Chapter 2 - Anatomy & Physiology of the Respiratory System

Written by - AH Kendrick & C Newall

2.1 Introduction 2.2 Gross Anatomy of the Lungs, 2.3 Anatomy of the Thorax, 2.4 Anatomy and Histology of the Respiratory Tract, 2.5 Development and Ageing of the Respiratory System 2.6 Lung Volumes and Ventilation 2.7 Mechanics of Ventilation (including Surfactant) 2.8 Airflow 2.9 Diffusion of Gases in the Lungs 2.10 Pulmonary, Bronchial and Systemic Circulation 2.11 Gas Transport in Blood 2.12 Ventilation and Perfusion Relationships 2.13 Acid - Base Status 2.14 Neurochemical Control of Respiration, 2.15 Pharmacological Control of the Airways 2.16 Non-Respiratory Functions of the Lung 2.17 Further Reading

Professional Skills

Upon completion of this chapter, the reader will be expected to: • Describe the main anatomical and histological features of the lungs and chest wall • Describe the main features of the development of the lungs from conception onwards • Define the static and dynamic volumes of the lungs • Understand the basics of the mechanics of ventilation, including the terms total

ventilation, alveolar ventilation and physiological dead space • Describe briefly the role and importance of surfactant • Understand the basics of airflow and airway resistance within the tracheobronchial tree • Understand the diffusion of gases within the lungs and how both oxygen and carbon

dioxide are transported to and from the lungs • Describe briefly the concepts of ventilation and of perfusion of the lungs, and the need

for matching of ventilation and perfusion • Describe briefly the regulation of acid-base balance • Describe the basics of the control of ventilation, including neurochemical, regulation of

airway smooth muscle and the influences of higher centres • State the non-respiratory functions of the lungs and pulmonary circulation • Know all equations in bold text

2.1 Introduction Respiratory systems in different classes of animals have a number of common features. 1. There is a large surface area for gas exchange that separates blood from the air or water. 2. The barrier separating the blood from the air or water is thin - about 50 times thinner than a

sheet of airmail stationery - providing minimal resistance to gas transfer. 3. The flow of O2 across the barrier separating the blood from air or water occurs by diffusion

down a pressure gradient from a point of high PO2 to a point of lower PO2. This gradient exists throughout the respiratory system.

The combination of an efficient external gas exchanger, a developed circulatory system and the operational capabilities of the mitochondria in the cells results in a system that can convert O2 and carbohydrates into energy needed by the animal to meet the demands of daily living. The lungs and the pulmonary circulation have other functions apart from gas exchange – 1. Pulmonary defence: The nasal passages warm inspired air and filter out large dust particles.

The sticky mucous layer lining the airways removes smaller particles. The olfactory system detects noxious smells and initiates action to prevent substances such as sulphur dioxide, ammonia and hydrogen sulphide from entering the lungs. Within the airways and alveoli, cells of the immune system protect against inhaled micro-organisms, and anti-oxidants in the alveoli protect against harmful oxidants such as ozone and hydrogen peroxide.

2. Metabolic function: The lungs produce substances for local use, such as surfactant, which is

essential in stabilizing the alveoli. Other substances produced by the lungs are released into the circulation. The lungs also metabolise a variety of substances.

3. Pulmonary circulation: This acts as a reservoir for the left ventricle, a filter for a variety of

substances, and is involved in fluid exchange and drug absorption from the circulation. This acts as a protective mechanism for the systemic circulation.

The human respiratory system consists of the lungs and the mitochondria linked by the circulatory system. Failure of any part of this system, due to either functional or structural failures, results in the system failing. Thus, the lungs, circulation and mitochondria form an integrated system with each being dependent on the others for successful gas exchange. This chapter concentrates on the anatomy and physiology of the lungs, as this is what you will be assessing within the respiratory laboratory on a day-to-day basis. The circulatory system will be outlined, but whilst it is important, no further discussion on cellular respiration will be given. 2.2 Gross Anatomy of the Lungs The surface anatomy of the lungs is shown in Figure 2.1 in anterior and posterior view. There are a number of key features that allow positioning of the lungs within the chest. The suprasternal notch is located at the apex of the sternal bone and between the left and right clavicle. The sternal angle (angle of Louis) is located at the second rib, and is a small ridge near the apex of the sternum. At the base of the sternum is the xiphoid process. The nipple in the male lies in the fourth intercostal space, but may be at a different level in the female.

