Tubular reabsorption
Reabsorption plays a major role than the secretion in the formation of urine. More than 90% of the water in the GF is reabsorbed as it passes through the tubules. Some substances such as sodium, glucose and amino acids are almost completely reabsorbed so that their concentration decreases almost to zero before the fluid becomes urine so that they are conserved by the body and not excreted and lost by the urine.
Basic mechanisms of absorption is by two process – Active transport and Passive transport.
Active transport through the tubular wall
Tubular epithelial cells have a brush border at the luminal surface. It is composed of microvilli which increases surface area of the lumen. The base of the cell rests on the basement membrane. Basal channels in the basement membrane increases the surface area. Epithelial cells are attached to each other near the brush border forming tight junction or zona occludens.
Sodium, glucose, amino acid, calcium, potassium, phosphate and urate ions are actively transported. They are transported through carrier proteins.
Uniport
Transport by a carrier for a single compound (e.g., sodium) and is unidirectional.
Symport or co-transport
Transport of two compounds on the same carrier in the same direction (e.g., sodium plus glucose, or sodium plus amino acid).
Antiport or counter transport
Movement of a compound in one direction driven by the movement of a second compound in opposite direction (e.g., Na2+ and H+ antiport).
Sodium reabsorption
About 65% of Na2+ is reabsorbed in the proximal tubule. The energy required for this mechanism is derived from the Na2+-K+ ATPase (sodium pump) located in the basolateral membrane of the proximal tubule epithelial cells.
Electrochemical Gradient
Sodium ions are actively transported from the interior of the tubular epithelial cells to the peritubular space across the basal membrane. Therefore, intra-cellular Na2+ is lowered, creating a chemical gradient for Na2+ (lumen concentration higher) between the tubular lumen and tubular epithelial cell. It also causes a low negative intracellular electrical potential (-70 mv) which in turn causes the Na2+ to diffuse from the tubular lumen into the cell through the brush border. This mechanism is uniport or unidirectional Na2+ transport. Chloride ion readily diffuses from the tubular lumen to the peritubular space through tight junction between tubular epithelial cells because of transepithelial electrical potential difference (lumen negative) created by Na2+ transport.
Antiport or counter transport of sodium ion
Diffusion of Na2+ because of electrochemical gradient is coupled through a carrier protein with H+ diffusing in the opposite direction from the cell interior to the tubular lumen. HCO3– in the cell can diffuse through the basolateral membrane to the peritubular space or move into the tubular lumen in counter transport to Cl– diffusion into the cell.
Chloride driven sodium ion transport
As more of HCO3– is being reabsorbed in to the peritubular space Cl– gradient favours diffusion of Cl– through the leaky tight function from the tubular lumen into the peritubular space and is accompanied by diffusion of Na2+ in the same direction to maintain electrical neutrality.
Glucose and amino acid reabsorption
These are reabsorbed by symport or co-transport. They are coupled with specific carriers that require Na2+ binding and diffuse into the cell. Inside the cell Na2+ and glucose or amino acid separates from the carrier. The Na2+ is actively transported by Na2+– K+ ATPase to the peritubular space. Glucose and amino acids are then transported by facilitated diffusion.
Passive transport of water and other solutes
After the diffusion of solute (Na2+, Cl– , HCO3–, glucose, amino acid) into the peritubular space, an osmotic gradient is established, where by a greater osmotic pressure is present in the peritubular space. Therefore, water diffuses from the peritubular lumen and tubular cells into the peritubular space.
As 65% of Na2+ is reabsorbed, similarly 65% of water is reabsorbed from the proximal tubule (an additional amount for other osmotically active substances such as glucose, amino-acid).
As water is reabsorbed, urea and other non-actively reabsorbed solutes get concentrated in the tubular lumen.
A chemical concentration gradient is established for these substances and they are reabsorbed down their concentrated gradient. The extent of their reabsorption depends on the permeability of the tubular epithelium for the solute.
Urea permeability for the proximal tubule is much less than that of water and more than 50% of the amount of urea in the GF continues beyond the proximal tubule. There is no permeability of tubular membrane for reabsorption of creatinine, inulin, mannitol and sucrose and therefore once these are filtered, their total quantity appears in the urine.
Reabsorption of proteins and peptides
Proteins with a molecular weight of less than 69,000 are reabsorbed in the proximal tubule and not lost in the urine. They are reabsorbed by endocytosis and subsequently degraded by cellular lysosomes to amino acids. The amino acids move from the cell to the peritubular space by facilitated diffusion. Small peptides are hydrolyzed at the luminal brush border of proximal tubule and the resultant amino acids is taken into the cell by co-transport mechanism.