Mechanisms of Ion Reabsorption
The primary ion determining osmotic pressure and water reabsorption is sodium ($Na^+$).
Sodium reabsorption occurs in two steps:
- Passive entry: At the apical membrane (facing the tubular lumen), $Na^+$ enters passively down its concentration gradient. The membrane potential is 69 mV.
- Active extrusion: At the basolateral membrane (facing the peritubular capillary), $Na^+$ is actively pumped out of the cell by $Na^+,K^+$-ATPase. The basolateral membrane potential is 70 mV.
Total energy expenditure for sodium transport is low because the net potential difference between blood and urine is only 1 mV.
Other ions are reabsorbed through various mechanisms:
- Potassium ($K^+$) is actively taken up at the apical membrane and exits into the blood via diffusion.
- Mechanisms for $Ca^{2+}$, $Mg^{2+}$, $SO_4^{2-}$, and $HPO_4^{2-}$ are similar to those of sodium, potassium, and chloride transport.
- Anions (e.g., $Cl^-$, $HCO_3^-$) return to the blood passively, following positively charged cations ($Na^+$, $K^+$).
Transport of Organic Solutes and Water
The proximal convoluted tubules achieve complete reabsorption of essential organic compounds such as glucose, amino acids, vitamins, low-molecular-weight proteins, and trace elements.
Transport of organic molecules includes:
- Active entry: Substances cross the apical membrane via facilitated diffusion (co-transport) coupled with sodium ions. This process requires energy (high-energy phosphates).
- Passive exit: Substances diffuse across the basolateral membrane into the blood along a concentration gradient.
- Water is reabsorbed passively, following reabsorbed solutes (primarily sodium) down an osmotic gradient. As water is reabsorbed, the concentration of dissolved solutes in the tubular fluid increases, facilitating their passive reabsorption as well.
Threshold Substances
Certain substances are completely reabsorbed into the blood only up to a specific plasma concentration, known as the renal threshold. If the concentration exceeds this threshold, transport systems become saturated, and the substance begins to appear in the final urine.
A classic example is glucose:
- At normal blood concentrations (3.8–7.1 mmol/L), glucose is entirely filtered and returned to the bloodstream.
- If glucose levels exceed 7.1 mmol/L, a portion remains in the renal tubules.
- The presence of glucose in the urine is termed glucosuria.
Tubular Secretion and pH Regulation
Parallel to reabsorption is tubular secretion, which involves the active transport of substances from the blood into the tubular lumen. The primary purpose is the removal of metabolic waste products (choline, ammonia, p-aminohippuric acid) and drug molecules that were not filtered at the glomerulus.
Key processes in the proximal tubule epithelium include:
- Ammoniagenesis: Epithelial cells take up glutamine and use the enzyme glutaminase to cleave it into glutamic acid and ammonia. Ammonia is secreted and eliminated as ammonium salts.
- Acid-base regulation: The enzyme carbonic anhydrase breaks down carbonic acid. Bicarbonate ($HCO_3^-$) is reabsorbed into the blood to maintain the alkaline reserve, while protons ($H^+$) are secreted into the urine, where they bind to sodium hydrogen phosphate ($Na_2HPO_4$) to form dihydrogen phosphate ($NaH_2PO_4$). This prevents acidosis and acidifies the final urine.