In the first step of urine formation kidneys reabsorb the water, small molecules and ions which are required from the blood. These molecules are removed from the blood (excluding large proteins and corpuscles) through diffusion or osmosis and the filtrate then flows into Bowman’s capsule. This process is termed as glomerular filtration. After filtration, the filtrate passes from the glomerulus to the nephron.
During the process of Reabsorption molecules such as water, glucose, Na+ and other nutrients move from the renal tubules back to the peritubular capillaries that surrounds the renal tubules. Reabsorption of glucose, amino acids, NaCl, potassium ions, water and bicarbonate occurs in the proximal convoluted tubule. Reabsorption of water and NaCl occurs in loop of Henle and collecting ducts whereas reabsorption of water, NaCl, bicarbonate and hydrogen ions occurs in Distal convulated tubule.
The concentrated urine moves back into the renal tubules from where it is excreted. Uric acid, drugs and hydrogen ions are secreted into the urine by proximal convulated tubule and potassium and hydrogen ions are secreted into urine by the distal convulated tubule.