Reabsorption and Secretion in the PCT

The proximal convoluted tubules, or PCTs, reabsorb most filtered water, ions, glucose, and amino acids.

Sodium ion transport is a critical process in the PCT.

The Na⁺–glucose symporter transports glucose and Na+ ions from the filtrate into the tubule cells. While glucose exits these cells by facilitated diffusion, Na⁺ ions are actively transported by sodium-potassium pumps.

Similarly, other Na⁺ symporters reabsorb phosphate and sulfate ions, amino acids, and lactic acid.

Conversely, Na⁺-H⁺ antiporters carry filtered Na⁺ ions into the cell while moving H⁺  ions into the lumen, resulting in H⁺ secretion and Na⁺ reabsorption.

Solute reabsorption promotes osmosis, moving water from tubular fluid to the peritubular capillaries through the paracellular and transcellular routes.

At the distal end of PCTs, electrochemical gradients facilitate the diffusion of chloride, potassium, calcium, and magnesium ions into the peritubular capillaries.

Additionally, the PCTs move wastes such as ammonia and urea in the opposite direction, secreting them into the tubular fluid.