Overview of the Distal Convoluted Tubule (DCT)

• Location: Part of the nephron in the kidney, located between the loop of Henle and the collecting duct.

• Function: Fine-tunes the composition of urine by selective reabsorption and secretion of ions, water, and other substances.

• Regulation: Highly responsive to hormonal signals (e.g., aldosterone, parathyroid hormone, antidiuretic hormone).

• Structure: Lined with simple cuboidal epithelium, fewer microvilli than proximal tubule, prominent mitochondria for active transport.

Reabsorption in the DCT

• Definition: Process of moving substances from the tubular filtrate back into the peritubular capillaries (blood).

• Major Substances Reabsorbed:

  • Sodium (Na⁺): Reabsorbed via Na⁺/Cl⁻ cotransporters and Na⁺ channels, regulated by aldosterone.

  • Chloride (Cl⁻): Co-transported with Na⁺ or via Cl⁻ channels.

  • Calcium (Ca²⁺): Reabsorbed under the influence of parathyroid hormone (PTH) via Ca²⁺ channels.

  • Water: Limited reabsorption, dependent on aquaporin channels and antidiuretic hormone (ADH) presence (less permeable than collecting duct).

• Mechanism: Primarily active transport for ions, with water following passively via osmosis (if permeable).

• Significance: Adjusts electrolyte balance and pH, contributes to blood pressure regulation (via Na⁺ reabsorption).

Secretion in the DCT

• Definition: Movement of substances from the peritubular capillaries (blood) into the tubular filtrate for excretion.

• Major Substances Secreted:

  • Potassium (K⁺): Secreted via K⁺ channels, regulated by aldosterone (increases K⁺ excretion).

  • Hydrogen (H⁺): Secreted to regulate blood pH, via H⁺-ATPase or Na⁺/H⁺ exchangers.

  • Ammonium (NH₄⁺): Secreted to buffer urine pH and excrete excess nitrogen.

  • Certain drugs/toxins: e.g., penicillin, via organic anion transporters.

• Mechanism: Active transport (e.g., K⁺, H⁺) or facilitated diffusion, driven by electrochemical gradients.

• Significance: Maintains acid-base balance, regulates K⁺ levels, and clears waste or drugs.

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Hormonal Regulation

• Aldosterone: Increases Na⁺ reabsorption and K⁺ secretion, triggered by low blood pressure or high K⁺ levels.

• Parathyroid Hormone (PTH): Enhances Ca²⁺ reabsorption, reduces phosphate reabsorption.

• Antidiuretic Hormone (ADH): Increases water reabsorption (more significant in collecting duct but affects DCT permeability).

• Atrial Natriuretic Peptide (ANP): Inhibits Na⁺ reabsorption, promoting natriuresis.

Clinical Relevance

• Diuretics: Thiazide diuretics inhibit Na⁺/Cl⁻ cotransporter in DCT, increasing Na⁺ and water excretion (used in hypertension, edema).

• Disorders:

  • Gitelman Syndrome: Defect in Na⁺/Cl⁻ cotransporter, leading to hypokalemia, hypomagnesemia.

  • Liddle Syndrome: Overactive Na⁺ channels, causing hypertension and hypokalemia.

• Acid-Base Imbalance: Impaired H⁺ secretion can lead to renal tubular acidosis.

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Reabsorption and Secretion in the Urinary System

Below is a chart summarizing the key processes of reabsorption and secretion across different segments of the nephron in the urinary system.

Multiple Choice Questions (MCQs) on Distal Convoluted Tubule, Reabsorption, and Secretion

1. Which hormone primarily regulates sodium reabsorption and potassium secretion in the distal convoluted tubule?
   a) Antidiuretic hormone (ADH)
   b) Parathyroid hormone (PTH)
   c) Aldosterone
   d) Atrial natriuretic peptide (ANP)
   Answer: c) Aldosterone

2. What is the primary mechanism for calcium reabsorption in the DCT?
   a) Passive diffusion via aquaporins
   b) Active transport regulated by parathyroid hormone
   c) Co-transport with potassium
   d) Facilitated diffusion with chloride
   Answer: b) Active transport regulated by parathyroid hormone

3. Which substance is primarily secreted in the DCT to regulate blood pH?
   a) Sodium (Na⁺)
   b) Hydrogen (H⁺)
   c) Calcium (Ca²⁺)
   d) Chloride (Cl⁻)
   Answer: b) Hydrogen (H⁺)

4. Thiazide diuretics primarily affect the DCT by inhibiting which transporter?
   a) Na⁺/K⁺/2Cl⁻ cotransporter
   b) Na⁺/Cl⁻ cotransporter
   c) Na⁺/H⁺ exchanger
   d) Ca²⁺ channel
   Answer: b) Na⁺/Cl⁻ cotransporter

5. In the DCT, water reabsorption is:
   a) Highly dependent on aldosterone
   b) Limited and regulated by antidiuretic hormone
   c) Completely absent
   d) Equivalent to that in the proximal tubule
   Answer: b) Limited and regulated by antidiuretic hormone