Mention the three major steps involved in the production of urine.
Topic: Steps in urine formation
Answer
- ULTRAFILTRATION — in the Malpighian capsule, blood is filtered under high pressure and water, glucose, amino acids, urea and salts pass into Bowman’s capsule, while blood cells and plasma proteins are held back.
- SELECTIVE REABSORPTION — as the filtrate passes along the tubule, useful substances (ALL the glucose and amino acids, and most of the water and salts) are reabsorbed into the blood.
- TUBULAR SECRETION — additional wastes and excess ions such as potassium, hydrogen and creatinine are actively secreted FROM the blood INTO the tubule. What remains is urine.
One mark per step, and the names must be exact: ultrafiltration, selective reabsorption, tubular secretion.
THE WORD SELECTIVE matters in step 2, because it is what makes the kidney intelligent rather than merely a sieve. Filtration is indiscriminate — it removes useful substances along with the wastes — so the tubule must then recover what the body needs and leave the rest.
That is why glucose does not normally appear in urine: ALL of it is reabsorbed. Glucose in the urine (glycosuria) means there was more in the blood than the tubule could recover, which is the sign of DIABETES MELLITUS.
Step 3 runs in the OPPOSITE direction to step 2 — out of the blood into the tubule — which is the detail candidates most often get backwards.