The SNS causes the kidneys to constrict when
a) blood pressure rises.
b) fluids are released.
c) blood pressure drops.
d) fluids are retained.
The Correct Answer and Explanation is:
The correct answer is c) blood pressure drops.
Explanation:
The sympathetic nervous system (SNS) plays a crucial role in regulating blood pressure and fluid balance. One of its key functions is to respond to a drop in blood pressure by triggering a series of physiological responses that help restore normal pressure levels. This is part of the body’s broader mechanism for maintaining homeostasis, specifically through the baroreceptor reflex.
When blood pressure drops (such as during dehydration, blood loss, or shock), the baroreceptors in the carotid arteries and aortic arch detect this decrease and send signals to the brainstem. In response, the SNS is activated, and the body initiates compensatory mechanisms to increase blood pressure, including constricting blood vessels. These actions include:
- Vasoconstriction: The SNS stimulates the release of norepinephrine and epinephrine, which bind to receptors on smooth muscle cells in blood vessels, causing them to constrict. This increases peripheral vascular resistance and raises blood pressure.
- Renal response: The kidneys also play a significant role in blood pressure regulation. In response to SNS activation, the kidneys constrict the afferent arterioles, reducing the flow of blood into the glomeruli. This helps to maintain blood pressure by limiting the amount of fluid filtered into the urine, thus conserving fluid.
- Renin-Angiotensin-Aldosterone System (RAAS): SNS activation also triggers the release of renin from the kidneys. Renin converts angiotensinogen into angiotensin I, which is further converted into angiotensin II in the lungs. Angiotensin II has several effects, including vasoconstriction and the stimulation of aldosterone release, leading to sodium and water retention in the kidneys, further helping to raise blood pressure.
Thus, when blood pressure drops, the SNS causes the kidneys to constrict in order to conserve fluid and maintain adequate perfusion to vital organs, which helps restore blood pressure to normal levels.