Therefore, contraction of smooth muscle can greatly increase airway resistance and diminish the flow of air into the lungs by decreasing size of the small airways. The cross-sectional area of all the bronchioles is much larger than the cross-sectional area of the biggest airway. Similarly, in the lungs, contraction of smooth muscle in the bronchioles determines the air flow. Smooth muscle surrounds other tubular structures in the human body, such as arteries, where smooth muscle contraction dictates the flow of blood to vital organs. Without the support of cartilage, when smooth muscle contracts, the airways become increasingly narrow. These smaller tubes are called bronchi and bronchioles. However, as the tubes get smaller, these cartilaginous rings disappear and only a layer of smooth muscle remains (Fig. In the lung, the larger tubes such as the trachea and main bronchus are supported by both cartilaginous rings and smooth muscle. The characteristics of the larger tubes as compared to the smaller tubes are very different. If one thinks of the lungs as a series of tubes that continues to divide, the tubes get smaller and smaller until they end in small air sacks (called alveoli), where the exchange of gas occurs. Before describing the abnormal features of the asthmatic airway, we must first briefly describe the basic features of the normal airway.Īsthma is thought to be a disease of the small airways. Although there have been many advances in the treatment of asthma, death from this airways disease is still an unfortunate outcome in a minority of patients. The autopsies of patients who have died of asthma gave researchers the first clues as to the possible etiology of this disease. We will also explore how understanding the pathophysiology of asthma can help us to understand the symptoms and presentation of asthma, as well as the best strategies for diagnosing this disease. In this chapter, we will briefly explore some major theories of asthma pathogenesis, both new and old. Asthma is a heterogeneous disease process with varying phenotypes and presentations. Although we understand the basic clinical features of asthma, the links between symptoms, physical signs, and underlying pathophysiological mechanisms are still being delineated. We know that the basic features of asthma include episodic airways inflammation, airways hyperresponsiveness, and mucous hypersecretion. Extensive research over the last few decades has yielded a better understanding of asthma. 2006) or 23 million Americans, the pathogenesis of this disease remains to be fully elucidated. Although asthma is a common disorder affecting approximately 7.8% of the United States population (Schiller et al.
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