Distal Tubule

What is the primary role of the distal tubule in the nephron

The "fine-tuning" of final urine composition.

How does the function of the distal tubule differ from earlier segments like the proximal tubule

While the proximal tubule handles bulk reabsorption, the distal tubule determines specific amounts for excretion based on homeostasis.

Which key substances does the distal tubule regulate for excretion

Water, sodium (Na+), potassium (K+), and hydrogen ions (H+).

What specific salt does the distal tubule variably reabsorb

Sodium chloride (NaCl).

How is sodium reabsorption in the distal tubule regulated

It is highly regulated by hormonal signals, not constant.

Why is regulated sodium reabsorption in the distal tubule vital

For maintaining proper plasma water volume and regulating body osmolality.

What is the primary hormonal system that regulates Na+ reabsorption in the distal tubule

The Renin-Angiotensin-Aldosterone System (RAAS).

What cells initiate the RAAS and what do they release

Granular renin-producing cells in the Juxtaglomerular Apparatus (JGA) release renin.

What is the substrate upon which renin acts, and what is it converted to

Renin acts on angiotensinogen (from the liver) to produce Angiotensin I.

Where is Angiotensin I converted to Angiotensin II, and by what

In the lungs, by a converting enzyme.

What is the role of Angiotensin II in the RAAS

It stimulates the adrenal gland to release aldosterone.

What is the principal hormone that acts directly on the distal tubule to increase Na+ reabsorption

Aldosterone.

What two main effects does aldosterone have

It promotes sodium (and water) retention to elevate blood pressure, and controls potassium (K+) secretion for excretion.

What hormones antagonize the RAAS by inhibiting Na+ reabsorption

Atrial and brain natriuretic peptides (ANP and BNP).

Besides salt transport, what other critical role does the distal tubule play in homeostasis

Acid-base balance, through the secretion of hydrogen ions (H+).

What other substances does the distal tubule help eliminate through secretion

Metabolic waste products, toxins, and drugs, via organic anions and catio