What is the function of the kidneys regarding urine formation?

The kidneys play a critical role in urine formation through a multifaceted process that includes filtration, reabsorption, and secretion of substances. Filtration occurs in the glomeruli, where blood plasma is filtered, allowing water, ions, and small molecules to pass into the renal tubules while retaining larger proteins and blood cells.

After initial filtration, the kidneys reabsorb essential nutrients, such as glucose, amino acids, and a significant amount of water and electrolytes, back into the bloodstream, ensuring that the body retains what it needs for proper function. Additionally, the secretion process allows the kidneys to actively transport waste products and excess ions into the tubular fluid, further refining the composition of urine. This comprehensive process is vital for maintaining homeostasis, regulating electrolyte balance, and excreting waste products from metabolism.

The other options do not capture the full scope of kidney function. While blood filtration is a key part, the role of the kidneys extends beyond just filtering. Hormone secretion is also an important function of the kidneys, particularly in the production of erythropoietin and renin, but this does not directly define urine formation. Likewise, regulation of blood glucose levels is primarily managed by the liver and endocrine system, not specifically by the