On the posterior wall, the spinous processes of the thoracic vertebrae can be palpated along the midline. The scapula (shoulder blade) is flat and triangular and is located on the upper part of the posterior surface of the thorax (Figure 2.1). Apart from the first rib, which lies deep to the clavicle, the ribs can be palpated from below the clavicle. The apex of the lung projects into the neck.

Figure 2.1. Anterior and Posterior views of the thorax of a male. The major location points on the surface of the thorax are indicated, along with the sites of the major muscles covering the surface of the chest wall. 2.3 Anatomy of the Thorax The thoracic cage is a closed compartment. It is bounded at the neck by muscles and connective tissue and separated from the abdomen by a large dome-shaped sheet of skeletal muscle and connective tissue, known as the diaphragm. The outer walls of the thoracic cage are made up of twelve thoracic vertebrae, the sternum, twelve pairs of ribs, and large amounts of elastic connective tissue (Figure 2.2).

Figure 2.2. Diagram of the anterior and posterior views of the rib cage, showing the major

features.

There are seven pairs of ‘true’ ribs and five pairs of ‘false’ ribs. True ribs are those that are attached directly to the sternum; false ribs insert on to the xiphoid cartilage below the sternum. Lying between the ribs are the external and internal intercostal muscles. Within the thorax are two lungs separated by the mediastinum in which the heart is situated. The lungs are divided into lobes by folds of membrane (known as fissures). The left lung is divided into two lobes (upper and lower) and the right lung into three lobes (upper, middle and lower) (Figure 2.3). The lungs are separated from the abdominal cavity by the diaphragm; immediately beneath which on the right hand side is the liver. The left lung is narrower than the right due to a depression in its surface to accommodate the heart - known as the cardiac notch.

Figure 2.3. Chest x-ray of a normal male showing the main features. The relationship of the lobes of the right and left lungs to the trachea and main bronchi. The carina (bifurcation of the trachea into left and right main bronchi) is shown, as is the cardiac notch. Two pleural membranes, the parietal pleura attached to the inner wall of the thorax and the visceral pleura attached to the lungs themselves, surround the lungs. Between the two pleurae is the pleural fluid, which reduces friction between the membranes as they move over each other during respiration. 2.4 Anatomy and Histology of the Respiratory Tract Air enters the respiratory passages via the nose or mouth at the entrance to the nasal passage (external nares). Small hairs (called vibrissae) act as a filter removing any large particles in the inspired air. The nose warms and humidifies air via a system of nasal turbinate bones, which cause the inspired airflow to become turbulent and more efficiently mixed as it passes to the pharynx (Figure 2.4). The pharynx is involved in both the digestive and respiratory tracts, food being channelled through the oesophagus to the stomach and air passing to and from the lungs. The vocal cords and several cartilages are found in the larynx. The largest cartilage is the thyroid cartilage, which produces the bulge on the front of the neck, called the “Adam’s Apple”. Another cartilage called the epiglottis lies on top of the larynx to prevent food entering the trachea on swallowing.

Figure 2.4. The upper airway showing the major features. The trachea leads into the thorax and branches at the carina into left and right bronchi (Figure 2.5). The bronchi divide into secondary bronchi, which extend into the lobes of the lung. In each lobe, the secondary bronchi divide into tertiary bronchi. These further branch into a fine network of bronchioles, the terminal bronchioles, which terminate in the alveolar air sac (Figure 2.5). Figure 2.5. A) Resin cast of a human lung, showing the branching pattern of the bronchial tree (B) originating from the trachea (T). In the left lung, the pulmonary arteries (A) and veins (V) are shown. The inset shows the peripheral airways. B) Organization of the human airways, showing the assignment to generation (z) of the dichotomous branching. Each time the bronchial tree divides, it is referred to as a new generation (Figure 2.5). There are twenty-three generations of airways; gas exchange occurs in the last eight. At each division, the cross-sectional area of each airway becomes less. However, with the increasing number of airways the total cross-sectional area is increased (Figure 2.6). The total surface area of the alveolar gas exchange units is about 70 - 80 m2, i.e. about the size of a tennis court. The terminal bronchioles, alveolar ducts, alveoli and surrounding blood and lymphatic vessels are surrounded by sheets of elastic connective tissue, which divide the lobes of the lung into lobules.

A B