Question

The chemoreceptors in the medulla are directly stimulated by a. \(\mathrm{CO}_{2}\) from the blood. b. \(\mathrm{H}^{+}\)from the blood. c. \(\mathrm{H}^{+}\)in brain interstitial fluid that is derived from blood \(\mathrm{CO}_{2}\). d. decreased arterial \(\mathrm{P}_{\mathrm{O}_{2}}\).

Short answer

The chemoreceptors in the medulla are directly stimulated by \(\mathrm{H}^{+}\) in brain interstitial fluid that is derived from blood \(\mathrm{CO}_{2}\).

Step-by-step solution

Understand the chemoreceptors in the medulla

Chemoreceptors are specialized cells that detect changes in the chemical composition of body fluids. In the medulla, they are responsible for monitoring levels of \(\mathrm{CO}_{2}\), \(\mathrm{H}^{+}\), and \(\mathrm{O}_{2}\) and sending signals to the respiratory center to regulate breathing. Understanding this basic function will help us eliminate incorrect options.

Interpret each given option

We will analyze each option and use our understanding of chemoreceptors in the medulla to determine their validity: a. \(\mathrm{CO}_{2}\) from the blood: Since chemoreceptors monitor \(\mathrm{CO}_{2}\) levels in body fluids, this could be a possible stimulator. b. \(\mathrm{H}^{+}\) from the blood: This could be a stimulator as well since an increase in acidity (high \(\mathrm{H}^{+}\) concentration) is closely related to an increase in \(\mathrm{CO}_{2}\) levels. c. \(\mathrm{H}^{+}\) in brain interstitial fluid that is derived from blood \(\mathrm{CO}_{2}\): This option seems plausible because an increase in blood \(\mathrm{CO}_{2}\) would lead to an increase in \(\mathrm{H}^{+}\) in brain interstitial fluid. d. Decreased arterial \(\mathrm{P}_{\mathrm{O}_{2}}\): Although chemoreceptors monitor \(\mathrm{O}_{2}\) levels, they are more sensitive to changes in \(\mathrm{CO}_{2}\) and \(\mathrm{H}^{+}\) concentrations. Thus, this option is less likely than the others.

Choose the correct option

Based on our analysis of each option, the option that best fits is: c. \(\mathrm{H}^{+}\) in brain interstitial fluid that is derived from blood \(\mathrm{CO}_{2}\) This is because chemoreceptors in the medulla are directly stimulated by \(\mathrm{H}^{+}\) in the brain interstitial fluid derived from blood \(\mathrm{CO}_{2}\), meaning they respond to changes in \(\mathrm{CO}_{2}\) levels through their impact on the acidity of the brain interstitial fluid.

Key concepts

Regulation of Breathing

Breathing is an essential life process that provides oxygen to the body's cells and removes carbon dioxide. The regulation of breathing involves a complex interplay between various physiological systems, but a key player in this process is the medulla oblongata, a part of the brainstem.

The medulla houses the respiratory center, which controls the rate and depth of breathing. This center receives critical information about the body's needs from chemoreceptors, which are sensitive to the levels of carbon dioxide (CO2), oxygen (O2), and the pH of body fluids. When CO2 levels rise, it leads to increased acidity in the form of hydrogen ions (H+) in the brain's interstitial fluid, which in turn stimulates the chemoreceptors in the medulla. As a result, the respiratory rate increases, helping to exhale excess CO2 and restore homeostasis.

Blood CO2 Levels

Carbon dioxide is a byproduct of cellular metabolism and is transported in the blood. It is critical to maintain proper CO2 levels as it can directly impact the acidity of blood and other body fluids.

Chemoreceptors both in the medulla and located peripherally in the body can detect changes in blood CO2 levels. When these levels become elevated, CO2 can diffuse into the brain, where it is converted to hydrogen ions, increasing the acidity. The central chemoreceptors in the medulla oblongata respond to this acidity, not the CO2 itself, by stimulating the respiratory center to increase breathing rate and depth, enabling the exhalation of more CO2 to normalize these levels.

Brain Interstitial Fluid Acidity

The interstitial fluid of the brain is the narrow space that surrounds the brain's cells. It has a crucial role in maintaining a stable environment for neurons. When CO2 levels rise in the bloodstream, the gas can easily cross the blood-brain barrier and enter the brain interstitial fluid.

Once in the interstitial fluid, CO2 is converted to carbonic acid, which dissociates into bicarbonate and hydrogen ions (H+), leading to an increase in acidity. The medullary chemoreceptors are highly responsive to changes in this acidity, and their activation prompts the respiratory center to initiate corrective measures in the form of faster and deeper breathing to expel excess CO2 and reduce the fluid's acidity.

Respiratory Center Function

The respiratory center consists of several groups of neurons located in the medulla and the pons. Its primary function is to regulate the rate and rhythm of breathing, adapting to the body's metabolic demands.

Central chemoreceptors located on the surface of the medulla detect changes in the pH of the cerebral spinal fluid, which reflects the acidity of brain interstitial fluid. This information is used to adjust the respiratory rate. Other peripheral chemoreceptors located in the carotid arteries and aorta also respond to changes in blood pH, CO2, and O2 levels. Together, central and peripheral chemoreceptors provide a full-picture assessment to the respiratory center, ensuring appropriate adjustments to breathing to maintain homeostasis.