Renal Tubule Transport – Proximal Convoluted Tubule

These vocabulary flashcards summarize the key transport mechanisms, structures, and concepts involved in solute and water handling within the proximal convoluted tubule, as discussed in the lecture notes.

Proximal Convoluted Tubule (PCT)

The first segment of the renal tubule where the majority of reabsorption occurs: ~100 % of glucose, amino acids, proteins, 65 % of water, Na⁺, K⁺, Ca²⁺, 80 % of HCO₃⁻, and half of Cl⁻ and urea.

Reabsorption (renal)

Movement of substances from the tubular fluid back into the blood, returning filtered solutes and water to the body.

Secretion (renal)

Transfer of substances from the blood into the tubular fluid for elimination in urine.

Paracellular Transport

Passive movement of molecules between adjacent tubular cells through tight-junction pathways.

Transcellular Transport

Movement of substances through the cell, crossing both apical and basolateral membranes.

Obligatory Water Reabsorption

Water reabsorption that automatically follows solute reabsorption; not hormonally regulated.

Facultative Water Reabsorption

Water reabsorption that is hormonally regulated (e.g., by ADH) and occurs mainly beyond the PCT.

Sodium–Glucose Symporter (SGLT)

Membrane protein in the PCT that simultaneously transports Na⁺ down its gradient and glucose against its gradient into the cell.

Indirect Active Transport

Transport that relies on an ion gradient created by ATP-driven pumps (e.g., Na⁺ gradient driving glucose uptake via SGLT).

Sodium–Potassium Pump (Na⁺/K⁺-ATPase)

Basolateral enzyme that uses ATP to move 3 Na⁺ out and 2 K⁺ into the cell, maintaining low intracellular Na⁺.

Facilitated Diffusion

Carrier-mediated passive movement of a substance down its concentration gradient (e.g., glucose exit from PCT cells).

Sodium–Hydrogen Antiporter (Na⁺/H⁺ Exchanger)

PCT membrane transporter that brings Na⁺ into the cell while secreting H⁺ into the tubular fluid.

Bicarbonate Reabsorption

Process in which filtered HCO₃⁻ is reclaimed in the PCT via conversion to CO₂ and H₂O, re-formation inside the cell, and diffusion into blood.

Hydrogen Ion Secretion

Active export of H⁺ from PCT cells into tubular fluid, helping regulate blood pH.

Carbonic Acid Reaction

CO₂ + H₂O ⇌ H₂CO₃ ⇌ H⁺ + HCO₃⁻; central to H⁺ secretion and HCO₃⁻ reabsorption in the PCT.

Peritubular Capillaries

Blood vessels surrounding the renal tubules that receive reabsorbed water and solutes.

Tubular Fluid (Lumen)

The filtrate inside the nephron where reabsorption and secretion take place.

Aquaporins

Water-channel proteins that facilitate rapid transcellular water movement, aiding bulk flow in the PCT.

Osmotic Gradient

Difference in solute concentration that drives water movement from tubular fluid to blood after solute reabsorption.

Passive Transport

Movement of substances down a concentration or electrochemical gradient without direct ATP use (e.g., diffusion).

Active Transport

Energy-requiring movement of substances against their gradient, often via ATP hydrolysis (e.g., Na⁺/K⁺-ATPase).

Osmolarity

Measure of solute concentration; increases in blood after solute reabsorption draw water from the tubule.

Paracellular Pathway

Route between cells through tight junctions for ions/water during passive reabsorption.

Transcellular Pathway

Route through the cell, involving specific transporters or channels on both membranes.

Symporter

Membrane carrier that moves two substrates in the same direction across a membrane (e.g., Na⁺ and glucose).

Antiporter

Membrane carrier that exchanges two substrates in opposite directions (e.g., Na⁺/H⁺ exchanger).