Control Of Ventilation

The body contains several physiologic control systems to maintain arterial pH (pHa) within normal limits, to meet the oxygen demands of the tissues, to minimize the mechanical work of breathing, and to prevent lung injury by environmental agents. This means that limitations in lung function or gas exchange can be masked by the physiologic control systems acting to maintain homeostasis. Therefore, knowledge of the normal physiology of respiratory control is critical for understanding respiratory responses to activity, the environment, and pulmonary disease. This knowledge is also necessary to distinguish abnormalities in the respiratory control system from primary disturbances in lung function.

Reflexes and Negative Feedback

Breathing originates from a respiratory rhythm generated in the brain, as described next in the Respiratory Rhythm Generation section. However, homeostasis requires reflexes to modulate the timing and amplitude of the basic breathing rhythm in response to changes in respiratory system function.

The reflex control system has three components (Fig. 1). First, a sensory system detects and transmits information about the level of some physiologic variable in the body, called the physiologic stimulus. This sensory information is called afferent input. Afferent nerves code sensory information with frequency of action potentials (see Chapter 49). Second, central integration processes afferent input from multiple sensory systems and determines an appropriate response. Such complex

Central integration medulla, pons

Central integration medulla, pons

Afferent input

Higher CNS center (behaviour) Lung receptors

(mechanics) Chemoreceptors (arterial blood)

Efferent output

Ventilation Bronchial muscle Secretory glands

Negative Feedback Gas exchange Mechanics

FIGURE 1 Three components of negative feedback ventilatory control by reflexes as described in the text. Lung receptors and chemoreceptors sense changes in mechanics and arterial blood, and cause reflex changes in ventilation and airways. These efferent responses affect gas exchange and lung mechanics and also provide negative feedback.

Efferent output

Ventilation Bronchial muscle Secretory glands

Afferent input

Higher CNS center (behaviour) Lung receptors

(mechanics) Chemoreceptors (arterial blood)

Negative Feedback Gas exchange Mechanics

FIGURE 1 Three components of negative feedback ventilatory control by reflexes as described in the text. Lung receptors and chemoreceptors sense changes in mechanics and arterial blood, and cause reflex changes in ventilation and airways. These efferent responses affect gas exchange and lung mechanics and also provide negative feedback.

information processing occurs in the central nervous system (CNS) and involves the neurophysiologic and neurochemical mechanisms of synaptic transmission (Chapter 6). Central integration of respiratory reflexes occurs in the medulla and pons, in proximity to the areas that generate the respiratory rhythm. Third, efferent output occurs when an effector system executes the appropriate response. Negative feedback occurs when the efferent output has an effect on the original stimulus and afferent input, such as ventilation changing arterial PCO2.

Figure 1 shows the important components of respiratory reflexes. Reflexes are involuntary by definition, but notice that breathing can be changed voluntarily by neural commands for behaviors from higher centers in the CNS. For example, breathing may be changed during speech, which requires a voluntary command from the cortex. However, such voluntary commands are integrated with all other afferent information to generate an appropriate efferent response. This explains why it is so difficult to speak in a normal voice immediately after hard exercise, when other factors are strongly stimulating ventilation.

Two main classes of sensory systems convey afferent information about the function of the respiratory system: the chemoreceptors and the mechanoreceptors (see Fig. 1). Arterial Po2, Pco2, and pH are collectively referred to as arterial blood gases, and they provide a good index of gas exchange and lung mechanics, as described in previous chapters. Chemoreceptors monitor changes in arterial blood gases and cause reflex changes in ventilation that return arterial blood gases toward normal values. For example, increases in Paco2 or decreases in PaO2 stimulate reflex increases in ventilation. Mechanoreceptors monitor pressure and volume in the lungs and airways to provide afferent information about pulmonary mechanics. Generally, mechanoreceptors induce reflex changes in the rate and depth of breathing to minimize the work of breathing under different mechanical conditions and at different levels of ventilation. Mechanoreceptors (and other sensory nerves from the lungs and airways) are also involved in airway smooth muscle and secretory responses that defend the lungs from environmental insult. Note that, in this context, the term receptor refers to a specialized sensory nerve ending and not a neurotransmitter or drug receptor.

All of the reflexes described in this chapter are examples of negative feedback control. Negative feedback describes the effect of the response (efferent output) on the stimulus. For example, increased PaCO2 will stimulate chemoreceptors to cause a reflex increase in ventilation and this, in turn, decreases PaCO2. Negative feedback reduces the original deviation in the stimulus from its normal level. In control systems terminology, the measured input from a regulated variable (PaCo2) is held constant by changes in the output of a controlled variable (ventilation). In most physiologic control systems, increases or decreases in a regulated variable stimulate the appropriate response from a controlled variable to minimize the initial disturbance. For example, decreases in PaCo2 will decrease chemoreceptor stimulation and cause a reflex decrease in ventilation. The resulting increase in PaCo2 is also an example of negative feedback.

Note that the most common stimulus for increased ventilation is exercise. However, despite years of study, we still cannot explain exercise hyperpnea based on the reflexes described in this chapter. The topic is discussed more in the chapter on exercise.

Respiratory Rhythm Generation

Although breathing is similar to the heartbeat in terms of being automatic and being continuous during sleep or general anesthesia, the skeletal muscles driving ventilation do not contract spontaneously like cardiac muscle does. Rhythmic breathing results from periodic activation of the ventilatory muscles by motor nerves from the CNS. A so-called central pattern generator, composed of networks of neurons, generates this basic respiratory rhythm. The central pattern generator is located in the medulla near other respiratory centers that integrate afferent information for ventilatory reflexes to fine-tune the motor output to ventilatory muscles.

Figure 2 shows the areas of the brain that include the (1) central pattern generator of respiration, (2) the motor nuclei for respiratory muscles, and (3) the sites of central integration for respiratory reflexes. The neural structures responsible for these functions have been identified by experiments on animals and studies of patients with various brain injuries. Modern imaging techniques hold promise for a better understanding of human respiratory centers, but the current technology does not have the resolution necessary to study these small complexes of neurons in the brain stem.

The central pattern generator for ventilation has been isolated in experimental animals to a very small region of the medulla called the pre-Botzinger complex. It has been known for centuries that ventilation begins in the CNS. Galen observed that Roman gladiators with injuries in the neck stopped breathing, but breathing continued if the same injury was below the neck. In the 1800s, physiologists identified most of the important

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