Читать книгу Understanding Anatomy and Physiology in Nursing - John Knight - Страница 159

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.