Nephrons form urine

1. A nephron is a microscopic kidney tubule that filters the blood and then reabsorbs useful substances.
2. The nephron begins with a glomerulus enclosed by Bowman's capsule.
3. The filtrate then travels along a tubule before entering a collecting duct.
4. Capillaries surround the tubule so substances can move from the filtrate back into the blood.
5. Urine formation has two linked stages: ultrafiltration followed by selective reabsorption.

High pressure filters small molecules

1. Ultrafiltration is high-pressure filtration of the blood at the glomerulus.
2. Blood pressure forces water and small dissolved molecules through the filtration barrier into Bowman's capsule.
3. The filtrate contains water, glucose, urea and mineral ions because these molecules are small enough to pass.
4. Blood cells remain in the capillaries because they are too large to cross the filtration barrier.
5. Large plasma proteins remain in the blood because their molecules are too large to pass through the barrier.

The glomerulus produces filtrate

1. The glomerulus is a knot of capillaries inside Bowman's capsule.
2. Its capillary network provides a large surface area for filtration.
3. Bowman's capsule collects the liquid and small solutes that leave the glomerular blood.
4. The first liquid is filtrate rather than urine because it still contains useful glucose, water and ions.

Selective reabsorption returns glucose

1. Selective reabsorption returns useful substances from the nephron filtrate to the blood.
2. All glucose is normally reabsorbed from the first part of the tubule into the surrounding capillaries.
3. Active transport moves glucose against its concentration gradient when necessary and requires energy released by respiration.
4. Tubule cells contain many mitochondria to supply energy for active transport.
5. Microvilli increase surface area so glucose can be reabsorbed rapidly.

Water returns to the blood

1. Water is reabsorbed from the filtrate into the blood by osmosis.
2. The amount reabsorbed varies according to the water content of the blood and the permeability of the collecting duct.
3. Useful quantities of ions are reabsorbed while excess ions remain in the tubule.
4. Urea remains in the filtrate together with excess water and ions, forming urine.

Structure explains each movement

1. Bowman's capsule surrounds the glomerulus and receives the filtrate directly.
2. A long tubule provides time and surface area for selective reabsorption.
3. A close capillary supply carries reabsorbed glucose and water away and helps maintain concentration gradients.
4. The collecting duct provides a final route in which water reabsorption can be adjusted before urine leaves the kidney.