Question

Define excretion. Explain mechanism of urine formation.

Answer 1

Excretion can be defined as the process of removal of nitrogenous waste produced by the metabolic activity of the cells. In humans, urea is the major nitrogenous waste. Uric acid and creatinine are also expelled in the process of excretion.

Mechanism of urine formation: It occurs in three steps as follows:

1. Glomerular Filtration- the water, minerals, ions, and solutes are diffused out of the blood and move into the Bowman's capsule forming a filtrate.

2. Tubular reabsorption: the necessary substances such as water, glucose, amino acids, vitamins, mineral ions in the filtrate are reabsorbed by the tubules of the nephrons. The loop of Henle regulates the water reabsorption

3. Tubular secretion: K${}^{+}$, H${}^{+}$, creatinine and excessive water are secreted out from the tubules into the filtrate. Such a filtrate is called the urine.