The excretion of large amounts of H+ (on occasion as much as 500 mEq/day) in the urine is accomplished primarily by combining the H+ with buffers in the tubular fluid. The most important buffers are phosphate buffer and ammonia buffer. There are other weak buffer systems, such as urate and citrate, that are much less important.
When H+ is titrated in the tubular fluid with HCO3-, this results in the reabsorption of one HCO3- for each H+ secreted, as discussed earlier. But when there are excess H+ in the urine, they combine with buffers other than HCO3-, and this results in the generation of new HCO3- that can also enter the blood. Thus, when there is excess H+ in the extracellular fluid, the kidneys not only reabsorb all the filtered HCO3- but also generate new HCO3-, thereby helping to replenish the HCO3-lost from the extracellular fluid in acidosis. In the next two sections, we discuss the mechanisms by which phosphate and ammonia buffers contribute to the generation of new HCO3-.
Buffering of secreted hydrogen ions by filtered phosphate (NaHPO4-). Note that a new bicarbonate ion is returned to the blood for each NaHPO4- that reacts with a secreted hydrogen ion.
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This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.