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 |
|---|---|---|---|
| Descending limb | Water | 15 | Obligatory water reabsorption |
| Ascending limb | Na+ | 20–30 | Secondary active transport; paracellular diffusion |
| K+ | 20–30 | Secondary active transport; paracellular diffusion | |
| Cl− | 35 | Secondary active transport; paracellular diffusion | |
| HCO3− | 10–20 | Secondary active transport with Na+ | |
| Ca2+ | variable* | Passive paracellular diffusion | |
| Mg2+ | variable* | Passive paracellular diffusion |
Reabsorption in the Distal Tubule and Collecting Duct
Because 80 percent of filtered water is reabsorbed by the time tubular filtrate enters the distal convoluted tubules, the flow of this fluid has slowed to a rate of approximately 25 mL/min (1 ounce/min). Most reabsorption in this segment occurs in the initial portion of the distal convoluted tubule. When the fluid reaches the end of the distal tubule, 90 to 95 percent of solutes and water have been reabsorbed. Both principal and intercalated cells located here and in the collecting duct. Principal cells reabsorb sodium ions (and secrete potassium ions), while the intercalated cells reabsorb potassium ions and bicarbonate ions (and secrete hydrogen ions). The needs of the body determine how much of these solutes, and of water, are reabsorbed in the late distal convoluted tubule and collecting duct.
| Region | Reabsorbed Substances | Amount Absorbed(Percent) | Mechanism |
|---|---|---|---|
| Distal convoluted tubule | Water | 10–15 | Obligatory water reabsorption |
| Na+ | 5 | active Na+ transport | |
| Cl− | 5 | active Na+ transport | |
| Collecting duct | |||
| Water | variable* | Facultative water reabsorption; antidiuretic hormone required to insert aquaporins (water channels) | |
| Na+ | 3 | Primary active transport (requires aldosterone) | |
| Na+, H+, HCO3−, Cl−, hydrogen ions (H+) | variable* | Primary active transport of Na+ and the medullary gradient create the conditions for passive transport of some HCO3− and Cl− and co-transport of H+, Cl−, and HCO3− | |
| K+ | variable* | K+ is both reabsorbed and secreted (aldosterone dependent), usually resulting in net K+ secretion | |
| Urea | variable* | Facilitated diffusion in response to concentration gradient in the deep medulla region; recycles and contributes to medullary osmotic gradient |
(Note: Not necessary to memorize the tables, but most numbers are variable to what the body needs at the time.)