**Core Concept**
The respiratory center, a group of neurons in the medulla oblongata, is responsible for regulating breathing patterns. It is intricately connected with other centers in the brainstem to ensure coordinated respiratory and cardiovascular functions.

**Why the Correct Answer is Right**
The respiratory center is connected with the cardiac center through the pneumotaxic and slowly adapting pulmonary stretch receptors, which provide feedback on lung inflation and deflation. This connection allows for the integration of respiratory and cardiac functions, enabling the body to respond appropriately to changes in oxygen and carbon dioxide levels, as well as blood pressure. The cardiac center, located in the medulla, receives input from the respiratory center and adjusts heart rate and contractility accordingly.

**Why Each Wrong Option is Incorrect**
**Option A:** The respiratory center is primarily stimulated by an increase in PaCO2 (carbon dioxide partial pressure), not a decrease in PaO2 (oxygen partial pressure). A decrease in PaO2 would stimulate peripheral chemoreceptors in the carotid and aortic bodies.

**Option C:** Swallowing does not directly inhibit the respiratory center. However, during swallowing, breathing is temporarily suspended to prevent aspiration of food or liquids into the airways. This is due to the activation of the swallowing center, which temporarily inhibits the respiratory center.

**Option D:** The respiratory center is primarily located in the medulla oblongata, but it extends into the pons. The midbrain is not the primary location of the respiratory center.

**Clinical Pearl / High-Yield Fact**
The pneumotaxic center, located in the pons, helps regulate the rate of breathing by limiting the duration of inspiration. This center is often affected in patients with respiratory disorders, such as chronic obstructive pulmonary disease (COPD).

**✓ Correct Answer: B. Connected with cardiac center**