Transport in the various elements of the tubular system of the nephron. Renal clearance.
Following glomerular filtration, the tubular system of the nephron selectively reabsorbs essential substances and water back into the bloodstream while secreting certain waste products. This intricate process involves various transport mechanisms across different segments of the nephron, ensuring the formation of urine with the appropriate composition and volume.
Reabsorption: A Two-Step Process-
Tubular reabsorption: 65% in proximal tubule, 25% in loop of henle, 8% in distal part.
Tubular reabsorption, the retrieval of substances from the filtrate back into the bloodstream, involves a two-step process:
- 1. Movement of substances from the tubular fluid (filtrate) across the luminal membrane of the tubular epithelial cells.
- 2. Transport of substances across the basolateral membrane of the tubular epithelial cells into the interstitial fluid, followed by movement into the peritubular capillaries.
Reabsorption can occur via two pathways:
- Transcellular: Through the epithelial cells.
- Paracellular: Between adjacent epithelial cells through tight junctions. The permeability of these junctions varies in different nephron segments.
Mechanisms of Tubular Transport
Passive Transport: Movement of substances down their electrochemical gradients, requiring no energy expenditure.
- Diffusion (simple/facilitated): Movement across the membrane based on concentration gradients, potentially facilitated by carrier proteins.
- Osmosis: Water movement across a semipermeable membrane from a region of low solute concentration to a region of high solute concentration.
- Solvent drag: Water carrying dissolved solutes as it moves across the membrane.
Active Transport: Movement of substances against their electrochemical gradients, requiring energy (ATP).
- Primary active transport: Directly utilizes ATP hydrolysis for transport, as exemplified by the Na+/K+ ATPase pump, which actively pumps sodium out of the cell and potassium into the cell, establishing a crucial electrochemical gradient.
- Secondary active transport: Utilizes the energy stored in the electrochemical gradient of one substance (usually sodium) to transport another substance against its gradient.
- Cotransport (symport): Both substances move in the same direction across the membrane (e.g., Na+-glucose cotransport in the proximal tubule).
- Countertransport (antiport/exchange): Substances move in opposite directions (e.g., Na+-H+ exchange in the proximal tubule).
The Sodium Gradient: A Driving Force for Reabsorption
The Na+/K+ ATPase pumps located on the basolateral membrane of tubular cells constantly pump sodium out of the cells and into the interstitial fluid. This active transport creates a low intracellular sodium concentration and a sodium gradient across the luminal membrane. This gradient serves as the primary driving force for many reabsorptive processes in the nephron.