Question

A large quantity of fluid is filtered every day by the nephrons in the kidneys. Only about $1\% $ of it is excreted as urine. The remaining $99\% $ of the filtrate
A. Gets collected in the renal pelvis
B. Is lost as sweat
C. Is stored in urinary bladder
D. is reabsorbed into the blood

Answer 1

Hint: The normal GFR is $125\,ml/\min $, which equals to $180$ litres per day. The urine output is $800 - 2000\,ml$ which comes to about $2$ litres per day. Thus, the urine output is very less than the amount of blood filtered by the kidneys.

Complete answer:
The kidneys filter blood by a series of three steps - ultrafiltration, selective reabsorption and tubular secretion. In ultrafiltration, the entire blood is filtered under pressure in which only the fluid part comes out into the tubules leaving behind the cellular part along with the proteins.
When the fluid travels in the tubules, a huge portion of it is absorbed back into the circulation through selective reabsorption. This includes most of the water and certain ions. This is mainly responsible for the reduction in the volume of the excreted fluid.
It is in the PCT that glucose is also absorbed back into the circulation. In the process of tubular secretion, some of the ions are secreted back to the tubule from the circulation. These include ions like potassium, calcium, hydrogen ion, and urea.

Hence, the correct answer is option (D).

Note: Nephrons are the structural and functional units of the kidneys. These processes take place at the level of the nephrons. During the process of absorption or secretion, water and sodium ions are co-transported. Water and sodium ions are almost entirely absorbed back into the circulation.