So in this part of tubules, the distal and the cortical collecting tubules, K+ can be reabsorbed or secreted depending on the demand. So we can take in per day about 100mEq of K+, some of it is eliminated in feces (8mEq) but the rest has to be handled by the kidneys (the remaining 92mEq). About a 1/3 of that 92mEq will be secreted by those end sections, the distal and cortical collecting tubules. Why is this mechanism so important?
This mechanism is so sensitive that we can actually secrete more than we filter so if need arises we can get rid of more K+, if our kidneys are working well, than we actually filter.