Carotid bodies are stimulated by

Options

A. `CO_2` concentration in venous blood
B. `O_2` concentration in arterial blood
C. `CO_2` concentration in arterial blood
D. `O_2` concentration in venous blood

Answer

**Step-by-Step Solution:**

1. **Understanding Carotid Bodies**: Carotid bodies are small clusters of chemoreceptors located near the bifurcation of the carotid arteries. They play a crucial role in monitoring the levels of certain gases in the blood.

2. **Function of Chemoreceptors**: The primary function of these chemoreceptors is to detect changes in the concentration of specific substances in the blood, particularly carbon dioxide (CO2) and hydrogen ions (H+).

3. **Stimuli for Carotid Bodies**: The carotid bodies are stimulated primarily by:
- An increase in the concentration of carbon dioxide (CO2) in the arterial blood.
- An increase in hydrogen ion concentration (which indicates a decrease in pH).

4. **Analyzing the Options**: The options provided in the question include:
- CO2 concentration in venous blood
- Oxygen concentration in arterial blood
- CO2 concentration in arterial blood
- O2 concentration in venous blood

5. **Eliminating Incorrect Options**:
- **Oxygen concentration in arterial blood**: This does not stimulate the carotid bodies directly; they are more sensitive to CO2 and H+ levels.
- **CO2 concentration in venous blood**: While CO2 is present in venous blood, the carotid bodies primarily respond to arterial blood levels.
- **O2 concentration in venous blood**: This is not relevant to the stimulation of carotid bodies.

6. **Identifying the Correct Answer**: The only option that correctly identifies a stimulus for the carotid bodies is the **CO2 concentration in arterial blood**. This is because the carotid bodies respond to changes in arterial blood gases, particularly increases in CO2.

7. **Conclusion**: Therefore, carotid bodies are stimulated by the CO2 concentration in arterial blood.

**Final Answer**: Carotid bodies are stimulated by CO2 concentration in arterial blood.

---