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 the role of respiratory gases and blood pH in the urge to breathe. The body's respiratory system is closely monitored by chemoreceptors that respond to changes in the concentration of gases and the pH of the blood.
2. Recognize that CO2 levels in the blood affect the pH. When CO2 levels rise, it reacts with water in the blood to form carbonic acid, which dissociates into hydrogen ions and bicarbonate, lowering the pH of the blood.
3. Identify that a lower blood pH (more acidic) due to increased CO2 is detected by central chemoreceptors in the brain. These chemoreceptors respond to the change in pH and send signals to increase the respiratory rate.
4. Note that while low oxygen levels (O2) can also stimulate the urge to breathe, this typically occurs under more severe conditions of oxygen deprivation. The primary drive under normal circumstances is the CO2 levels and the resulting pH change.
5. Conclude that the most immediate and sensitive response that leads to the urge to breathe is related to the rising levels of CO2 in the blood, which lowers the pH, rather than changes in oxygen levels.

Key Concepts

Carbon Dioxide (CO2) Levels

When you hold your breath, carbon dioxide (CO2) levels in the blood begin to rise due to continued cellular respiration. This increase in CO2 leads to a decrease in blood pH, making the blood more acidic. The body has chemoreceptors that detect these changes, and the urge to breathe is primarily triggered by elevated CO2 levels rather than low oxygen levels.

Oxygen (O2) Levels

Oxygen levels in the blood are also important for regulating the urge to breathe, but they play a secondary role compared to CO2 levels. While a significant drop in O2 can eventually stimulate breathing, the body is more sensitive to changes in CO2. Therefore, the urge to breathe is typically felt before oxygen levels become critically low.

Blood pH and Acidity

Blood pH is a measure of acidity in the blood, which is influenced by the levels of CO2. An increase in CO2 results in a decrease in pH (more acidic), which is detected by chemoreceptors in the body. This drop in pH is a key factor that signals the respiratory center in the brain to initiate breathing, making it a crucial concept in understanding the urge to breathe.