A huge surface
A microscopic image scan of a cross-section of a mouse kidney. Different microscopic images are digitally stitched together to create this image. The bright fluorescent green structures are the brush borders within the kidney.
Both kidneys together filter 200 liters of fluid every 24 hours. The filtrate is further processed and concentrated to the 1,5 liter of urine we normally pee throughout the day. The filtrate is processed in specialized tubes (nephrons). A human kidney has about one million nephrons, and the total length of the nephrons in the kidney is about 40 miles! The inside of these tubes is lined with a brush border (the green structures). This brush border is made up by microvilli which increase the surface area of the, already long, tubes. The huge surface created by the brush border within the tubes is necessary to take-up and transport the filtered nutrients, water and ions back to the blood.