4.8 - Kidneys - The Nephron

Ultrafiltration and selective reabsorption

Inside each nephron, urine is made in two stages: ultrafiltration, then selective reabsorption.

Stage 1 - Ultrafiltration

• Blood arriving at the glomerulus is under high pressure, because the arteriole leaving the glomerulus is narrower than the one entering it.
• This high pressure squeezes the liquid part of the blood - water, plus the small dissolved molecules glucose, urea, and ions - out of the glomerulus and into the Bowman's capsule.
• The membranes between the glomerulus and the capsule act like filters with very small holes:
  • Larger molecules such as proteins, and the blood cells, are too big to be squeezed out and stay behind in the blood.
• The high-pressure filtration is called ultrafiltration, and the filtered liquid now in the Bowman's capsule is the glomerular filtrate.

Stage 2 - Selective reabsorption

• As the filtrate flows along the tubule, substances the body needs are taken back into the blood, while waste is left behind.
• It's called selective reabsorption because only some substances are reabsorbed:

From filtrate to urine

1. The substances not reabsorbed - urea, excess water, and excess ions - stay in the tubule and form urine.
2. Urine passes through the collecting duct, into the ureter, and down to the bladder, where it's stored before being released through the urethra.