The Extracellular concentration of potassium (K) is between 3.5-5.0 mmol/l. 98% of the total potassium in the body is in the Intracellular fluid (approx 3700mmol for a 70kg man) while only 2% is in ECF (approx 65mmol). The typical daily intake of potassium is 50-150mmol which exceeds total amount of potassium in extracellular compartment.
The distribution of potassium between Intracellular and Extracellular compartment is determined by the balance between inward and outward movement of potassium. Uptake of potassium into the cell is dependent of Na+,k+-ATPase, where as efflux from cell is passive.
As earlier stated potassium intake in meal in a day is more than the total potassium content of the ECF compartment, the amount of K+ in a single meal could cause lethal hyperkalemia.
Two mechanism are used to keep K+ within normal range in the body
Kidney response is the most effective regulator of K+, but the mechanism is very slow. In the interim cellular buffering acts to keep the K+ level within range. As ECF K+ rises, its uptake into the cell increases and when ECF K+ falls it shift out of the cell. it uses passive for outward movement and active for inward movement. The Na+,k+-ATPase which actively transport k+ into cell is stimulated by an elevation in k+ and inhibited by a decrease in ECF k+ concentration.
Other factors that can affect k+ shift in and out of the cell are
• ACIDEMIA: increase H+ concentration in acidosis increases potassium concentration in the extracellular fluid. This occurs as a result of exchange of extracellular hydrogen ion for intracellular potassium.
• INSULIN: Insulin promote uptake of potassium by skeletal muscle and liver cells. insulin stimulates Na+,k+-ATPase
• ALDOSTERONE: acts on K+ transport in epithelia cell aside from its kidney function
• Other factors like tissue trauma, hemolysis, severe exercise, ischemia moves K+ outside the cell
Typically K+ excretion is around 50-150mmol/day but it varies base on K+ level in the body. sometimes during excessive K+ intake, amount of K+ excreted can exceed filtered level of K+ which signifies active secretion along the tube of nephron.
̰60% of filtered K+ is reabsorbed at the proximal tubule
̰30% of filtered K+ is reabsorbed at the loop of henle (mostly ascending loop)
̰10% of filtered K+ reach distal convoluted tubule
This little K+ concentration at the distal tubule necessitate active secretion to get rid of excess K+ . An increase in K+ intake causes increase K+ secretion at the collecting duct and tube, increase surface area of basolateral membrane and also increases density of Na+,k+-ATPase.
The responsibility of Potassium balance rest on the cortical collecting duct which control the amount of K+ secreted into the filterate.
• When ECF K+ is low: K+ moves from tissue cell into ECF and renal principal cell conserve K+ by reducing its secretion and excretion to minimum.