The respiration rate (RR) is the number of breaths taken per minute. This rhythm of inhalation and exhalation ensures gas exchange, allowing oxygen to enter the bloodstream and carbon dioxide to be removed from the body. Breathing is managed by a precisely regulated control system that operates largely without conscious thought. This system constantly adjusts the rate and depth of breathing to match the body’s metabolic needs. Control of the respiration rate involves neural centers that establish the rhythm, chemical sensors that adjust it, and mechanical influences that modify it.
The Respiratory Rhythm Generator
The fundamental rhythm of breathing is generated within specialized groups of neurons located in the brainstem, specifically in the medulla oblongata and the pons. This collection of nerve cells acts as the central pattern generator for respiration, ensuring a consistent, involuntary cycle of inspiration and expiration. The medulla contains two main groups responsible for this rhythm: the Dorsal Respiratory Group (DRG) and the Ventral Respiratory Group (VRG).
The Dorsal Respiratory Group (DRG) is the primary center for quiet, resting inspiration, controlling the muscles that initiate the breath. Neurons in the DRG send signals to the diaphragm and the external intercostal muscles, causing them to contract. When these neurons stop firing, the inspiratory muscles relax, and the recoil of the lung tissue allows for passive expiration. The DRG also serves as an integrating center, receiving sensory input from various receptors that monitor blood gas levels and lung mechanics.
The Ventral Respiratory Group (VRG) is typically inactive during quiet breathing. It becomes active during forced or strenuous breathing, such as during intense exercise or respiratory distress. The VRG contains both inspiratory and expiratory neurons that stimulate accessory muscles for forceful inhalation and active exhalation. This group increases the volume and speed of breathing beyond the basic rhythm set by the DRG.
Working in concert with the medullary groups is the Pontine Respiratory Group, located in the pons. This center fine-tunes the medullary rhythm by smoothing the transition between inspiration and expiration. It includes the pneumotaxic and apneustic centers, which help coordinate the timing and depth of breaths. By limiting the duration of inspiration, the pontine centers contribute to a more efficient and regulated breathing pattern.
Chemical Regulation and Feedback Loops
While the brainstem establishes the basic pace, adjustments to the respiration rate are driven by chemical feedback loops that maintain homeostasis of blood gases. The body employs specialized sensors called chemoreceptors to monitor the levels of carbon dioxide, oxygen, and hydrogen ions (acidity) in the blood and cerebrospinal fluid. These sensors relay information back to the respiratory rhythm generator.
Carbon dioxide (CO2) is the primary chemical stimulus influencing the respiration rate. Central chemoreceptors, located near the surface of the medulla oblongata, are sensitive to changes in the CO2 concentration. Carbon dioxide readily diffuses from the blood into the brain’s interstitial fluid, where it reacts with water to form carbonic acid.
The formation of carbonic acid releases hydrogen ions, lowering the pH and increasing the acidity of the fluid surrounding the central chemoreceptors. This increase in acidity triggers the chemoreceptors to send signals to the medullary centers, resulting in hyperventilation (increased rate and depth of breathing). This faster, deeper breathing expels the excess CO2 from the body, raising the pH back to its normal level.
Peripheral chemoreceptors, found in the carotid bodies and the aortic bodies, also monitor blood chemistry. These receptors are sensitive to CO2 and hydrogen ions, but they primarily monitor arterial oxygen levels. Under normal conditions, oxygen has a minimal influence on the respiration rate because the blood maintains a high saturation of oxygen.
Oxygen becomes a driver of the respiratory rate when its partial pressure in the arterial blood drops below 60 millimeters of mercury (mmHg). In this state of hypoxemia, the peripheral chemoreceptors become the primary mechanism to increase ventilation. This mechanism ensures oxygen delivery when the CO2-driven system is insufficient or impaired. The peripheral receptors also adjust respiration when the body experiences sudden, non-CO2 related changes in blood pH, such as those caused by metabolic disturbances.
Modifying the Rate: Mechanical and Voluntary Influences
Beyond the automatic chemical adjustments, the respiration rate is modified by mechanical inputs and voluntary control from the cerebral cortex. The cortex allows for voluntary control over breathing, enabling actions like holding one’s breath, speaking, or singing. This conscious override bypasses the brainstem’s automatic rhythm generator. However, the chemical drive, particularly rising CO2 levels, limits this voluntary control, eventually forcing an involuntary breath.
Mechanical adjustments are triggered by proprioceptors, sensory receptors located in the muscles and joints. During physical activity, such as walking or running, these receptors detect movement and send signals to the brainstem to increase ventilation. This anticipatory response allows the body to increase the respiration rate at the start of exercise, often before chemical changes in blood CO2 or O2 have occurred.
The lungs contain specialized sensors that contribute to the regulation of breathing depth. Pulmonary stretch receptors, located in the smooth muscle of the airways, detect excessive stretching of the lung tissue during large inspirations. Once activated, these receptors initiate the Hering-Breuer reflex. This reflex sends inhibitory signals via the vagus nerve to the inspiratory center in the medulla, halting inhalation and promoting expiration to prevent over-inflation. In adult humans, this reflex is limited to instances where the volume of air inhaled is substantially large, such as during very deep breathing or intense exertion.
Other Neural Factors
Other factors from the nervous system can temporarily affect the breathing pattern. Signals from the limbic system, which controls emotion, can cause the rapid, shallow breathing associated with fear or anxiety. Sudden pain or a rise in body temperature can also prompt the respiratory control centers to increase the respiration rate.