Elemental K+ is the major intracellular cation of the body. The normal intracellular concentration is 100 to 150 meq/L, while the normal extracellular concentration is 3.5 to 5.0 meq/L. The total body K+ store ranges from 35 to 55 meq/kg or 3500 meq in a healthy 70-kg man. Approximately 70 to 75 percent of total body K+ is found in muscle tissue; therefore, in patients with severe muscle wasting, total body stores may be as low as 20 to 30 meq/kg. Daily intake of K + ranges from 50 to 150 meq. Foods high in K+ include oranges, grapefruit, tomatoes, bananas, avocados, and raisins. K+ is excreted predominantly by the kidneys (90 percent) and with some loss in the stool and by sweating. K+ is filtered freely through the glomerulus and then reabsorbed in the proximal and ascending tubules. It is secreted in the distal tubule in exchange for Na+. In healthy individuals, the kidneys are able to excrete up to 6 meq/kg per day. Even in severe K + deficit, 5 to 15 meq/L may be excreted.
In the absence of an acid-base disturbance, measuring extracellular [K+] is a reasonably accurate way to assess total body K+ in relatively healthy patients. A decrease in measured serum [K+] from 4 to 3 meq/L represents a total body deficit of approximately 200 to 400 meq. An increase in measured serum [K+] from 4 to 5 meq/L represents a total body K+ increase of approximately 100 to 200 meq/L. Extracellular K+ represents about 2 percent of total body K+, or 70 meq, and is influenced by two important variables: total body K+ stores, and distribution between the ICF and ECF spaces. Significant and rapid intracellular to extracellular shifting occurs in response to severe injury (i.e., surgical stress, trauma, or burns), acid-base imbalance, catabolic states, increased extracellular osmolality, or insulin deficiency. These shifts are important when viewed in light of the role of K + in maintaining the resting membrane potential. The normally balanced intracellular-extracellular gradient facilitates propagation of electrical impulses. This is particularly important in the functioning of the heart.
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