Tubular secretion and reabsorption are two critical processes in the nephron tubule of the kidneys. When the fluid filtered from the glomerulus enters the proximal convoluted tubule, it is referred to as filtrate, and its composition changes due to tubular reabsorption and secretion.

Tubular reabsorption is a selective process that starts when the filtrate enters the proximal tubules. It involves substances traveling through the transcellular route (through the tubule cell and peritubular capillaries) or the paracellular route (between the tubule cells). The latter is limited by tight junctions connecting the cells, but these junctions are "leaky" in the proximal nephron, allowing certain ions to pass. The reabsorption process can be either active (requiring ATP) or passive (involving diffusion, facilitated diffusion, or osmosis).

Tubular secretion, in contrast, works in reverse. It's a method of clearing the plasma of unwanted substances by not reabsorbing them from the filtrate or actively secreting them. This process is essential for disposing of substances tightly bound to plasma proteins, eliminating undesirable substances reabsorbed passively, ridding the body of excess potassium, and controlling blood pH.

For instance, when blood pH becomes too acidic, renal tubule cells secrete more hydrogen ions and ammonium while retaining bicarbonate ions, increasing blood pH and removing excess hydrogen ions through urine. In response to alkalosis, the kidneys excrete bicarbonate while retaining hydrogen ions to lower blood pH. Chloride ions are reabsorbed to maintain electroneutrality but do not directly replace bicarbonate.