**Core Concept**
The blood pressure regulatory system involves multiple mechanisms to maintain homeostasis. The baroreceptor system plays a crucial role as a buffer system to counteract rapid changes in blood pressure. Baroreceptors are specialized sensors located in the walls of the carotid sinus and aortic arch that detect changes in arterial pressure.

**Why the Correct Answer is Right**
The baroreceptor system acts as a buffer system by modulating heart rate and vascular resistance in response to changes in blood pressure. When blood pressure increases, baroreceptors are stretched, leading to the activation of the vagus nerve and subsequent decrease in heart rate and peripheral resistance. Conversely, when blood pressure decreases, baroreceptor firing rates decrease, allowing for an increase in heart rate and peripheral resistance to restore blood pressure. This mechanism helps to maintain a relatively constant blood pressure despite changes in blood volume or resistance.

**Why Each Wrong Option is Incorrect**
**Option B:** Chemoreceptors are primarily involved in detecting changes in the levels of oxygen, carbon dioxide, and pH in the blood, which can indirectly affect blood pressure. However, they do not directly buffer changes in blood pressure.
**Option C:** The kidneys play a crucial role in long-term regulation of blood pressure through the renin-angiotensin-aldosterone system (RAAS), but they do not function as a buffer system to rapidly counteract changes in blood pressure.
**Option D:** The CNS ischemic response is a mechanism that occurs in response to inadequate blood flow to the brain, leading to vasodilation and increased blood flow. While this response can affect blood pressure, it is not a buffer system designed to counteract changes in blood pressure.

**Clinical Pearl / High-Yield Fact**
The baroreceptor reflex is a critical mechanism for maintaining blood pressure homeostasis. It is often tested in clinical settings using maneuvers such as the Valsalva maneuver or head-up tilt to assess its function.

**✓ Correct Answer: A. Baroreceptor**