The figure given below shows a part of a nephron. Answer the questions that follow: The figure given below shows a part of a nephron. Answer the questions that follow:
Why is fluid X not called urine? Justify your answer.
Topic: Glomerular filtrate is not urine
Answer
Because fluid X is the GLOMERULAR FILTRATE, and it still contains a great deal of USEFUL material — all the glucose, the amino acids, and most of the water and salts. These are reabsorbed further along the tubule. Only after selective reabsorption and tubular secretion is the remaining fluid called URINE.
The reason must be that the filtrate is NOT YET WASTE — it contains substances the body needs and intends to recover.
WHY FILTRATION ALONE IS NOT ENOUGH: ultrafiltration is INDISCRIMINATE. It removes anything small enough to pass through, useful or not. The tubule must then recover what the body needs and leave the rest — which is why the second step is called SELECTIVE reabsorption.
THE PROOF that reabsorption works: glucose does not normally appear in urine at all, although it is present in the filtrate. ALL of it is recovered. Glucose in the urine (glycosuria) means there was more in the blood than the tubule could recover — the sign of diabetes mellitus.