The prime function of respiration is to exchange CO₂ and O₂. In mammals, the gas exchange takes place in the lungs. During inspiration air rich in oxygen is inhaled to the lungs. During expiration, carbon dioxide produced during oxidative processes of the body is exhaled from the lungs. Both gases are transported by the blood. Therefore, both the respiratory and the cardiovascular systems are involved in supplying body cells with O₂ and eliminating their waste product, CO₂.

Air flows to the lungs through a series of air passages connecting the lungs to the nose and mouth. Air, when it gets through the nose, passes over a complex series of surfaces, the nasal septum and the nasal turbinates. These surfaces clean the air of big dust particles. From the nose, warmed and moistened air flows through the common passages for air and food, the pharynx, then the larynx, and via the trachea, and bronchi, reaches the periphery of the lungs. The lungs and the airways share the chest cavity with the heart, the great vessels, and the oesophagus.
The airways consist of a series of tubes that branch and become narrower, shorter and more numerous as they penetrate into the lungs. The trachea divides into two main bronchi. The main bronchi divide into lobar and segmental bronchi (Fig. 1). The right main bronchus has three lobar bronchi (the right lung has three lobes). The left one has only two bronchi (the left lung has two lobes). The segmental bronchi divide further, into smaller branches. The smallest airways without alveoli are the terminal bronchioles. All of these airways, from the mouth and nose openings to the terminal bronchioles, constitute the conducting airways. They conduct air from the atmosphere to the respiratory part of the lungs. Since the conductive airways do not contribute to the gas exchange, they compose the anatomical dead space. The respiratory part of the lungs begins where the terminal bronchioles divide into respiratory bronchioles which have some alveoli opening to their lumena. Beyond the respiratory bronchioles, there are the alveolar ducts lined with alveoli. The alveolated region of the lungs is the site of the gas exchange and is called the respiratory zone. Because of such abundant branching of the airways, the respiratory zone makes up most of the lungs. The smallest physiological unit of the lungs, distal to the terminal bronchiole is called the acinus (Fig. 2).
 

 

Fig. 2.  Anatomical relationship between the pulmonary artery and airways. The smallest physiological lung unit, acinus, originates from the terminal bronchole (TB) and is composed of the respiratory bronchole (RB), alveolar ducts (AD), and alveoli (A). Terminal broncholi are the smallest conducting airways. B: bronchus. Arrows indicate direction of blood flow.

In the respiratory zone, gas exchange takes place between the air in the alveoli and the blood in the pulmonary capillaries. Blood to the pulmonary capillaries is supplied via the pulmonary artery, originating from the right ventricle. Branches of the pulmonary artery run with airways (Fig. 2). When the alveoli are reached, arterioles divide into a capillary bed. The pulmonary arteries supply all capillaries within the alveolar walls, which constitute the respiratory surface of the lungs where the gas exchange takes place. Oxygenated blood, from the alveolar capillaries, returns to the left heart by the pulmonary veins.

There are about 300 million alveoli in the human lungs. Each alveolus may be associated with as many as 1000 capillaries. Alveoli are lined by epithelial type I cells interspersed with large type II cells. Together, all the alveolar epithelial cells form a complete epithelial layer sealed by tight junctions. Little is known about the specific metabolic activities of type I cells. Type II cells produce surfactant, a substance decreasing the surface tension of the alveoli.

The endothelial cells that constitute the wall of the pulmonary capillary, may be as thin as 0.1 micron. Between the epithelium and endothelium there is a thin layer of interstitial fluid. Therefore, to get from the alveolar space to the blood in the pulmonary capillary, oxygen must cross the epithelial cell, interstitial fluid and the endothelial cell (i.e. the alveolar-capillary membrane, see Fig. 7). Carbon dioxide, delivered to the pulmonary capillaries by the venous blood stream, must cross the same distance in the opposite direction. However, the blood within the capillary is separated from the air within the alveolus only by an extremely thin barrier (0.2 micron), while the alveolar diffusion area is 50-100m².

The lung tissue is elastic but is unable to expand or contract by itself. Air has to be sucked into the lungs. This function is powered by the respiratory muscles of the chest wall. The diaphragm is the main muscle for inspiration. Contraction of the diaphragm causes its dome to descend and the chest to expand longitudinally. At the same time, because of the vertically oriented attachments of the diaphragm to the costal margins, its contractions also elevate the lower ribs. Contraction of the external intercostal muscles (Fig. 3) also raises the ribs during inspiration. As the ribs are elevated, the anterio-posterior and transverse dimensions of the chest enlarge.
 

In addition to the diaphragm and the external intercostal muscles, the neck muscles may assist inspiration. Their major contribution is during high levels of ventilation. Contractions of these muscles are also apparent during severe asthma and other disorders that obstruct the movements of air into the lungs. The neck muscles elevate and fix the uppermost part of the rib cage, elevate the sternum and slightly enlarge the anterio-posterior and longitudinal dimensions of the chest.

In contrast to inspiration, expiration is passive as a result of recoil of the lungs and the chest wall. It becomes active at higher levels of ventilation (exercise), or in pathological states when expiratory resistance increases and the movement of airflow out of the lungs is impeded. Muscles involved in active expiration include the internal intercostal muscles and the abdominal muscles. Contractions of these latter muscles compress the abdominal contents, depress the lower ribs and pull down the anterior part of the lower chest. In effect, they force the diaphragm upwards. They are essential for several body functions - eg. coughing, talking, singing, vomiting. Forced maximal contraction of the expiratory muscles against a closed glottis (Valsalva's maneuver) can result in an enormous increase in pressure (up to100 mm Hg) in the thoracic cage and abdomen. If sustained, this would lead to a decrease in venous return to the heart, thus also to a decrease in cardiac output.

Clinically, it is useful to be able to measure the volume of air inhaled during inspiration under a number of different circumstances. Subdivisions of lung volume can be determined by means of a spirometer, a simple gas volume recorder. Fig. 4
illustrates a spirogram.

Tidal Volume - is the volume of air that enters the lungs during inspiration and leaves the lungs during expiration during normal breathing (V_T). The volume of air in the lungs at the end of expiration during normal breathing is referred to as the functional residual capacity (FRC).

FRC is composed of two volumes: expiratory reserve volume (ERV) and residual volume (RV). ERV is the maximal additional volume of air that can be exhaled following a normal expiration to FRC. RV is the volume of air remaining in the lungs at the end of a maximal exhalation.

Inspiratory capacity (IC) - is the maximal volume of air that can be inhaled from FRC and is made up of two subdivisions: V_T and inspiratory reserve volume (IRV).

Total lung capacity (TLC) - is the volume of air in the lungs following a maximal inspiration: RV, ERV, V_T, IRV.

Vital capacity (VC) - is the maximal volume of air that can be exhaled from the lungs following a maximal inspiration, and includes : IRV, V_T and ERV.

All the lung volumes are expressed in terms of Body Temperature, ambient Pressure and Saturation with water vapour (BTPS). Since gas volume measured in the spirometer is at Ambient Temperature, Pressure and Saturation, (ATPS), appropriate correction must be made.
The spirometric technique allows one to determine only volumes of air which can be inhaled or exhaled, i.e. changes in lung volumes, not absolute values. Thus RV cannot be measured. In practice, first FRC is measured by other techniques, then RV is calculated as FRC-ERV.

Lung volumes depend on the size, sex, age and body position. In seated adult males the following parameters may be estimated:
                                                  vital capacity = 1 x height³