Question

Explain, using a flowchart, how a rise in blood \(\mathrm{P}_{\mathrm{CO}_{2}}\) stimulates breathing. Include both the central and peripheral chemoreceptors in your answer.

Short answer

A rise in blood \(P_{CO_{2}}\) stimulates breathing through the involvement of central and peripheral chemoreceptors. When \(P_{CO_{2}}\) increases, CO2 diffuses into the cerebrospinal fluid and forms carbonic acid, which dissociates into HCO3- and H+. This increase in H+ is detected by central chemoreceptors located in the medulla oblongata. Peripheral chemoreceptors in the carotid and aortic bodies also detect changes in \(P_{CO_{2}}\), \(P_{O_{2}}\), and H+ concentration and send signals to the respiratory center. The respiratory center in the medulla oblongata and pons integrates these signals and sends action potentials to the diaphragm and intercostal muscles, leading to an increased breathing rate. This allows the body to take in more oxygen and remove more CO2, restoring normal blood gas levels.

Step-by-step solution

Increased Blood \(P_{CO_{2}}\)

When \(P_{CO_{2}}\) levels in the blood rise, it represents an increase in carbon dioxide concentration. This increase indicates that body cells produce more CO2, either due to increased activity or decreased elimination of CO2 by the respiratory system.

Central Chemoreceptors

Central chemoreceptors are located in the medulla oblongata of the brainstem. When there is an increase in \(P_{CO_{2}}\), more CO2 diffuses into the cerebrospinal fluid (CSF) surrounding the brain. CO2 reacts with water (H2O) to form carbonic acid (H2CO3), which then dissociates into bicarbonate (HCO3-) and hydrogen ion (H+). The increase in H+ concentration in the CSF is detected by central chemoreceptors.

Peripheral Chemoreceptors

Peripheral chemoreceptors are primarily located in the carotid and aortic bodies. These receptors can detect changes in blood \(P_{CO_{2}}\), \(P_{O_{2}}\), and H+ concentration. When there is an increase in \(P_{CO_{2}}\), these receptors become more sensitive to changes in \(P_{O_{2}}\) and H+ concentration. Peripheral chemoreceptors send signals to the respiratory center when they detect a deviation from normal levels.

Respiratory Center Stimulation

The signals from both the central and peripheral chemoreceptors are sent to the respiratory center, located in the medulla oblongata and the pons. The respiratory center then integrates these signals and sends action potentials to the diaphragm and intercostal muscles.

Increased Breathing Rate

As a result of the received action potentials, the diaphragm and intercostal muscles contract more frequently and strongly, leading to an increase in breathing rate. This increase in breathing rate allows the lungs to take in more oxygen and remove more CO2 from the body, thus restoring the normal blood \(P_{CO_{2}}\), \(P_{O_{2}}\), and H+ levels.

Key concepts

Central Chemoreceptors

Central chemoreceptors are essential components of the respiratory system, located in the medulla oblongata—the lower part of the brainstem. These specialized receptors play a vital role in regulating breathing.
When carbon dioxide (\(CO_2\)) levels increase in the blood, they diffuse into the cerebrospinal fluid (CSF) surrounding the brain. In this fluid, \(CO_2\) combines with water to form carbonic acid (\(H_2CO_3\)). This acid further breaks down into bicarbonate (\(HCO_3^-\)) and hydrogen ions (\(H^+\)), leading to a higher concentration of \(H^+\) in the CSF.
The central chemoreceptors are sensitive to these changes in hydrogen ion concentration rather than the carbon dioxide itself. When the concentration of \(H^+\) ions in the CSF increases, the central chemoreceptors detect this change and prompt the brain to increase the breathing rate.

• This increased breathing rate works to expel more \(CO_2\) from the lungs.
• As a result, normal levels of \(H^+\) ions and carbon dioxide are restored.

Peripheral Chemoreceptors

Peripheral chemoreceptors are another crucial element in the control of respiration. Unlike central chemoreceptors, these are located outside the brain in structures known as the carotid and aortic bodies. They monitor blood directly circulating through the body rather than the cerebrospinal fluid.
These receptors can detect changes in the partial pressure of carbon dioxide (\(P_{CO_2}\)), as well as the levels of oxygen (\(P_{O_2}\)) and hydrogen ions (\(H^+\)). When \(P_{CO_2}\) levels rise, peripheral chemoreceptors become more sensitive to these changes, particularly the \(H^+\) concentration.

• The increased \(P_{CO_2}\) leads to a more significant detection of \(H^+\), prompting these chemoreceptors to send signals to the respiratory centers in the brain.
• Peripheral chemoreceptors thereby ensure a faster response to changes in blood gas levels, complementing the role of the central chemoreceptors.

Carbon Dioxide

Carbon dioxide (\(CO_2\)) is a byproduct of cellular respiration, the process by which cells produce energy. The body needs to maintain precise levels of \(CO_2\); any imbalance can affect acidity levels in the blood, crucial for maintaining homeostasis.
When there's an increase in blood \(P_{CO_2}\), it indicates either an increase in production or a problem in eliminating \(CO_2\) from the body. This could happen due to heightened cell activity or impaired function of the respiratory system.
The body responds to elevated \(P_{CO_2}\) through the actions of both central and peripheral chemoreceptors. The stimulation of these receptors triggers increased breathing rates to achieve two main goals:

• Increase the intake of \(O_2\) and removal of \(CO_2\) from the bloodstream.
• Restore the balance of blood \(CO_2\), oxygen and hydrogen ion levels back to normal.

Proper breathing regulation through close monitoring of \(CO_2\) levels ensures that the body's physiological processes remain optimal.