How does the kidney work?

The kidney is the body's filtration system, and works by separating the fluid and cells that make up blood, then selectively reabsorbing what is still useful, so that we don't lose anything valuable but can still get rid of waste products. The vast majority of what the kidneys remove from the blood gets put back!The blood is first ultra filtered to separate the fluid from the cells, and this 'filtrate' is passed along the proximal convoluted tubule (PCT), which has lots of receptors to grab everything useful that just got filtered out, and deposit it right back into the bloodstream. The remaining filtrate is passed through the Loop of Henle, which is the main way to reabsorb water. It does this by osmosis (more detail in a full explanation). The remaining filtrate then passes through the distal convoluted tubule to selectively reabsorb just as in the PCT. Finally, the filtrate passes through the collecting duct which can be used to regain even more water if the brain sends the hormone anti-diuretic hormone (ADH) to utilise aquaporins on the inner surface of the collecting duct. :)