What is the primary function of the nephrons in the kidneys?

The primary function of nephrons, which are the functional units of the kidneys, is the filtration of blood. Each nephron consists of a glomerulus and a renal tubule, and together they perform the critical task of filtering the blood to remove waste products and excess substances. During this filtration process, blood enters the glomerulus under pressure, allowing water, salts, and small molecules to pass through while larger molecules and blood cells remain in circulation. The filtered fluid then travels through the renal tubule, where selective reabsorption occurs, meaning the kidneys can reclaim essential substances like glucose and certain ions to keep in the body while concentrating waste products into urine for excretion.

This function is essential for maintaining homeostasis, regulating electrolyte levels, and controlling the volume of body fluids. While other options like regulating blood pressure and absorption of nutrients are indirectly related to kidney function, especially through its influence over the body's fluid balance and electrolyte concentration, they do not represent the primary activity of the nephrons themselves. Additionally, the kidneys do not store waste; instead, they continuously filter blood to produce urine that is then transported to the bladder for temporary storage before excretion.