Decrease in BP causes:
**Core Concept**
The question tests the understanding of the body's response to a decrease in blood pressure, specifically the baroreceptor reflex. This reflex is crucial for maintaining **blood pressure homeostasis**. The **autonomic nervous system** plays a key role in this reflex.
**Why the Correct Answer is Right**
A decrease in blood pressure activates the baroreceptor reflex, which involves the **sympathetic nervous system** to increase heart rate and vascular tone, thereby increasing blood pressure. This reflex also involves the **parasympathetic nervous system**, specifically the **vagus nerve**, to decrease heart rate, but the sympathetic response dominates in this scenario. The **baroreceptors** in the **carotid sinus** and **aortic arch** detect the decrease in blood pressure and send signals to the **brainstem**, which then activates the sympathetic nervous system.
**Why Each Wrong Option is Incorrect**
**Option A:** Incorrect because a decrease in blood pressure would not directly cause an increase in parasympathetic activity to decrease heart rate further.
**Option B:** Incorrect as it is not directly related to the immediate response to decreased blood pressure.
**Option C:** Incorrect because the body's immediate response to decreased blood pressure is not to decrease sympathetic activity.
**Option D:** Incorrect as it is the correct answer choice that is missing.
**Clinical Pearl / High-Yield Fact**
A key point to remember is that the baroreceptor reflex is a critical mechanism for maintaining blood pressure homeostasis, and its dysfunction can lead to **orthostatic hypotension**. Understanding this reflex is essential for managing patients with blood pressure disorders.
**Correct Answer:** D. Increase in sympathetic activity