Describe the process of urine formation …

Urine is formed in the nephron of kidneys. Nephron is the structural and functional unit of the kidney. Blood at high pressure travels into these  tubules by the tuft of blood capillaries called glomerulus contained in Bowman's capsule.

The following steps are involved in the process:

1.  Filtration: Blood enters the glomerulus through the afferent arterioles It passes under high pressure that results in the filtration of blood. Water and small molecules are forced out of glomerular capillary walls and Bowman’s capsule. Large molecules remain in the blood of the glomerulus.
2. Selective reabsorption: Some molecules are selectively reabsorbed into the blood. The glomerular filtrate flows through the proximal convoluted tubule, the U-shaped Henle’s loop and distal convoluted tubule. The useful substances such as glucose, amino acids and salts which require energy are reabsorbed by a process called selective reabsorption. Hence, the filtrate now contains urea, some salts and water. Reabsorption of solutes increases the water concentration of the filtrate. Water is then reabsorbed into the blood by osmosis.
3. Tubular secretion: Some nitrogenous waste products like creatinine and some other substances like K⁺ are removed from the blood by DCT (Distal Convoluted Tubule) and are passed to blood. The urine thus formed is collected in the urinary bladder.