Draw a labelled diagram showing reabsorption and secretion of major substances at different parts of the nephron. 

Reabsorption:

A comparison of the volume of the filtrate formed per day (180 litres per day) with that of the urine released (1.5 litres), suggest that nearly $99 \%$ of the filtrate can be reabsorbed by the renal tubules. This process is called reabsorption.

This process takes place in the various parts of uriniferous tubules. eg. substances like glucose, amino acids, $\mathrm{Na}^{+}$, etc., in the filtrate are reabsorbed actively, whereas the nitrogenous wastes are absorbed by passive transport in the PC.T.

Filterate flowing away from PCT gets concentrated in the Henle's loop. Urine is more concentrated if the length of Henle's loop is more.

Secretion :

The process occurs in the DCT of the tubular cells secrete substances like $\mathrm{H}^{+}, \mathrm{K}^{+}$ and ammonia into the filtrate. These substances are not filtered during glomerular filteration. Ammonia, uric acid, $\mathrm{H}^{+}$ions and penicillin like medicinal drugs are secreted in this way.

Thus, urine is formed which is transported to the collecting tubule water is absorbed in the collecting tubule and much concentrated urine is emptied into the renal pelvis.