Feedback mechanisms, electrolyte levels

Figure 1-3 Example of feedback mechanism for regulation of electrolyte levels.

In order for the feedback mechanism to be effective, the organs or systems responsible for absorption and excretion (gastrointestinal) or reabsorption and excretion (renal) must function adequately. If the intestinal track is damaged or illness causes diarrhea or vomiting, absorption and excretion of electrolytes can be affected, and the feedback mechanism will malfunction. For example, in malabsorption syndrome, electrolytes are not absorbed through the tissue of the intestines to the degree needed, even though the levels of electrolytes are low. 

Similarly, if renal system function is insufficient or nonexistent (failure), reabsorption and excretion of electrolytes may occur without response to the feedback mechanism or consideration of current levels of electrolytes. For example, in renal failure, potassium may be not be excreted and may even be reabsorbed, although the potassium level is already high because there is a failure of the usual feedback mechanism. Table 1-3 is a summary of regulation mechanisms for representative electrolytes. 

From the point of view of potassium balance, there is increased renal excretion of potassium, loss of potassium in the vomitus and no potassium being delivered for absorption in the alimentary tract. All these factors contribute to a severe depletion of the body s total potassium content. Yet another factor contributes to potassium loss. A drop in volume of the circulating blood leads to aldosterone secretion via the renin-angiotensin mechanism which, in turn, promotes sodium reabsorption in the renal tubule this contributes further to excessive renal loss of potassium and hydrogen ions. The acidity of the urine is inappropriate as a response to metabolic alkalosis, but the preservation of electrolyte and fluid volume takes precedence over the acid-base disturbance. These various efiects all combine to yield a positive feedback system driving the metabolic alkalosis which, if not treated, reaches lethal levels in a few days. 

---