What are the primary stages of urine formation?

The primary stages of urine formation involve glomerular filtration, tubular reabsorption, and tubular secretion.

During glomerular filtration, blood is filtered in the glomeruli of the kidneys, allowing water, ions, and small molecules to pass into the nephron while retaining larger molecules like proteins and blood cells. This process initiates the formation of filtrate.

Next, tubular reabsorption occurs as the filtrate moves through the renal tubules. In this stage, essential substances such as water, glucose, and certain ions are reabsorbed back into the bloodstream, ensuring that the body retains necessary nutrients and maintains homeostasis. The nephron selectively reabsorbs these substances based on the body's needs.

Lastly, tubular secretion is where additional waste products and excess ions are secreted from the blood into the tubular fluid. This process helps to fine-tune the composition of urine, further eliminating unwanted substances from the body.

Understanding these stages highlights how the kidneys effectively regulate the composition and volume of blood while producing urine. The other choices refer to processes that may be involved in the kidney's function but do not accurately represent the primary stages of urine formation.