What is the primary feedback used by the brain to control breathing?
a. Heart rate
b. Partial pressure of O₂
c. Blood pH, which indicates O₂ level
d. Blood pH, which indicates CO₂ level

Verified step by step guidance

1. Understand the role of feedback mechanisms in breathing: The brain regulates breathing primarily through feedback mechanisms that monitor the levels of certain gases and pH in the blood.
2. Recall the relationship between CO2 and blood pH: Carbon dioxide (CO2) dissolves in the blood and reacts with water to form carbonic acid (H2CO3), which dissociates into hydrogen ions (H⁺) and bicarbonate ions (HCO3⁻). This process lowers blood pH, making it more acidic.
3. Identify the primary feedback signal: The brain, specifically the medulla oblongata, is highly sensitive to changes in blood pH. A drop in pH (indicating an increase in CO2 levels) triggers the brain to increase the rate and depth of breathing to expel more CO2.
4. Eliminate incorrect options: Heart rate (option a) is not directly used to control breathing. Partial pressure of O2 (option b) is monitored but is a secondary feedback mechanism. Blood pH indicating O2 level (option c) is incorrect because pH changes are primarily linked to CO2 levels, not O2.
5. Conclude the correct answer: The primary feedback used by the brain to control breathing is blood pH, which indicates CO2 levels (option d).

Key Concepts

Homeostasis

Homeostasis refers to the body's ability to maintain stable internal conditions despite external changes. In the context of breathing, it involves regulating levels of oxygen (O2) and carbon dioxide (CO2) in the blood to ensure that cellular processes function optimally. The brain monitors these levels to adjust breathing rates accordingly, ensuring that the body meets its metabolic demands.

Chemoreceptors

Chemoreceptors are specialized sensory receptors that detect changes in the chemical composition of the blood, particularly the levels of O2 and CO2. These receptors are primarily located in the carotid arteries and the brainstem. They play a crucial role in the respiratory control center of the brain, providing feedback that influences the rate and depth of breathing based on the body's needs.

Blood pH and CO2 Levels

Blood pH is a measure of acidity or alkalinity in the blood, which is influenced by the concentration of CO2. When CO2 levels rise, it leads to an increase in carbonic acid, lowering blood pH and signaling the brain to increase breathing rate to expel CO2. This feedback mechanism is vital for maintaining acid-base balance and ensuring proper respiratory function.