When a person moves from a supine position to a standing position, which of the following compensatory changes occurs?
## **Core Concept**
The question tests understanding of physiological responses to postural changes, specifically orthostatic hypotension and the body's compensatory mechanisms. When a person moves from a supine (lying down) position to a standing position, gravity causes a transient decrease in venous return to the heart, which can lead to a temporary drop in blood pressure. The body compensates for this through various mechanisms to maintain blood pressure and cerebral perfusion.
## **Why the Correct Answer is Right**
The correct compensatory change involves an increase in sympathetic tone and a decrease in parasympathetic tone. When standing up, there's a pooling of blood in the lower extremities, reducing venous return and stroke volume. The body responds by increasing heart rate and vascular resistance. This is primarily mediated by the **baroreceptor reflex**, which senses the decrease in blood pressure and triggers the **sympathetic nervous system** to increase heart rate (tachycardia) and peripheral resistance, while also reducing **parasympathetic (vagal) tone**, which normally acts to slow the heart.
## **Why Each Wrong Option is Incorrect**
- **Option A:** This option might suggest a decrease in heart rate, which is the opposite of what occurs as a compensatory mechanism. The heart rate increases to compensate for the decreased venous return and maintain cardiac output.
- **Option B:** This could imply a decrease in peripheral resistance, which would worsen the hypotension rather than compensate for it. Peripheral resistance increases to help maintain blood pressure.
- **Option C:** This might suggest no change or an inappropriate response, which does not align with the known physiological compensatory mechanisms.
## **Clinical Pearl / High-Yield Fact**
A key clinical pearl is that in individuals with autonomic dysfunction (e.g., diabetic neuropathy), the body's ability to compensate for postural changes is impaired, leading to **orthostatic hypotension**. This condition can cause dizziness or fainting upon standing. A simple clinical test for this is measuring blood pressure and heart rate in both supine and standing positions.
## **Correct Answer:** . Increased sympathetic tone and decreased parasympathetic tone.