47 Reabsorption in the Proximal Convoluted Tubule
The majority of solute and water reabsorption occurs in the proximal convoluted tubule. Sodium ions, which are the most plentiful cation (positively charged ion) in the tubular fluid, participate in most solute reabsorption. Sodium ion carriers co-transport all filtered glucose, amino acids, lactic acid, water-soluble vitamins, and other nutrients. The reabsorption of sodium ions into the interstitial fluid and then into the blood creates the osmotic gradient needed for water reabsorption. An osmotic gradient is produced when the solute concentration in the interstitial fluid is higher than that in the tubular fluid.
| Region | Reabsorbed Substances | Amount Absorbed(Percent) | Mechanism |
|---|---|---|---|
| Proximal convoluted tubule | Water | 65 | Obligatory water reabsorption |
| Sodium ions (Na+) | 65 | Primary active transport | |
| Potassium ions (K+) | 65 | Passive transport; paracellular route | |
| Glucose, amino acids, and most other organic solutes | 100 | Secondary active transport with Na+ | |
| Chloride ions (Cl−) | 50 | Passive transport; paracellular diffusion | |
| Bicarbonate ions (HCO3−) | 80-90 | Secondary active transport with Na+ | |
| Urea | 50 | Passive diffusion | |
| Calcium ions (Ca2+) | variable* | Passive transport; paracellular route | |
| Magnesium ions (Mg2+) | variable* | Passive transport; paracellular route |
Reabsorption in the Nephron Loop
Because all glucose, amino acids, and other nutrients are reabsorbed in the proximal convoluted tubule, fluid flows into the nephron loop (loop of Henle) at a rate of 40 to 45 milliliters per minute (mL/min) (1.5 ounces/min), down from 125 mL/min (4.5 ounces/min) in the proximal convoluted tubule. The chemical composition of the tubular filtrate at this point is very different from that of the glomerular filtrate even though the osmolarity is similar. The fluid’s osmolarity is still comparable to that of blood, because water was reabsorbed as solutes were reabsorbed throughout the proximal convoluted tubule. Since part of the nephron loop is comparatively water-impermeable, the reabsorption of water in this region is not necessarily linked to the reabsorption of solutes. Water is reabsorbed by osmosis in the water-permeable descending limb, but not in the ascending limb. This water reabsorption in the water-permeable descending limb can occur because the descending limb dips down toward, or into, the renal medulla, where the osmolarity of the interstitial fluid increases. This creates a gradient for this water reabsorption to occur. Essentially no solutes are reabsorbed in the descending limb, while solutes are reabsorbed via both active and passive mechanisms in the ascending limb. The permeability differences in these two limbs plays an important role in the ability of the kidneys to form either dilute or concentrated urine.
| Region | Reabsorbed Substances | Amount Absorbed(Percent) | Mechanism |
|---|---|---|---|
| Desce