The Formation of Urine

Ultrafiltration

Blood are brought into the glomerulus through the renal artery.

The high pressure in the glomerulus forces water, urea, glucose, amino acids, mineral salts and other small molecules (glomerular filtrate) except red blood cells and plasma proteins into the capsular space.
 
Reabsorption
Reabsorption occurs along the renal tubule (PCT, LOH,DCT) and the collecting duct.
At the PCT, glucose, amino acids, Na+, Cl- are reabsorbed into blood capillaries through active transport. Water is reabsorbed through osmosis.
At the LOH, water is also reabsorbed through osmosis. Na+ and Cl- are reabsorbed through active transport.
At the DCT, more water, Na+ and Cl- are reabsorbed.
When the filtrate reaches the CD, very little salt (Na+ and Cl-) is left and 99% of water has been reabsorbed. Only little water will be reabsorbed here.

Secretion
Secretion occurs along the renal tubules and the collecting duct (actively at the DCT). It occurs through PT and AT.
The substances secreted include H+, K+, NH3, urea, creatinine, toxins and certain drugs.
Secretion helps eliminate and increase waste removal.
Urine mainly consists of water, nitrogenous wastes (urea, uric acid), salts (H+, K+, HCO3-), drugs or toxins.