Fisiología Renal - Reabsorción y secreción tubular renal(Túbulo Distal y Colector)(IG:@doctor.paiva)

Introduction to Renal Physiology: Reabsorption and Secretion

Overview of the Distal Tubule and Collecting Duct

• The class focuses on renal physiology, specifically reabsorption and secretion in the distal tubule and collecting duct.

• Topics include the initial and final portions of the distal tubule, as well as the cortical and medullary collecting ducts.

• Previous classes covered proximal tubule absorption; this session will delve into segments of the distal convoluted tubule.

Initial Portion of the Distal Tubule

• The initial segment resembles the thick ascending limb of Henle's loop, featuring a sodium-potassium pump in its basolateral membrane.

• This pump extrudes sodium into the interstitium while bringing potassium into cells, leading to low intracellular sodium levels that activate a sodium-chloride transporter.

• Key ions reabsorbed here include sodium, potassium, chloride, calcium, and magnesium; however, water is not reabsorbed in this segment.

Final Portion of Distal Tubule and Cortical Collecting Duct

• The final portion shares functional characteristics with the cortical collecting duct; both contain principal cells and intercalated cells.

• Principal cells are responsible for sodium reabsorption (mediated by aldosterone), water reabsorption (regulated by ADH), and potassium secretion.

Intercalated Cells Functionality

• Intercalated cells play a crucial role in acid-base regulation; they comprise 30% to 40% of cell types present in these segments.

• Type A intercalated cells absorb potassium and bicarbonate while secreting hydrogen ions; Type B do the opposite.

Hormonal Regulation Mechanisms

• Aldosterone stimulates sodium-potassium pumps in principal cells; increased aldosterone leads to greater sodium reabsorption and potassium secretion.

• Aldosterone is lipophilic, entering tubular cells to bind mineralocorticoid receptors which then influence gene expression related to ion transport proteins.

Role of Antidiuretic Hormone (ADH)

Mechanism of Action

• ADH is produced in the hypothalamus but stored in neurohypophysis. It regulates blood osmolarity by increasing water permeability in principal cells when osmolarity rises.

Clinical Implications

Carbonic Acid and Ion Transport Mechanisms

Carbonic Acid Formation and Ion Exchange

• Carbon dioxide (CO2) combines with water to form carbonic acid, which dissociates into hydrogen ions and bicarbonate.

• Hydrogen ions are expelled from the tubular cells via hydrogen ion pumps, while potassium is transported into the cells through potassium channels in the basolateral membrane.

• Intercalated type B cells perform opposite functions; CO2 enters these cells, forming carbonic acid that releases hydrogen ions into the interstitial space.

Role of Intercalated Cells

• Type A intercalated cells secrete hydrogen ions while reabsorbing bicarbonate and potassium, crucial for maintaining acid-base balance.

• In acidosis, these cells secrete more hydrogen ions and reabsorb bicarbonate to counteract pH changes.

Distal Tubule Functionality

• The final portion of the distal tubule is impermeable to urea but allows sodium reabsorption and potassium secretion influenced by aldosterone.

• The medullary collecting duct absorbs less than 10% of filtered water and sodium but plays a key role in urine processing.

Hormonal Influence on Water Reabsorption

• Aldosterone mediates sodium chloride reabsorption while promoting potassium secretion; antidiuretic hormone (ADH) enhances water absorption in response to body needs.

• Under normal conditions, type A intercalated cells manage acidosis by secreting hydrogen ions while absorbing bicarbonate.

Diuretics and Their Mechanisms

• Diuretics like hydrochlorothiazide act at the initial part of the distal tubule, inhibiting sodium-chloride transporters but are less potent than loop diuretics.

• Potassium-sparing diuretics such as spironolactone block sodium channels competitively at mineralocorticoid receptors, preventing potassium secretion.

Conclusion on Potassium-Sparing Diuretics

• Spironolactone increases blood potassium levels due to its mechanism of action against aldosterone.