Fluid & Electrolyte Q 73

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A rise in arterial pressure causes the baroreceptors and stretch receptors to signal an inhibition of the sympathetic nervous system, resulting in:

  

    A. Decreased sodium reabsorption

    B. Increased sodium reabsorption

    C. Decreased urine output

    D. Increased urine output

    

    

Correct Answer: D. Increased urine output

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Arterial baroreceptors and stretch receptors help maintain fluid balance by increasing urine output in response to a rise in arterial pressure. Baroreceptors are sensitive to the rate of pressure change as well as to the steady or mean pressure. Therefore, at a given mean arterial pressure, decreasing the pulse pressure (systolic minus diastolic pressure) decreases the baroreceptor firing rate. This is important during conditions such as hemorrhagic shock in which pulse pressure, as well as mean pressure, decreases. The combination of reduced mean pressure and reduced pulse pressure amplifies the baroreceptor response.

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Option A: During chronic increased dietary sodium intake, arterial baroreceptors buffer against sustained increases in arterial pressure, and renal sympatho inhibition contributes importantly to the maintenance of sodium balance by decreasing renal tubular sodium reabsorption and increasing urinary sodium excretion.

Option B: Arterial baroreceptors function to inform the autonomic nervous system of beat-to-beat changes in blood pressure within the arterial system. Rapid decreases in blood pressure, such as in orthostatic hypotension, resulted in decreased stretching of the artery wall and decreased action potential frequency, ultimately resulting in increased cardiac output and vasoconstriction resulting in increased blood pressure.

Option C: Nerve impulses from cardiopulmonary baroreceptors are also tonically active and increase their rate of firing secondary to increased blood volume and mean arterial pressure results in decreased sympathetic outflow to the sinoatrial node and decreased heart rate and cardiac output. In a notable difference, sympathetic outflow to the kidney increases, which increases renal blood flow and urine production, thereby decreasing the fluid volume of the body.

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