The tubules and the counter current

Each part of the tubule has its own job.

• PCT: lined with cells with a brush border (tiny projections) that give a large surface. It reabsorbs nearly all the useful nutrients and 70-80% of the salts and water. It also secretes H⁺ and ammonia and reabsorbs bicarbonate (HCO₃⁻), helping keep the pH right. So most reabsorption happens here.
• Loop of Henle: the descending limb lets water out but hardly any salts, so the filtrate becomes more concentrated as it goes down. The ascending limb does the opposite: it lets salts out but not water, so the filtrate becomes more dilute as it goes up. Together, the loop keeps the fluid around it in the medulla very salty.
• DCT: reabsorbs Na⁺ and water as needed (conditional reabsorption), reabsorbs HCO₃⁻, and secretes H⁺, K⁺ and NH₃.
• Collecting duct: can reabsorb large amounts of water to make urine concentrated. It also lets a little urea into the medulla, and secretes H⁺ and K⁺.

How does the kidney make concentrated urine? Through the counter current mechanism. In the loop of Henle, the filtrate flows down one limb and up the other, in opposite directions, close together. Blood in the vasa recta also flows in opposite directions in its two limbs. This arrangement builds up and keeps a rising saltiness in the medulla, from 300 mOsmol per litre in the cortex to about 1200 mOsmol per litre in the inner medulla. The gradient is made mainly by NaCl and urea.

Because the medulla is so salty, water is easily drawn out of the collecting duct as urine passes through it. As a result, human urine can be nearly four times as concentrated as the original filtrate.