Structure of the respiratory system: lungs, 1 airways and dead space

(a) Lung lobes

RU (b) The airways LU RM RL LL Nasal cavity

Right lateral Left lateral Pharynx aspect aspect Epiglotti s

Larynx C6 Cricoi d C7 Sternal angl e T1 (angle of Louis) T2 RU Tr achea (generation 0) T 3 LU Manubrium T 4 Carina RM T 5 RL R and L main bronchi (generation 1) LL Body T6 Anterior aspect Bronchi (generations 2–11) T7 Sternum Bronchioles (gener ations 12–16) T8

T9 Respiratory bronchioles (generations 17–19) Xiphoi d T10 LU RU process Alveolar ducts and sacs Diaphragm T 11 (generations 20–23) RU = Right upper RM = Right middl e T12 LL RL RL = Right lowe r LU = Left upper LL = Left lowe r Posterior aspect

(c) Bohr equation for measuring Anatomical dead space, End-tidal = dead space Vo lume = V alveolar gas D In an expired breath none of the CO 2 expi re d came from the dead space region Anatomical dead space, Mixed expired gas: Vo lume = V ; ∴ Vo lume = V D T Quantity of CO 2 Mixed expired CO 2 fraction = FECO2 in mixed expired air = quantity of CO 2 from alveolar region

Respiratory zone: V x F CO = (V –V ) x F CO Alveolar CO 2 fraction = FACO2 T E 2 T D A 2 ∴ VD = V T ( FACO2 –F ECO2 )/ FACO2 CO - free gas CO - containing gas End of inspira tion 2 2 End of expira tion

10 Structure and function Lungs, airways and dead space

WWTR01.inddTR01.indd 1010 224/5/20064/5/2006 10:33:3410:33:34 Lungs increased numbers more than make up for their reduced size. Genera- The respiratory system consists of a pair of lungs within the thoracic tions 5–11 are small bronchi, the smallest being 1 mm in diameter. cage (Chapter 2). Its main function is gas exchange, but other roles The lobar, segmental and small bronchi are supported by irregular include speech, fi ltration of microthrombi arriving from systemic veins plates of cartilage, with bronchial smooth muscle forming helical and metabolic activities such as conversion of angiotensin I to an- bands. Bronchioles start at about generation 12 and from this point on giotensin II and removal or deactivation of serotonin, bradykinin, cartilage is absent. These airways are embedded in lung tissue, which norepinephrine, acetylcholine and drugs such as propranolol and holds them open like tent guy ropes. The terminal bronchioles (gen- chlorpromazine. The right lung is divided by transverse and oblique eration 16) lead to respiratory bronchioles, the fi rst generation to fi ssures into three lobes: upper, middle and lower. The left lung has an have alveoli (Chapter 5) in their walls. These lead to alveolar ducts oblique fi ssure and two lobes (Fig. 1a). Vessels, nerves and lymphatics and alveolar sacs (generation 23), whose walls are entirely composed enter the lungs on their medial surfaces at the lung root or hilum. Each of alveoli. lobe is divided into a number of wedge-shaped bronchopulmonary The bronchi and airways down to the terminal bronchioles receive segments with their apices at the hilum and bases at the lung surface. nutrition from the bronchial arteries arising from the descending Each bronchopulmonary segment is supplied by its own segmental aorta. The respiratory bronchioles, alveolar ducts and sacs are supplied bronchus, artery and vein and can be removed surgically with little by the pulmonary circulation (Chapter 13). bleeding or air leakage from the remaining lung. The airways from trachea to respiratory bronchioles are lined with The pulmonary nerve plexus lies behind each hilum, receiving ciliated columnar epithelial cells. Goblet cells and submucosal fi bres from both vagi and the second to fourth thoracic ganglia of the glands secrete mucus. Synchronous beating of cilia moves the mucus sympathetic trunk. Each vagus contains sensory afferents from lungs and associated debris to the mouth (mucociliary clearance) (Chapter and airways and parasympathetic bronchoconstrictor and secretomo- 18). Epithelial cells forming the walls of alveoli and alveolar ducts tor efferents. Sympathetic fi bres are bronchodilator but relatively are unciliated, and largely very thin type I alveolar pneumocytes sparse. (alveolar cells; squamous epithelium). These form the gas exchange Each lung is lined by a thin membrane, the visceral pleura, which is surface with the capillary endothelium (alveolar-capillary mem- continuous with the parietal pleura, lining the chest wall, diaphragm, brane). A few type II pneumocytes secrete surfactant which reduces pericardium and mediastinum. The space between the parietal and vis- surface tension and prevents alveolar collapse (Chapters 6 & 18). ceral layers is tiny in health and lubricated with pleural fl uid. The right and left pleural cavities are separate and each extends as the costodia- Dead space phragmatic recess below the lungs even during full inspiration. The The upper respiratory tract and airways as far as the terminal bronchi- parietal pleura is segmentally innervated by intercostal nerves and by oles do not take part in gas exchange. These conducting airways form

the phrenic nerve, and so pain from pleural infl ammation (pleurisy) the anatomical dead space (VD), whose volume is normally about is often referred to the chest wall or shoulder tip. The visceral pleura 150 mL. These airways have an air-conditioning function, warming, lacks sensory innervation. fi ltering and humidifying inspired air. Lymph channels are absent in alveolar walls, but accompany small Alveoli that have lost their blood supply —for example because of a blood vessels conveying lymph towards the hilar bronchopulmonary pulmonary embolus —no longer take part in gas exchange and form nodes and from there to tracheobronchial nodes at the tracheal bifur- alveolar dead space. The sum of the anatomical and alveolar dead cation. Some lymph from the lower lobe drains to the posterior medi- space is known as the physiological dead space, ventilation of which astinal nodes. is wasted in terms of gas exchange. In health, all alveoli take part in gas The upper respiratory tract consists of the nose, pharynx and exchange, so physiological dead space equals anatomical dead space.

larynx. The lower respiratory tract (Fig. 1b) starts with the trachea at The volume of a breath or tidal volume (VT), is about 500 mL at rest. the lower border of the cricoid cartilage, level with the sixth cervical Resting respiratory frequency (f) is about 15 breaths/min, so the vertebra (C6). It bifurcates into right and left main bronchi at the volume entering the lungs each minute, the minute ventilation (V˙ ) is

level of the sternal angle and T4/5 (lower when upright and in inspira- about 7500 mL/min (=500 × 15) at rest. Alveolar ventilation (V˙ A) is tion). The right main bronchus is wider, shorter and more vertical than the volume taking part in gas exchange each minute. At rest, dead the left, so inhaled foreign bodies enter it more easily. space volume = 150 mL and alveolar ventilation is 5250 mL/min (=(500–150) × 15). Airways The Bohr method for measuring anatomical dead space is based on

The airways divide repeatedly, with each successive generation the principle that the degree to which dead space gas (0% CO2) dilutes

approximately doubling in number. The trachea and main bronchi alveolar gas (about 5% CO2) to give mixed expired gas (about 3.5%) have U-shaped cartilage linked posteriorly by smooth muscle. Lobar depends on its volume (Fig. 1c). Alveolar gas can be sampled at the bronchi supply the three right and two left lung lobes and divide to give end of the breath as end-tidal gas. The Bohr equation can be modifi ed

segmental bronchi (generations 3 and 4). The total cross-sectional to measure physiological dead space by using arterial Pco2 to estimate

area of each generation is minimum here, after which it rises rapidly, as the CO2 in the gas-exchanging or ideal alveoli.

Lungs, airways and dead space Structure and function 11

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