Urine formation includes glomerular filteration (ultra filteration), selective reabsorption and tubular secretion that takes place in different parts of the nephron.

Glomerular filteration involves the filteration of blood, which is carried out by glomerulus. Selective reabsorption is the absorption of filtrate through renal tubules either actively or passively.

Tubular secretion involves secretion through tubular cells in urine in order to maintain ionic and acid-base balance of body fluids.

Actively transported substances during reabsorption of GFR- Glucose, amino acids, $\mathrm{Na}^{+}$.

Passively transported substances during reabsorption of GFR- Nitrogenous wastes, water.

(a) Urinary excretion $=$ tubular reabsorption + tubular secretion - filtration

(b) Dialysis fluid = plasma - nitrogenous wastes.

The concentration gradient in medullary interstitium is established primarily by renal tubules of loop of Henle and the blood vessels surrounding them (vasa recta) in a process called countercurrent exchange.

The substances that exit from tubules for maintenance of such gradient are mainly sodium chloride $(\mathrm{NaCl})$, water and urea (contaning $\mathrm{H}^{+}, \mathrm{K}^{+}$and $\mathrm{NH}_3^{+}$).