Studying the control of breathing in man

The control system of breathing can be considered as a closed-loop system, consisting of two subsystems: the controlling system and the controlled system. Both subsystems are defined by their input-output relationships. In the controlling system the input is the blood gas value; the output is some parameter of ventilation. The controlled system is characterized by an input of ventilation, and an output of blood gas values. In the closed-loop situation the control of breathing can be influenced by outside "disturbances", threatening to disrupt the regulation of the constancy of the internal environment. When studying the control of breathing, and therefore studying the strengths or defects of this homeostatic system, one has to decide whether one intends to investigate the closed-loop or the open-loop situation, and which defect in a subsystem may be the cause of a disrupted homeostasis. What non-feedback stimuli may be active at the moment of the investigation? How can they be kept constant or eliminated? What possible effects from drugs, beverages, nutrients (possibly consumed hours earlier) may still be present? In particular, the output parameters of the controlling system should be carefully chosen to represent that part of the system that one intends to investigate. Disruptions of the control of breathing may have serious consequences for several categories of patients, e.g. those with chronic obstructive pulmonary disease (COPD), asthma, sleep apnoea, sudden infant death syndrome, several neurological syndromes, and the hyperventilation syndrome. Adequate investigation of the control of breathing in these patients is of great importance for their treatment.