Function of the tubules

PCT: lined by simple cuboidal brush border epithelium, which gives a larger surface for reabsorption. Nearly all essential nutrients and 70–80 per cent of the electrolytes and water are reabsorbed here.

The PCT also helps keep the pH and ionic balance of body fluids: it selectively secretes H⁺ and ammonia into the filtrate and absorbs HCO₃⁻ from it.

Henle's loop: its ascending limb takes back very little, yet the loop matters greatly, because it keeps the interstitial fluid of the medulla at high osmolarity.

The descending limb is permeable to water but almost impermeable to electrolytes, so the filtrate becomes more concentrated as it goes down.

The ascending limb is impermeable to water but lets electrolytes pass out, actively or passively, so the concentrated filtrate becomes diluted as it goes up.

DCT: conditional reabsorption of Na⁺ and water happens here. The DCT can also reabsorb HCO₃⁻ and selectively secrete H⁺, K⁺ and NH₃, keeping the pH and the sodium–potassium balance of blood.

Collecting duct: a long duct running from the cortex to the inner medulla. Large amounts of water can be reabsorbed here to make concentrated urine.

The collecting duct lets a little urea leak into the medullary interstitium, which helps hold the osmolarity there. It also secretes H⁺ and K⁺ selectively, helping the pH and ionic balance of blood.