Role of Atrial Natriuretic Peptide in Controlling Renal Excretion
Thus far, we have discussed mainly the role of sodium-and water-retaining hormones in controlling extracel-lular fluid volume. However, several different natri-uretic hormones may also contribute to volume regulation. One of the most important of the natri-uretic hormones is a peptide referred to as atrial natri-uretic peptide (ANP), released by the cardiac atrialmuscle fibers. The stimulus for release of this peptide appears to be overstretch of the atria, which can result from excess blood volume. Once released by the cardiac atria, ANP enters the circulation and acts on the kidneys to cause small increases in GFR and decreases in sodium reabsorption by the collecting ducts. These combined actions of ANP lead to increased excretion of salt and water, which helps to compensate for the excess blood volume.
Changes in ANP levels probably help to minimize changes in blood volume during various disturbances, such as increased salt and water intake. However, excessive production of ANP or even complete lack of ANP does not cause major changes in blood volume because these effects can easily be overcome by small changes in blood pressure, acting through pressure natriuresis. For example, infusions of large amounts of ANP initially raise urine output of salt and water and cause slight decreases in blood volume. In less than 24 hours, this effect is overcome by a slight decrease in blood pressure that returns urine output toward normal, despite continued excess of ANP.