Tubular Secretion

Tubular secretion runs in the opposite direction to reabsorption: rather than recovering substances from the filtrate, the tubular cells actively move certain substances out of the peritubular blood and into the tubular fluid so they can be flushed out in the urine. This is how the body disposes of substances that either were not filtered efficiently at the glomerulus or need to be cleared faster than filtration alone allows, and it is also the mechanism the kidney uses to fine-tune blood pH. Hydrogen ions (H+), potassium ions (K+), ammonium ions (NH4+), creatinine and various organic acids and drugs are all secreted this way. The H+ secretion is especially important for acid-base balance: for every H+ secreted into the tubular fluid, a Na+ is reabsorbed in exchange, and the secreted H+ combines with buffers such as bicarbonate, phosphate and ammonia in the fluid so that it stays fixed there rather than being reabsorbed back into the blood - this one-way trapping is what lets the kidney permanently excrete …