Understanding Anatomy and Physiology in Nursing. John Knight

      Figure 4.4 Branching structure of the respiratory tract

      The carina

      Located within the carina are densely arranged sensory receptors which continually monitor the airway for debris. These receptors respond to mechanical stimulation and are ideally positioned since inhaled particles will have to pass through the carina before travelling into the primary bronchi and the lungs. Once activated, these receptors initiate a vigorous coughing reflex where explosive expiration of air will usually clear irritating particulates. Unfortunately, as we grow older, the receptors of the airway become less sensitive to irritation, and this together with age-associated losses in muscle mass and strength means that the coughing reflex is less effective. This contributes to an increased risk of respiratory tract infections in older people.

      The primary bronchi

      The right primary bronchus is angled at approximately 20 to 30 degrees to the trachea, while the left primary bronchus is angled at between 45 and 55 degrees. This means that foreign objects entering the trachea are more likely to end up in the right bronchus and right lung than the left. As in the trachea, the primary bronchi are reinforced and held open by cartilaginous rings, but rather than being C-shaped, these are complete rings. As the bronchi subdivide, becoming smaller bronchi and bronchioles, the cartilage rings are exchanged for irregular cartilaginous plates. The right lung consists of three lobes and the left lung two (Figure 4.4).

      The primary bronchi divide to form the lobar bronchi with each supplying a lobe of the lungs. The subdividing branching structure of the bronchial tree continues into each lung lobe, with each successive division (generation) of airway smaller than the one before.

      Bronchioles

      Bronchioles arise from the fourth generation of branching; these are small airways with a diameter of less than 1 mm and characteristically lack cartilaginous reinforcement. Structural support for the bronchioles is provided by the lung parenchyma. This is composed predominantly of the alveolar air sacs and elastic connective tissue, which attach to the external surface of these tiny airways, tethering the bronchioles (like guy ropes supporting a tent). This support is necessary to prevent airway collapse, particularly during forced inhalation and expiration.

      The bronchioles themselves subdivide into smaller and smaller airways with the terminal bronchioles having a diameter of 0.5 mm or less. Smooth muscle fibres are present in the bronchiolar walls; contraction of this smooth muscle layer leads to bronchoconstriction and a narrowing of the airway. This smooth muscle is physiologically very useful since coordinated bronchoconstriction and bronchodilation can regulate airflow within the lungs. However, uncoordinated bronchoconstriction or bronchospasm can lead to air becoming trapped in the lung, which is one of the key clinical features of asthma.

      The conduction zone

      The airway extending from the nose all the way through the bronchial tree to the terminal bronchioles is known as the conduction zone because its role is to conduct air to the regions of the lung where gaseous exchange takes place. Jake’s asthma case study at the beginning of this chapter perfectly illustrates the importance of a clear, unobstructed conduction zone and highlights why asthma must be carefully managed to ensure an adequate supply of conditioned air to the alveolar air sacs.

      The respiratory zone

      Each terminal bronchiole branches into several respiratory bronchioles and alveolar ducts which finally terminate in the alveolar air sacs. Collectively, these areas have a berry-like appearance (Figure 4.5) and are referred to as acini (Latin for berries). Acini are the functional units of the lungs where gas exchange takes place. In a healthy young adult, more than 30,000 of these acini are present in each lung, with each acinus having approximately 10,000 alveoli. Around 3 litres (3000 ml) of air is held within the lung acini of a typical adult male. This contrasts with the relatively small volume of air in the conducting airways, which totals only around 150 ml; this volume is known as the anatomical dead space since the air here does not reach the alveoli to participate in gas exchange.