Which of the following statements are correct? (i)...

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

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

Key Concepts

Active Reabsorption of Essential Nutrients

Many essential nutrients and solutes—such as glucose, amino acids, and certain electrolytes—are reabsorbed from the filtrate via active transport mechanisms. This active uptake, often mediated by specific transport proteins, ensures that valuable substances are not lost in the urine and helps sustain the body’s overall nutritional and metabolic balance.

Selective Secretion and Electrolyte Homeostasis

Selective secretion is the capacity of specific nephron segments, notably the distal convoluted tubule, to actively secrete certain ions (such as hydrogen, potassium, and ammonia) into the tubular fluid. This process is vital for fine-tuning blood pH and maintaining the balance of key electrolytes like sodium and potassium in the blood.

Conditional (Hormone?regulated) Reabsorption

Conditional reabsorption refers to the process by which the kidney modulates the reabsorption of substances, such as sodium and water, in response to hormonal signals, particularly in the distal portions of the nephron. Hormones like aldosterone and antidiuretic hormone (ADH) play key roles here by regulating transporter activity, thereby adjusting fluid and electrolyte balance as needed.

Nephron Functional Segmentation

The nephron is divided into distinct segments—such as the proximal tubule, loop of Henle, distal convoluted tubule (DCT), and collecting duct—each specialized for particular processes in filtration, reabsorption, and secretion. Recognizing the unique functions of these segments is important for understanding how substances like water, electrolytes, and metabolic wastes are handled differently along the nephron.

Passive and Active Transport

This concept distinguishes between the mechanisms by which substances cross cell membranes. In the kidney, passive transport (such as osmosis and diffusion) allows water and some solutes to move down their concentration gradients without energy expenditure, whereas active transport requires energy (ATP) to move substances against their gradient. Understanding this distinction is crucial for grasping how the nephron reabsorbs water and solutes.

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