Question

The hypoxic ventilatory response occurs when low arterial \(\mathrm{P}_{\mathrm{O}}\) stimulates the a. aortic bodies. b. carotid bodies. c. central chemoreceptors. d. all of these.

Short answer

The hypoxic ventilatory response is a reflex increase in breathing rate triggered by low arterial \(PO_2\) or low oxygen levels. In this case, the carotid bodies, a set of chemoreceptors found at the bifurcation of the carotid artery, play a significant role in sensing low oxygen levels in the arterial blood and sending signals to the brain, thereby triggering an increase in ventilation rate. Therefore, the correct answer is b. carotid bodies.

Step-by-step solution

Identify the hypoxic ventilatory response

Hypoxic ventilatory response is a reflex increase in breathing rate triggered by low arterial \(PO_2\) or low oxygen levels.

Role of aortic bodies

Aortic bodies are small clusters of chemoreceptor cells located in the aortic arch. They sense changes in oxygen, carbon dioxide, and pH levels in the blood and send signals to the brain, but they only play a minor role in the hypoxic ventilatory response.

Role of carotid bodies

Carotid bodies are a set of chemoreceptors found at the bifurcation of the carotid artery, near the neck. They play a significant role in sensing low oxygen levels in the arterial blood and then sending signals to the brain, thereby triggering an increase in ventilation rate.

Role of central chemoreceptors

Central chemoreceptors are located in the medulla oblongata of the brain. They primarily sense changes in the CO_2 and pH of the cerebrospinal fluid surrounding the brain and play a role in the regulation of ventilation rate.

Choose the correct answer

Based on the roles of these different parts, we can conclude that the hypoxic ventilatory response occurs when low arterial \(PO_2\) stimulates the carotid bodies. Therefore, the correct answer is: b. carotid bodies.

Key concepts

Aortic Bodies

Aortic bodies are small groups of chemoreceptor cells located near the aortic arch, which play an interesting role in monitoring blood chemistry. These clusters are sensitive to changes in the levels of key substances in the blood, such as oxygen \(O_2\), carbon dioxide \(CO_2\), and hydrogen ions \(H^+\), which together influence the pH level of the blood.

Aortic bodies send signals to the brainstem to help regulate breathing and maintain homeostasis—the body's stable internal environment. However, when it comes to the hypoxic ventilatory response, or the body's reflexive increase in breathing triggered by low arterial oxygen levels \(PO_2\), aortic bodies play a relatively minor role.

• They detect oxygen levels but are not the primary detectors.
• They contribute to the overall regulation of the difference that \(O_2\), \(CO_2\), and \(H^+\) levels make.

Thus, while aortic bodies have a part to play in general respiratory regulation, they are not the main actors during a hypoxic event.

Carotid Bodies

Carotid bodies are vital sensory organs responsible for detecting low oxygen levels in the blood, making them the primary players in the hypoxic ventilatory response. Located at the bifurcation of each carotid artery in the neck, these chemoreceptors are strategically positioned to monitor arterial oxygen levels effectively.

When a decrease in \(PO_2\) is detected, carotid bodies accelerate their signaling to the brain, particularly to the respiratory centers of the brainstem. This heightened neural activity triggers an increase in breathing rate, improving oxygen uptake and thus restoring oxygen levels in the blood.

• They act quickly to low \(O_2\) levels, providing immediate response.
• Their role is critical in conditions where oxygen supply is compromised.

These actions highlight why carotid bodies are considered key structures in the hypoxic ventilatory response, making them crucial for immediate respiratory adjustments.

Central Chemoreceptors

Central chemoreceptors are located within the brain, specifically in the medulla oblongata, and they play a significant role in respiratory control by responding primarily to changes in carbon dioxide \(CO_2\) and pH levels in the cerebrospinal fluid (CSF).

Unlike aortic and carotid bodies, which directly sense changes in blood chemistry, central chemoreceptors are sensitive to \(CO_2\) levels because carbon dioxide can diffuse into the brain.

• Higher \(CO_2\) levels lead to increased hydrogen ions, indicating acidity.
• They adjust breathing rate to stabilize \(CO_2\) and pH quickly.

While they play a secondary role in detecting low oxygen levels, their primary concern is managing \(CO_2\) and acidity to safeguard brain function. As a result, central chemoreceptors complement the carotid bodies' actions rather than taking the lead in the hypoxic ventilatory response.