How is ECF volume regulated?
If ECF volume is to remain constant, the amount of sodium ingested must be matched by the amount of sodium excreted by the kidneys. In the example above, expansion of the ECF volume must somehow be sensed. What is sensed is not ECF volume, but rather a portion of ECF called effective arterial volume. Effective arterial volume (also called effective circulating volume or sensed volume) is that portion of the ECF that is in the arterial tree and effectively perfusing tissues. An increase in effective arterial volume is sensed by baroreceptors in the aortic arch, carotid sinus, central veins, cardiac chambers, and afferent arterioles. In addition, an increase in effective arterial volume leads to an increase in renal tubular flow, which is sensed by the macula densa. Signals (suppression of renal sympathetic nerve activity and suppression of the renin angiotens in aldosterone system) are then sent to the kidneys, which lead to diminished sodium reabsorption by the renal tubules and increased sodium excretion by the kidneys. In the example above, 150 mmol of sodium will be excreted by the kidneys, returning ECF volume to normal.