What is the main function of the collecting duct in the kidneys?

The primary function of the collecting duct in the kidneys is to conserve water. This segment of the nephron plays a crucial role in the final stages of urine concentration. As the filtrate passes through the collecting duct, the permeability to water is regulated by hormones, particularly antidiuretic hormone (ADH). When ADH is present, the collecting duct becomes more permeable to water, allowing water to be reabsorbed back into the bloodstream. This reabsorption is vital for maintaining the body's fluid balance and preventing dehydration.

In terms of the other functions listed, while urine production occurs in the nephron, it is primarily the result of the combined actions of various nephron segments, including the glomerulus and the proximal convoluted tubule, rather than the function of the collecting duct alone. The filtering of blood occurs in the glomeruli and proximal tubules, where waste products and excess substances are removed from the blood. Finally, while the kidney does play a role in regulating blood flow through various mechanisms, the specific action of the collecting duct is not focused on this but rather on the reabsorption of water, thereby concentrating urine. Thus, conserving water is indeed the main function of the collecting duct.