Question

What is meant by excretion? Explain the process OR Describe the mechanism of urine formation.

Answer 1

Excretion is the removal of waste mainly urea with the help of urine by kidneys. Proteins are digested to form amino acids and the final by-product of the degradation of ammonia is urea. This urea containing blood (blood filtrate) is carried from the liver to the kidneys for its final excretion as urine. The formation of urine from blood filtrate is a complicated process involving thee major steps:

1. Glomerular filtration: During this process fluid is filtered from the plasma into the Bowman's capsule at a rate of about 120ml/min. The process helps in the removal of blood cells and proteins from the filtrate. The resulting filtrate is called the ultrafiltrate.
2. Tubular reabsorption and secretion: At various places in the proximal convulated tubule, loop of Henle, distal convulated tubule and collecting duct, sodium ions are reabsorbed actively from the filtrate into the blood vessels and chloride ions passively. During this process, water is also transported passively across the walls to maintain the osmotic balance. Substances such as potassium ions, hydrogen ions, ammonia and toxic compounds such as dyes and drugs enter the filtrate by the process of secretion. This causes changes in the composition of the glomerular filtrate resulting finally in the formation of urine.