**Core Concept**
The question tests understanding of **orthostatic changes** in blood pressure and circulation when transitioning from a recumbent to a standing position. This involves **baroreceptors**, **sympathetic nervous system**, and **renal responses**.

**Why the Correct Answer is Right**
When a person stands up, **gravity** causes blood to pool in the lower extremities, reducing **venous return** to the heart. This decrease in **cardiac preload** triggers **baroreceptors** in the **carotid sinus** and **aortic arch** to activate the **sympathetic nervous system**, increasing **heart rate** and **vascular tone** to compensate for the reduced blood pressure.

**Why Each Wrong Option is Incorrect**
**Option A:** Incorrect because **heart rate** typically increases to compensate for the decreased blood pressure.
**Option B:** Incorrect as **stroke volume** may decrease due to reduced **venous return**.
**Option C:** Incorrect because **systemic vascular resistance** increases to counteract the drop in blood pressure.

**Clinical Pearl / High-Yield Fact**
A key point to remember is that **orthostatic hypotension** can lead to **dizziness** or **syncope** if the compensatory mechanisms are inadequate.

**Correct Answer:** D. blood pressure decreases