Regulation of pulmonary ventilation

Maintaining homeostatic balance in blood PO₂, PCO₂, and pH requires a high degree of coordination between the respiratory and circulatory systems. Much of this coordination is accomplished by involuntary regulation of pulmonary ventilation. This control is not yet fully understood, although many of the intricate neural controls have been identified.

Mechanisms of regulation

The respiratory muscles are under the direct control of motor neurons, which are in turn regulated by respiratory centers(inspiratory and expiratory) located within the brain stem(in the medulla oblongata and pons). These centers establish the rate and depth of breathing by sending out periodic impulses to the respiratory muscles. The cortex can override these centers if voluntary control of respiration is desired. Additionally, input from other parts of the brain occurs under certain conditions.

The inspiratory area of the brain(dorsal respiratory group) contains cells that intrinsically fire and control the basic rhythm of ventilation. The expiratory area is quiet during normal quiet breathing(recall that expiration is a passive process at rest). However, during forceful breathing such as during exercise, the expiratory area actively sends signals to the muscles of expiration. Two other brain centers aid in the control of respiration. The apneustic area has an excitatory effect on the inspiratory center and results in prolonged firing of the inspiratory neurons. Finally, the pneumotaxic center inhibits or “switches off” inspiration, helping to regulate inspiratory volume.

The respiratory centers do not act alone in controlling breathing. Breathing also is regulated and modified by the changing chemical environment in the body. For example, sensitive areas in the brain respond to changes in carbon dioxide and H⁺ levels. The central chemoreceptors in the brain are stimulated by an increase in H⁺ ions in the cerebrospinal fluid. The blood-brain barrier is relatively impermeable to H⁺ ions or bicarbonate. However, CO₂ readily diffuses across the blood-brain barrier and then reacts to increase H⁺ ions. This, in turn, stimulates the inspiratory center, which then activates the neural circuitry to increase the rate and depth of respiration. This increase in respiration, in turn, increases the removal of carbon dioxide and H⁺.

Chemoreceptors in the aortic arch(the aortic bodies) and in the bifurcation of the common carotid artery(the carotid bodies) are sensitive primarily to blood changes in PO₂ but also respond to changes in H⁺ concentration and PCO₂. The carotid chemoreceptors are more sensitive to changes in H⁺ concentration and PCO₂. Overall, PCO₂ appears to be the strongest stimulus for the regulation of breathing. When carbon dioxide levels become too high, carbonic acid forms, then quickly dissociates, giving off H⁺. If H⁺ accumulates, the blood becomes too acidic(pH decreases). Thus, an increased PCO₂ stimulates the inspiratory center to increase respiration – not to bring in more oxygen but to rid the body of excess carbon dioxide and limit further pH changes.

In addition to the chemoreceptors, other neural mechanisms influence breathing. The pleurae, bronchioles, and alveoli in the lungs contain stretch receptors. When these areas are excessively stretched, that information is relayed to the expiratory center. The expiratory center responds by shortening the duration of an inspiration, which decreases the risk of overinflating the respiratory structures. This response is known as the Hering-Breuer reflex.

Many control mechanisms are involved in the regulation of breathing, as shown in the figure below. Such simple stimuli as emotional distress or an abrupt change in the temperature of the surroundings can affect breathing. But all these control mechanisms are essential. The goal of respiration is to maintain appropriate levels of the blood and tissue gases and to maintain proper pH for normal cellular function. Small changes in any of these, if not carefully controlled, could impair physical activity and jeopardize health.