The correct option is 1, 2 and 3 only.
Explanation
Tubular reabsorption is a vital process in urine formation where the renal tubules reabsorb essential substances from the glomerular filtrate back into the blood. This process occurs via active or passive mechanisms depending on the substance involved.
Statement-wise Analysis
- Statement 1 is Correct: The kidneys produce approximately 180 litres of filtrate per day (Glomerular Filtration Rate), but the urine released is only about 1.5 litres. This indicates that nearly 99% of the filtrate is reabsorbed by the renal tubules.
- Statement 2 is Correct: Substances of high physiological importance, such as glucose and amino acids, are reabsorbed actively (against the concentration gradient requiring energy) in the Proximal Convoluted Tubule (PCT).
- Statement 3 is Correct: Nitrogenous wastes, such as urea, are reabsorbed by passive transport (diffusion) along the concentration gradient. This helps maintain the osmolarity of the medullary interstitium.
- Statement 4 is Incorrect: Water is reabsorbed passively in the initial segments of the nephron (such as the PCT). This is known as obligatory water reabsorption, where water follows the active reabsorption of solutes like sodium and glucose via osmosis. It is not an active process.
Key Takeaway
Key Takeaway: In the nephron, nutrients like glucose and amino acids are reabsorbed via active transport, whereas water and nitrogenous wastes are reabsorbed via passive transport (osmosis and diffusion, respectively).