Sympathetic noradrenergic fibre
supplying airway smooth are sparse in humans, and the β2-adrenergic
receptors are stimulated by circulating catecholamines from the adrenal glands
(Chapter 7).
Each lung is lined by a thin membrane, the visceral pleura, which
is continuous with the parietal pleura, lining the chest wall,
diaphragm, pericardium and mediastinum. The space between the parietal and visceral
layers is tiny in health and lubricated with pleural fluid. The right and left
pleural cavities are separate and each extends as the costodiaphragmatic
recess below the lungs even during full inspiration. The parietal pleura is
segmentally innervated by intercostal nerves and by the phrenic nerve,
and so pain from pleural inflammation (pleurisy) is often referred to the chest wall or
shoulder-tip. The visceral pleura lacks sensory innervation.
Lymph channels are absent in alveolar walls, but accompany small
blood vessels conveying lymph towards the hilar bronchopulmonary nodes and
from there to tracheobronchial nodes at the tracheal bi- furcation. Some
lymph from the lower lobe drains to the posterior mediastinal nodes.
The upper respiratory tract consists of the nose, pharynx and
larynx. The lower respiratory tract (Fig. 1b) starts with the trachea at
the lower border of the cricoid cartilage, level with the sixth cervical
vertebra (C6). It bifurcates into right and left main bronchi at
the level of the sternal angle and T4/5 (lower when upright and in
inspiration). The right main bronchus is wider, shorter and more vertical than
the left, so inhaled foreign bodies enter it more easily.
Airways
The airways divide repeatedly, with each successive generation approximately
doubling in number. The trachea and main bronchi have U-shaped
cartilage, linked posteriorly by smooth muscle. Lo- bar bronchi supply the
three right and two left lung lobes and divide to give segmental bronchi (generations
3 and 4). The total cross- sectional area of each generation is minimum here,
after which it rises rapidly, as increased numbers more than make up for their
reduced size. Generations 5-11 are small bronchi, the smallest being 1 mm in diameter. The lobar, segmental and
small bronchi are supported by irregular plates of cartilage, with bronchial
smooth muscle forming helical bands. Bronchioles start at about
generation 12 and from this point onwards cartilage is absent. These airways
are embedded in lung tissue, which holds them open like tent guy ropes. The terminal
bronchioles (generation 16) lead to respiratory bronchioles, the
first generation to have
alveoli (Chapter 5) in their walls. These lead to alveolar ducts and alveolar
sacs (generation 23), whose walls are entirely composed of alveoli.
The bronchi and airways down to the terminal bronchioles receive
nutrition from the bronchial arteries arising from the descending aorta.
The respiratory bronchioles, alveolar ducts and sacs are supplied by the pulmonary
circulation (Chapter 13).
The airways from trachea to respiratory bronchioles are lined with ciliated
columnar epithelial cells. Goblet cells and submucosal glands secrete
mucus. Synchronous beating of cilia moves the mucus and associated
debris to the mouth (mucociliary clearance) (Chapters 18). Epithelial
cells forming the walls of alveoli and alveolar ducts are unciliated, and
largely very thin type I alveolar pneumocytes (alveolar cells, squamous
epithelium). These form the gas exchange surface with the capillary
endothelium (alveolar–capillary membrane). The type II pneumocytes make
up only a small proportion of the alveolar surface area and are mostly found at
the junction between alveoli. They are stem cells, which can divide following
lung damage. They secrete surfactant, which reduces surface tension and
has a role in lung immunity (Chapter 6 and 18). A similar substance is produced
by the non-ciliated Clara cells found in the bronchiolar epithelium close to
their junction with alveoli.
Dead space
The upper respiratory tract and airways as far as the terminal bronchioles
do not take part in gas exchange. These conducting airways form the anatomical
dead space whose volume (VD) is normally about 150 mL. These
airways have an air-conditioning function, warming, filterin and humidifying
inspired air.
Alveoli that have lost their blood supply- for example because of a pulmonary
embolus - no longer take part in gas exchange and form alveolar dead
space. The sum of the anatomical and alveolar dead space is known as the physiological
dead space, ventilation of which is wasted in terms of gas exchange. In
health, all alveoli take part in gas exchange, so physiological dead space
equals anatomical dead space.
The volume of a breath or tidal volume (VT) is about
500 mL at rest. Resting respiratory frequency (f) is about 15
breaths/min, so the volume entering the lungs each minute, the minute
ventilation (V' ), is about 7500
mL/min ( 500 15) at rest. Alveolar ventilation (V'
A) is the volume taking part in gas exchange each minute. At rest, with a dead-space
volume of 150
mL, alveolar ventilation
is about 5250 mL/min ( (500
150) 15).
The Bohr method for measuring anatomical dead space is based on
the principle that the degree to which dead-space gas (0% CO2)
dilutes alveolar gas ( 5% CO2) to give mixed expired gas ( 3.5%)
depends on its volume (Fig. 1c). Alveolar gas can be sampled at the end
of the breath as endtidal gas. The Bohr equation can be modifiet to measure physiological dead
space by using arterial Pco2 to estimate the CO2
in the gas-exchanging or ideal alveoli.
