Reabsorption of Na is controlled by Aldosterone.

Aldosterone is a hormone produced by the adrenal glands, specifically the outer layer called the adrenal cortex. It plays a crucial role in regulating the balance of electrolytes, particularly sodium and potassium, in the body.

Importance of Sodium Reabsorption:
Sodium is an essential electrolyte that plays a vital role in maintaining fluid balance, nerve function, and muscle contractions. Therefore, it is important to regulate the levels of sodium in the body to ensure proper physiological functioning.

Aldosterone and Sodium Reabsorption:
Aldosterone acts primarily on the renal tubules in the kidneys to enhance the reabsorption of sodium ions from the urine back into the bloodstream. This reabsorption process occurs in the distal convoluted tubules and collecting ducts of the nephrons.

How Aldosterone Works:
1. Release: The release of aldosterone is stimulated by various factors such as low blood sodium levels, high blood potassium levels, and the activation of the renin-angiotensin-aldosterone system (RAAS).
2. Binding to Receptors: Once released, aldosterone binds to mineralocorticoid receptors in the cells of the renal tubules.
3. Activation of Sodium Channels: The binding of aldosterone to these receptors leads to the activation of sodium channels on the luminal side of the tubular cells.
4. Sodium Reabsorption: Activation of these sodium channels facilitates the reabsorption of sodium ions from the tubular fluid into the tubular cells.
5. Potassium Excretion: At the same time, aldosterone also stimulates the secretion of potassium ions into the tubular fluid, promoting their excretion from the body.

Effect on Water Reabsorption:
The reabsorption of sodium ions by aldosterone indirectly affects the reabsorption of water. As sodium ions are reabsorbed, they create an osmotic gradient that promotes the passive reabsorption of water. Therefore, aldosterone indirectly increases water reabsorption as well.

Other Hormones:
Estrogen, glucocorticoids, and testosterone do not directly regulate the reabsorption of sodium. However, they may have some influence on the overall fluid and electrolyte balance in the body through their effects on other regulatory mechanisms.

Overall, aldosterone is the hormone primarily responsible for the reabsorption of sodium in the kidneys, thereby helping to maintain proper sodium levels and fluid balance in the body.