NEET Practice Questions (MCQs) with Answers & Solutions

According to NCERT, 'Distal Convoluted Tubule (DCT): Conditional reabsorption of Na$^+$ and water takes place in this segment. DCT is also capable of reabsorption of HCO$_3^-$ and selective secretion of hydrogen and potassium ions and NH$_3$ to maintain the pH and sodium-potassium balance in blood.'

The NCERT states, 'Collecting Duct: This segment allows passage of small amounts of urea into the medullary interstitium to keep up the osmolarity.'

As per NCERT, 'It (Collecting Duct) also plays a role in the maintenance of pH and ionic balance of blood by the selective secretion of H$^+$ and K$^+$ ions.'

The summary states, 'PCT is the major site of reabsorption and selective secretion.'

The summary highlights, 'HL primarily helps to maintain osmolar gradient (300 mOsmolL$^{ ext{-1}}$ - 1200 mOsmolL$^{ ext{-1}}$) within the kidney interstitium.'

The summary mentions, 'DCT and collecting duct allow extensive reabsorption of water and certain electrolytes, which help in osmoregulation: H$^+$ , K$^+$ and NH$_3$ could be secreted into the filtrate by the tubules to maintain the ionic balance and pH of body fluids.' It further adds, 'DCT and collecting duct concentrate the filtrate about four times, i.e., from 300 mOsmolL$^{ ext{-1}}$ to 1200 mOsmolL$^{ ext{-1}}$ , an excellent mechanism of conservation of water.'

The NCERT text states, 'During urine formation, the tubular cells secrete substances like H$^+$ , K$^+$ and ammonia into the filtrate. Tubular secretion is also an important step in urine formation as it helps in the maintenance of ionic and acid base balance of body fluids.'

The NCERT section on 'Function of the Tubules' mentions, 'A comparison of the volume of the filtrate formed per day (180 litres per day) with that of the urine released (1.5 litres), suggest that nearly 99 per cent of the filtrate has to be reabsorbed by the renal tubules. This process is called reabsorption.' This process is essential for preventing excessive loss of water and solutes.

The NCERT states, 'For example, substances like glucose, amino acids, Na$^+$ , etc., in the filtrate are reabsorbed actively whereas the nitrogenous wastes are absorbed by passive transport. Reabsorption of water also occurs passively in the initial segments of the nephron (Figure 16.5).'

As stated in NCERT, 'The ascending limb is impermeable to water but allows transport of electrolytes actively or passively. Therefore, as the concentrated filtrate pass upward, it gets diluted due to the passage of electrolytes to the medullary fluid.'