Feedback is used when the kidneys remove h+ ion from the blood when the ph of the blood becomes too acidic. positive or negative feedback

 Negative feedback

A buffer can be defined as a substance that help to resist or prevents a radical change in fluid pH by absorbing excess hydrogen or hydroxyl ions. Thus, the kidneys help control acid-base balance by excreting hydrogen ions and generating bicarbonate that helps maintain blood plasma pH within a normal range. The kidney is said to control pH by controlling the renal threshold of HCO3  . However, HCO3- level in the blood cannot rise without changes in levels of other anions or cations in the blood. To do so without effects on Na+, Cl- or other ions would defy electrical neutrality, i.e. the blood would become negatively charged and the urine positive.