A student reads that the kidneys filter the blood and then selectively reabsorb useful substances back into the blood, while excreting waste products in urine. Explain how the kidneys carry out excretion, including what is filtered, what is reabsorbed, and what remains in the urine.

The kidneys filter blood under high hydrostatic pressure to produce a filtrate that contains water, ions, glucose, urea and other small molecules.
1. In the glomerulus, plasma is forced through the filtration membrane, leaving behind a filtrate.
2. In the proximal tubule and other segments of the nephron, useful substances such as glucose, amino acids and most water are selectively reabsorbed back into the bloodstream.
3. Waste products, mainly urea, are not reabsorbed and remain in the filtrate.
4. Urea is produced in the liver from the breakdown of excess amino acids and is therefore a metabolic waste product.
5. The amount of water reabsorbed is regulated by antidiuretic hormone, allowing the kidneys to control the water content of the blood and the concentration of urine.

This explanation covers all five marking points.