Distal Convoluted Tubule (DCT): Conditional reabsorption of Na+ and water takes place in this segment. DCT is also capable of reabsorption of HCO3– and selective secretion of hydrogen and potassium ions and NH3 to maintain the pH and sodium-potassium balance in blood.

Bcz it's absorption necessarily occurs in other parts but in adverse conditions to maintain ionic balance under the effect of hormones it absorbs Na+

Introduction:
In the nephron, the distal convoluted tubule (DCT) is responsible for the reabsorption of various substances, including sodium (Na+). The reabsorption of Na+ in the DCT is often referred to as "conditional reabsorption" due to its regulation being dependent on certain conditions and factors.

Regulation of Na+ Absorption in DCT:
The reabsorption of Na+ in the DCT is influenced by several factors, including hormonal regulation, luminal factors, and intracellular signaling.

Hormonal regulation:
1. Aldosterone: This hormone, released by the adrenal glands, plays a crucial role in regulating Na+ absorption in the DCT. It binds to mineralocorticoid receptors on the cells of the DCT, promoting the synthesis and insertion of Na+ channels (ENaC) and Na+/K+ ATPase pumps on the luminal and basolateral membranes, respectively. This increases the reabsorption of Na+ from the tubular fluid into the bloodstream.

Luminal factors:
1. Tubular fluid flow rate: The rate at which the tubular fluid flows through the DCT affects the reabsorption of Na+. Higher flow rates reduce the time available for Na+ reabsorption, resulting in decreased Na+ absorption.
2. Luminal concentration of Na+: Increased Na+ concentration in the tubular fluid can stimulate the reabsorption of Na+ in the DCT.
3. Luminal concentration of other ions: The presence of certain ions, such as potassium (K+) and calcium (Ca2+), can influence the reabsorption of Na+ in the DCT.

Intracellular signaling:
1. Parathyroid hormone (PTH): PTH, released by the parathyroid glands, increases the reabsorption of Ca2+ in the DCT. This indirectly affects Na+ reabsorption by altering the electrochemical gradient across the cell membrane, which can impact the movement of Na+.

Conditional Reabsorption:
The term "conditional reabsorption" is used to describe the regulation of Na+ absorption in the DCT due to the dependence on specific conditions and factors. Unlike the proximal convoluted tubule (PCT), which reabsorbs the majority of Na+ and other substances regardless of physiological conditions, the DCT selectively reabsorbs Na+ based on the body's needs.

The reabsorption of Na+ in the DCT is regulated based on factors such as hormonal signals, luminal concentrations, and intracellular signaling. This allows for fine-tuning of Na+ reabsorption, enabling the body to maintain electrolyte balance and respond to changing physiological conditions.

Conclusion:
The process of Na+ absorption in the DCT is referred to as "conditional reabsorption" due to its dependence on specific conditions and factors. Hormonal regulation, luminal factors, and intracellular signaling play crucial roles in regulating the reabsorption of Na+ in the DCT. This selective regulation allows the body to maintain electrolyte balance and adapt to changing physiological conditions.