How can you appreciate the role of scientists in developing the concept of respiration?
Answers
Traditional emphasis in respiratory physiology normally places emphasis on the respiratory pump muscles such as the diaphragm and the intercostal muscles in generating effective breathing. These muscles are indeed designated as the primary muscles of breathing because their contraction expands the thoracic cavity and brings air into the lungs, that is, they generate airflow. Before effective lung ventilation can occur, however, air has to pass through the region of the upper airway. The muscles in this region are considered secondary muscles of breathing because although their activation does not generate airflow per se, their activation modulates the resistance to airflow.
The upper airway is a collapsible tube vulnerable to closure during breathing because this region of the breathing apparatus is surrounded by a complex anatomical arrangement of skeletal muscles and soft tissues, an arrangement unlike other regions of the respiratory tract such as the trachea and bronchi that are supported by a more rigid cartilaginous structure. The muscular and soft tissue composition of the pharyngeal airway provides the necessary support for a variety of essential nonrespiratory functions such as vocalization, suckling, chewing, and swallowing, that is, behaviors that require dynamic changes in airway size to move air, liquids, and solids. However, this property of a collapsible tube compromises the essential respiratory function of the upper airway, that is, the airway must remain open during breathing, in all postures, to allow for adequate lung ventilation and gas exchange.
The major role of the upper airway in nonrespiratory functions indicates that the motoneurons driving these muscles receive prominent input from nonrespiratory sources, sources that for the reasons discussed in sections ‘The State-Dependent Chemical Brain’ and ‘Central Control of Respiratory Network Activity’ are those which are most suppressed in sleep. Suppression of upper airway muscle tone in sleep leads to a narrower and more collapsible upper airway, a physical change that manifests as an increased resistance to airflow. The magnitude of the increased upper airway resistance in sleep varies between individuals, but this increase contributes significantly to the hypoventilation and increased arterial CO2 levels of 3–5 mmHg normally observed in sleeping humans; elimination of this sleep-related increase in upper airway resistance reverses a large component of the sleep-induced hypoventilation. Importantly, in individuals with already anatomically narrow upper airways, this suppressant effect of sleep on pharyngeal muscle tone predisposes to significant reductions in inspiratory airflow and airflow limitation (i.e., hypopneas and snoring), and even cessation of airflow because of complete airway closure (i.e., obstructive sleep apnea), leading to periods of asphyxia. In contrast to the strong effects of sleep on the muscles of the upper airway, the diaphragm is less affected by sleep mechanisms because motoneurons to this muscle are almost exclusively driven by respiratory neurons which are tightly linked to the respiratory network, with minimal distortion of their activity by sleep and other influences unrelated to breathing.
This discussion, therefore, expands upon the old view that the diaphragm and thoracic respiratory pump muscles are the sole important muscles of breathing. In this respect, continuing contraction of the diaphragm in the face of an upper airway obstruction in sleep is futile for effective generation of airflow. The pharyngeal muscles therefore serve a crucial role in lung ventilation in addition to their other roles in more recognized functions such as phonation and alimentation.
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The scientists havee made an experiment called the Bell jar experiment which helps in explaining respiration concept so the have done a great job in explaining the respiration process