Excretion is a process of removal of waste which is produced by the metabolic activities of the cell. The main form of nitrogenous waste excreted by the organism are ammonia, urea, and uric acid.
The animals are of three types on the basis of their excretion that is ammonotelic, ureotelic, and uricotelic.
Mechanism of Urine formation: glomerular filtration, reabsorption, and secretion.
Glomerular filtration:
This is done by the glomerulus and therefore it is called glomerular filtration.
The pressure in the glomerular capillary causes the filtration of blood by the three layers(the endothelium layer of glomerular blood vessels the epithelium layer of Bowman’s capsule, and the basement membrane between the two layers).
Blood is filtered very finely because of the epithelial cells of Bowman’s capsule are arranged in such a manner that they leave some minute pores called filtration slits.
And so almost everything except proteins from plasma passes into Bowman’s capsule.
About 1100-1200ml of blood is filtered by the kidney per minute but the amount of filtrate forms is 125ml/minute.
Reabsorption:
Nearly 99% of the filtrate formed is reabsorbed into the renal tubule, and this is called reabsorption.
The epithelial cells in the different segments of the nephrons are different.
The PCT absorbs nearly 70-80% of electrolytes and water.
The Henle’s loop has the minimum reabsorption.
DCT absorbs sodium and water when required.
Secretion:
During the urine formation, different parts of nephrons secrete different substances like H+,K+,and NH3.
Secretion is important as it maintains the ionic and pH of the body fluids.
Final Answer:
Urine formation takes place so that the waste during the metabolic process can be removed from the body.