Question

Describe in brief how urine is produced in the human body.

Answer 1

Formation of urine is known as uropoiesis. This occurs in the kidney. The volume of urine excreted from our body is far less than the volume of the glomerular filtrate and the composition is also quite different as many useful materials are reabsorbed and many are excreted.

Complete answer:
Glomerular filtration: The glomerular capillaries along with the Bowman’s capsule are generally semipermeable due to presence of minute pores. They allow water molecules as well as certain small molecules present in the blood to pass through them. The force responsible for moving the fluid from the glomerular capillaries is known as filtration pressure. Because of ultrafiltration the fluid that is filtered out is known as glomerular filtrate.

Tubular reabsorption: Glucose, amino acids and certain ions like $Na^{+}$ and $K^{+}$ are reabsorbed by the active transport in proximal convoluted tubules. $HCO_{3}^{-}$ is also reabsorbed in PCT. In henle's loop, the descending part is freely permeable for water whereas the ascending limb is Impermeable to water along its entire length.

Tubular secretion: Removal of $H^{+}$ and $NH_{4}^{+}$ from the blood helps in maintaining PH of the blood. Creatinine , hippuric acid and various other substances are secreted into the filtrate. Potassium, hydrogen , ammonium and bicarbonate ion are secreted actively in the distal convoluted tubule.

Note: The separation of certain small molecules,ions and the cells in blood is known as ultrafiltration. About $1100$-$1200$ litres of blood flows through the kidney but excreted out urine is $1.5$ liters of urine in a day from our body which is one fourth of the produced urine.