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Hypobaric Hypoxia: Adaptation and Acclimatization
Published in Anthony N. Nicholson, The Neurosciences and the Practice of Aviation Medicine, 2017
John H. Coote, James S. Milledge
The large step between the inspired and the alveolar oxygen tensions, which at sea level is around 50 mm (6.7 kPa), is reduced to around 30 mm (4.0 kPa) at the altitude simulated in the diagram (Figure 7.4). This is achieved by an increase in ventilation. Such hyperventilation is of adaptive value, being functionally equivalent to a descent to a lower altitude (Rahn and Otis, 1949). Initially, the increase in ventilation in response to a decrease in ambient oxygen tensions results from the stimulation of carotid body chemoreceptors by the reduction in the tension of oxygen in arterial blood. This can be shown in humans by the use of the Dejours O2-test (Dejours, 1957), whereby an abrupt inhalation of 100 per cent O2 in a subject breathing at progressively increased levels of hypoxia leads to a decrease in ventilation. By plotting the steady-state ventilation for each level of hypoxia and observing the ventilation following the Dejours O2-test, two curves are obtained of ventilation at the different degrees of oxygenation shown in Figure 7.5. The magnitude of the difference between the hypoxia ventilation and that of the few breaths during pure oxygen is a measure of the peripheral chemoreceptor drive (Bouverot et al., 1965; Leitner et al., 1965a, 1965b; Guz et al., 1966).
Intra-carotid body inter-cellular communication
Published in Journal of the Royal Society of New Zealand, 2023
Liam P. Argent, Aabharika Bose, Julian F. R. Paton
The carotid bodies are examples of such sensors. Although best known as regulators of blood gases, the carotid bodies are in fact multi-modal sensors (Conde et al. 2014; Holmes et al. 2019; Prabhakar and Peng 2017; Shin et al. 2019). Located bilaterally at the bifurcation of the two common carotid arteries, they are innervated by the glossopharyngeal and vagal nerves, as well as by a sympathetic nerve that originates in the nearby superior cervical ganglion (Schulz et al. 2016). Petrosal ganglion neuron projections carried by the carotid sinus and then glossopharyngeal nerves act as the only afferents and route carotid body output to the brainstem (Schulz et al. 2016). Activation of the carotid body by various stimuli, including acidosis, hypoxia, hypercapnia, hypoglycaemia and others, drives an increase in minute ventilation, as well as an increase in sympathetic activity (via a reflex loop that includes the nucleus of the solitary tract and ventral lateral medulla areas of the brainstem) that results in elevated blood pressure (Marshall 1994). Interestingly, carotid body stimulation also activates the nucleus ambiguus, which induces bradycardia (Marshall 1994) and bronchoconstriction (Moraes et al. 2021) via the vagus nerve. Collectively, these changes constitute the classical primary peripheral chemoreflex response.