When you hold your breath, which of the following first leads to the urge to breathe?
a. Falling CO₂
b. Falling O₂
c. Falling pH of the blood
d. Rising pH of the blood

Verified step by step guidance

1. Understand that the urge to breathe is primarily regulated by the body's need to maintain homeostasis, particularly in terms of blood gas levels and pH.
2. Recall that carbon dioxide (CO2) is a byproduct of cellular respiration and is transported in the blood. When CO2 levels rise, it reacts with water to form carbonic acid (H2CO3), which dissociates into hydrogen ions (H⁺) and bicarbonate ions (HCO3⁻). This process lowers the pH of the blood, making it more acidic.
3. Recognize that the body has chemoreceptors, such as those in the medulla oblongata and carotid bodies, that are sensitive to changes in CO2 levels and blood pH. These receptors are more sensitive to rising CO2 levels and the associated drop in pH than to falling oxygen (O2) levels.
4. Understand that when you hold your breath, CO2 accumulates in the blood because it is not being exhaled. This leads to a decrease in blood pH (acidosis), which is detected by the chemoreceptors.
5. Conclude that the first physiological trigger for the urge to breathe is the falling pH of the blood caused by rising CO2 levels, making the correct answer 'c. falling pH of the blood.'

Key Concepts

Carbon Dioxide (CO2) Levels

The concentration of carbon dioxide in the blood is a critical factor in regulating the urge to breathe. When you hold your breath, CO2 levels rise due to continued cellular respiration without exhalation. This increase in CO2 leads to a decrease in blood pH, stimulating chemoreceptors that signal the brain to initiate breathing.

Oxygen (O2) Levels

Oxygen levels in the blood are also important for respiratory regulation, but they typically have a lesser immediate effect compared to CO2 levels. While a significant drop in O2 can trigger the urge to breathe, it usually occurs after CO2 levels have already risen, making it a secondary factor in the immediate response to breath-holding.

Blood pH and Chemoreception

The pH of the blood is influenced by the levels of CO2, as increased CO2 leads to the formation of carbonic acid, lowering pH. Chemoreceptors in the body detect changes in pH and are sensitive to acidosis (low pH), which is a primary driver for the urge to breathe. Thus, a falling pH due to rising CO2 is a key signal for initiating respiration.