Lecture 36- Friday, April 9 , 2010
Potassium (K) is the most abundant intracellular ion.
98% Intracellular fluid
2% Extracellular fluid
The K concentration in the extracellular fluid is extremely important for the function
of excitable tissues (nerve and muscle).
Reason: the resting membrane potentials of these tissues are directly related to the
relative intracellular and extracellular K concentrations.
Hyperkalemia: high concentration of K in the extracellular fluid (>5 mEq/L)
Hypokalemia: low concentration of K in the extracellular fluid (<3.5 mEq/L)
Both cause abnormal rhythms of the heart and abnormalities of skeletal muscle
contraction. Hyperkalemia is usually the most often reason for consultation, usually
from an emergency room. This is because it is life threatening.
When it becomes sinusoidal, that’s when an electrical tachycardia occurs and you
must administer a shock. Fruits and nuts contain a lot of potassium so when you’re
on dialysis, they could kill you!
Renal Regulation of Potassium
-K is freely filtered at glomerulus.
-Normally, the tubules reabsorb most of this filtered K so that very little of the
filtered K appear in the urine.
-However, unlike sodium or water, K can be secreted at the cortical collecting ducts. -Changes in K excretion are due mainly to changes in K secretion in the CCD (some
in the DCT).
K+ is freely filtered at the glomerulus, and some is reabsorbed. Net reabsorption
varies from 15-99%. 99% occurs if you’re not getting any K+ hardly at all. If you eat
normally, reabsorption is usually around 86%.
2 picture: occurring in the cortical collecting duct cells. Secretion of K+ in the CCD
is coupled with Na+ reabsorption. This whole process responds to aldosterone as
Regulation of Potassium Secretion
Potassium secretion is regulated by:
1. Dietary intake of potassium
2. Aldosterone ^Summarizes 1 and 2. You need this mechanism. However there are situations
where the mechanism works as a side effect; the body doesn’t need to secret
potassium, but other factors cause the secretion of K+, leading to hypokalemia.
[There is a mechanism that cancels K+ secretion. When you have a low volume,
amount of K+ delivered to cortical collecting duct decreases. This is advanced
• The conditions in which the adrenal hormone aldosterone is released in excess.
• The most common cause: adenoma of the adrenal gland that produces aldosterone
• Increased fluid volume, hypertension, hypokalemia. Renin is suppressed, hydrogen
ion secretion. Metabolic alkalosis is often seen. This is apathological condition usually associated with high blood pressure in young
people, or hypokalemia.
Hydrogen Ion Regulation
-Metabolic reactions are highly sensitive to the hydrogen ion concentration of the
-Thus, the hydrogen ion concentration of the extracellular fluid is tightly regulated.
-pH: ~7.4 ([H ]: ~40 nmol/L)
Mass reaction takes place in your body constantly. As long as you’re living, CO2 and
H2O is always generated, and gets converted to carbonic acid by carbonic
anhydrase. Carbonic acid gets dissociated ins