What transport system do kidneys use?

The kidneys use a transport system called the counter-current mechanism.

The name is based on the fact that concentration first increases in the direction of flow, then decreases as flow continues through the ascending parallel loop.

The mechanism relies on the adjacent, parallel loops of Henle.

Blood flows into the glomerulus ( a tuft of capillaries) from a very small artery and out by another small artery. This makes it different from any other capillary bed which has an artery in and a vein out.

This pushes water, amino acids, sodium, potassium and glucose out into Bowman's capsule.

From Bowman's capsule the proximal convoluted tubule (PCT) descends into the medulla of the kidney and is renamed the descending limb of the loop of Henle.

The descending limb can only remove water from the filtrate and no ions. The water is removed passively as the medulla is very salty.

The ascending loop of Henle actively pumps sodium and potassium out but water can not follow.

The ascending loop is renamed the distal convoluted tubule (DCT) as it passes by the glomerulus. At this spot another structure is found called the juxtaglomerular apparatus.

This regulates blood pressure. Eventually the afferent arteriole flows into a capillary bed and that flows into veins to carry the water and other nutrients back to the body.