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Chemical Factors Affecting Respiration Centers

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CO₂, O₂, and H⁺ ion levels affect the rate of respiration.

These changes are detected by the central chemoreceptors in the brain stem, and the peripheral chemoreceptors in the aortic arch and the bifurcations of the common carotid arteries.

In hypercapnia, an increased level of arterial P_CO2 drives the diffusion of CO₂ from the blood to the cells, forming carbonic acid.

This carbonic acid then dissociates, liberating H⁺ ions, which are detected by the central chemoreceptors in the brain, triggering an intensified breathing pace.

Conversely, peripheral chemoreceptors can detect increased P_CO2 directly and also respond to increased H⁺ ion levels.

Normally, blood oxygen levels influence breathing indirectly by altering chemoreceptor sensitivity to changes in CO₂.

The peripheral chemoreceptors respond to oxygen levels only when arterial P_O2falls below 60 mmHg.

At this point, the breathing rate increases, creating a feedback loop: increased breathing decreases P_CO2 and increases the blood pH, both of which inhibit respiration.

As a result, the breathing rate is slowly restored to normal.

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Chemical factors such as changing CO₂, O₂, and H⁺ levels in arterial blood play a critical role in influencing respiration depth and rates. These variations are detected by chemoreceptors—specialized sensors located in two primary body areas. Central chemoreceptors are found throughout the brain stem, including the ventrolateral medulla, while peripheral chemoreceptors are located in the aortic arch and carotid arteries.

CO₂ has a potent influence on respiration and is strictly regulated. Under normal conditions, the arterial pCO₂is maintained at 40 mm Hg with small fluctuations within 38-42 mm Hg. An increase in P_CO2 levels, known as hypercapnia, leads to an accumulation of CO₂ in the brain. This CO₂ is hydrated to form carbonic acid, which dissociates to liberate H⁺, causing a drop in pH. This rise in H⁺ stimulates central chemoreceptors, closely linked with respiratory regulatory centers. The result is an increase in the depth and rate of breathing facilitated by both central and peripheral chemoreceptors. This increased ventilation rapidly eliminates CO₂ from the blood, raising the blood's pH. Conversely, when pCO₂is abnormally low, respiration slows and becomes shallow.

Changes in arterial pH can alter the respiratory rate and rhythm, independent of CO₂ and O₂levels. If arterial pH falls, the respiratory system attempts to compensate by increasing the respiratory rate and depth to eliminate CO₂ from the blood. This process is mediated through the peripheral chemoreceptors, as H⁺ does not cross the blood-brain barrier.

Peripheral chemoreceptors are sensitive to arterial O₂ levels and become the primary driver of respiration when arterial pO₂falls below 60 mm Hg. The peripheral chemoreceptors respond by reflexively increasing ventilation. Although increased ventilation can lead to hypocapnia and an increase in blood pH, which could inhibit respiration, the drive to correct severe hypoxia typically prevails, maintaining elevated ventilation until O₂ levels are restored.

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