Question

Which of the following statements are correct? (i) Reabsorption of water occurs passively in the initial segment of nephron. (ii) Nitrogenous wastes are absorbed by passive transport. (iii) Conditional reabsorption of \(\mathrm{Na}^{+}\)and water takes place in DCT. (iv) DCT reabsorbs \(\mathrm{HCO}_{3}^{-}\) (v) \(\mathrm{DCT}\) is capable of selective secretion of \(\mathrm{H}^{+}, K^{+}\)and \(\mathrm{NH}_{3}\) to maintain \(\mathrm{pH}\) and \(\mathrm{Na}^{+}-\mathrm{K}^{+}\)balance in blood. (vi) Substances like glucose, amino acids, \(\mathrm{Na}^{+}\), etc in the filtrate are reabsorbed actively. (a) (i) and (ii) (b) (ii) and (iii) (c) (iv) and (v) (d) All of these

Short answer

(d) All of these.

Step-by-step solution

Assess Statement (i)

Assess the validity of the statement regarding water reabsorption. The initial segment of the nephron, particularly the proximal convoluted tubule, does in fact function in the passive reabsorption of water. Therefore, this statement is correct.

Assess Statement (ii)

Determine the accuracy of the second statement concerning nitrogenous waste reabsorption. Nitrogenous wastes are typically not reabsorbed back into the blood; they are meant to be excreted. Hence, this statement is incorrect.

Assess Statement (iii)

Evalute the correctness of sodium and water reabsorption in the DCT (distal convoluted tubule). The DCT does indeed carry out conditional reabsorption of \(\mathrm{Na}^{+}\) and water based on the body's needs, often regulated by hormones like aldosterone. This statement is correct.

Assess Statement (iv)

Examine if \(\mathrm{HCO}_{3}^{-}\) reabsorption occurs in the DCT. Bicarbonate reabsorption primarily occurs in the proximal convoluted tubule, but the DCT also contributes to this process. Thus, this statement is correct.

Assess Statement (v)

Check if the DCT performs selective secretion. The DCT participates in the selective secretion of ions such as \(\mathrm{H}^{+}\), \(\mathrm{K}^{+}\), and \(\mathrm{NH}_{3}\) to manage blood pH and electrolyte balance, making this statement correct.

Assess Statement (vi)

Confirm if substances like glucose, amino acids, and \(\mathrm{Na}^{+}\) are actively reabsorbed in the filtrate. Active reabsorption of these substances takes place mainly in the proximal convoluted tubule, which makes this statement correct.

Combine Valid Statements

Identify the combination of statements that are correct. Since statements (i), (iii), (iv), (v), and (vi) are correct, the correct answer would include all of these statements.

Key concepts

Passive Water Reabsorption

In the process of passive water reabsorption, water moves from the renal tubules back into the bloodstream. This is a crucial part of the kidney's function, where the movement of water is driven by osmotic gradients that exist in the nephron. The proximal convoluted tubule (PCT), which constitutes the initial segment of the nephron, is a major site for passive reabsorption of water as it follows the reabsorption of solutes like sodium ions (a^+)) and glucose.

The reabsorption is 'passive' because it does not require energy from ATP (adenosine triphosphate); the water moves due to the concentration gradient set up by the reabsorption of solutes. Moreover, the PCT's semi-permeable membrane allows water to pass back into the circulatory system while keeping larger molecules like proteins in the filtrate for eventual excretion.

Nitrogenous Waste Excretion

The kidneys play a central role in the excretion of nitrogenous wastes, substances that are produced through the breakdown of proteins and nucleic acids. These waste products, such as urea, creatinine, and uric acid, are toxic and need to be removed from the bloodstream.

Unlike substances such as glucose and certain ions, nitrogenous wastes are not reabsorbed into the blood; rather, they are specifically targeted for clearance through filtration at the glomerulus and subsequent elimination in urine. Thus, the statement that these wastes are absorbed by passive transport is incorrect; the body actively clears them to maintain homeostasis.

Distal Convulated Tubule (DCT)

The distal convoluted tubule (DCT) is a segment of the nephron that follows the loop of Henle and plays a vital role in the fine-tuning of water and solute reabsorption. One key task of the DCT is the conditional reabsorption of sodium ions (a^+)) and water, responding to the body's needs. This process is often regulated by hormones like aldosterone, which promotes the reabsorption of a^+ and the excretion of potassium ions (K^+).

The DCT is also involved in acid-base homeostasis as it reabsorbs bicarbonate ions (HCO_3^-) and selectively secretes hydrogen (H^+), potassium (K^+), and ammonia (NH_3) to maintain the pH and Na^+-K^+ balance in the blood. These precise adjustments in the DCT are crucial for maintaining the body's overall electrolyte balance and ensuring acid-base equilibrium.

Electrolyte Balance

Maintaining the electrolyte balance in the body is essential for many physiological processes, including nerve impulse transmission and muscle contraction. The kidneys regulate electrolyte levels by filtering blood plasma, reabsorbing necessary ions, and excreting excesses or wastes.

The role of the DCT in this delicate balance cannot be overstated. Besides its function in reabsorbing Na^+ and HCO_3^-, the DCT also adjusts the excretion of K^+ and H^+ to keep the electrolyte levels within a narrow range. Disruptions in this balance can have serious health implications, highlighting the indispensable function of the nephron's distal convoluted tubule.

Active Reabsorption

Active reabsorption is a process where substances are moved from the filtrate back into the blood against their concentration gradient. This movement requires energy, usually in the form of ATP.

Substances like glucose, amino acids, and sodium ions (a^+)) are typically reabsorbed actively, particularly in the proximal convoluted tubule. Active transport can occur through various mechanisms such as primary active transport, directly using ATP, and secondary active transport, where the movement of one substance down its gradient fuels the movement of another against its gradient. The selective and regulated nature of active reabsorption allows the kidneys to reclaim essential nutrients and ions, ensuring that they are not lost in urine and maintaining the body's homeostasis.