Key Points

Negative feedback systems control ventilation to maintain normal arterial blood gases and minimize the work of breathing in response to changes in the environment, activity, and lung function.

The basic respiratory rhythm is generated by neurons in the brain stem, and this rhythm is modulated by ventilatory reflexes. Arterial Pco2 is the most important factor determining ventilatory drive in resting humans. The ventilatory response to hypoxia depends strongly on arterial Pco2 and it is not large in normal individuals until arterial Po2 drops below 60 mm Hg.

Arterial chemoreceptors sense changes in arterial Po2, Pco2, and pH, but central chemoreceptors sense changes only in arterial Pco2.

Vagal nerve endings sensitive to stretch in the lungs mediate reflexes that fine-tune the rate and depth of breathing.

Vagal nerve endings sensitive to mechanical and chemical irritation of the airways and lungs stimulate coughing, mucous production, bron-choconstriction, and rapid shallow breathing. The autonomic nervous system and vagal sensory nerves in the airways are involved in local control of airway function. The most common cause of increased ventilation in healthy humans is exercise, but this cannot be explained by negative feedback ventilatory reflexes.

The ventilatory response to arterial blood gases changes with time during chronic hypoxemia at high altitude or with lung disease.

Essential Medical Physiology, Third Edition

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