Not every patient receiving heparin develops dangerous hyperkalemia. The kidneys are remarkably resilient; in a healthy individual, the contralateral kidney or secondary mechanisms can often compensate for temporary aldosterone blockade. The danger arises when "second hits" are present.
Heparin competitively inhibits aldosterone binding to its receptors in the kidney. Essentially, heparin acts as a pharmacological "shield," blocking aldosterone from exerting its effect. Without aldosterone activity, the kidneys retain potassium while excreting sodium. heparin cause hyperkalemia