Atrial natriuretic peptide (ANP) is a hormone produced by the cardiac atria that plays a crucial role in regulating blood pressure and fluid balance in the body. When blood volume increases, the atria of the heart stretch, leading to the release of ANP.
ANP influences water reabsorption primarily in the kidneys by acting on the renal tubules, specifically affecting the collecting ducts. The presence of ANP inhibits the reabsorption of sodium (Nat) in these tubules. As sodium reabsorption decreases, water (H₂O) reabsorption also decreases due to osmotic principles; as less sodium is reabsorbed, less water follows sodium back into the bloodstream.
By promoting natriuresis (the excretion of sodium through urine), ANP effectively increases the excretion of water, leading to a decrease in blood volume and blood pressure. This process counteracts the effects of other hormones, such as aldosterone, which typically promote sodium and water reabsorption. In summary, ANP reduces water reabsorption in the kidneys by inhibiting sodium reabsorption in the tubules, thus facilitating the excretion of excess water and sodium.