Final answer:
ANP's relaxation of mesangial cells increases GFR by expanding filtration surface area, thus facilitating higher fluid excretion, reducing blood volume and pressure.
Step-by-step explanation:
ANP, or atrial natriuretic peptide, is a hormone that plays a crucial role in blood volume and pressure regulation by acting on the kidneys. When ANP relaxes mesangial cells in the glomeruli of the kidneys, it causes an increase in the glomerular filtration rate (GFR). Essentially, by relaxing mesangial cells, ANP allows for an expansion of the surface area available for filtration, thus permitting more blood to be filtered and increasing the GFR. As a result, there is a decrease in blood volume and pressure because more fluid is excreted as urine due to the reduced reabsorption of Na+ by the nephrons. Furthermore, ANP inhibits the release of renin, aldosterone, and ADH, which further contributes to the reduction in blood volume and pressure by preventing water retention.