- State the normal balance and distribution of potassium between cells and extracellular fluid.
- Describe how potassium moves between cells and the extracellular fluid, and how, on a short-term basis, the movement protects the extracellular fluid from large changes in potassium concentration.
- State why plasma levels of potassium do not always reflect the status of total-body potassium.
- State how insulin and epinephrine influence the cellular uptake of potassium and identify the situations in which these hormonal influences are most important.
- State the relative amounts of potassium reabsorbed by the proximal tubule and thick ascending limb of Henle's loop regardless of the state of potassium intake.
- Describe how nephron segments beyond the thick ascending limb can manifest net secretion or reabsorption; describe the role of principal cells and intercalated cells in these processes.
- List inputs that control the rate of potassium secretion by the distal nephron.
- Describe the actions of ROMK and BK potassium channels in conditions of low, normal, and high potassium excretion.
- Describe how changes in plasma potassium influence aldosterone secretion.
- State the effects of most diuretic drugs on potassium excretion.
The vast majority of body potassium is freely dissolved in the cytosol of tissue cells and constitutes the major osmotic component of the intracellular fluid (ICF). Only about 2% of total-body potassium is in the extracellular fluid (ECF). This small fraction, however, is absolutely crucial for body function, and the concentration of potassium in the ECF is a closely regulated quantity. Major increases and decreases (called hyperkalemia and hypokalemia) in plasma values are cause for medical intervention. The importance of maintaining this concentration stems primarily from the role of potassium in the excitability of nerve and muscle, especially the heart. The ratio of the intracellular to extracellular concentration of potassium is the major determinant of the resting membrane potential in these cells. A significant rise in the extracellular potassium concentration causes a sustained depolarization. Low extracellular potassium may hyperpolarize or depolarize depending on how changes in extracellular potassium affect membrane permeability. Both conditions lead to muscle and cardiac disturbances.
Given that the vast majority of body potassium is contained within cells, the extracellular potassium concentration is crucially dependent on (1) the total amount of potassium in the body and (2) the distribution of this potassium between the extracellular and intracellular fluid compartments. Total-body potassium is determined by the balance between potassium intake and excretion. Healthy individuals remain in potassium balance, as they do in sodium balance, by excreting potassium in response to dietary loads and withholding excretion when body potassium is depleted. The urine is the major route of potassium excretion, although some is lost in the feces and sweat. Normally the losses via sweat and the gastrointestinal tract are small, but large quantities can be lost from the ...