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PHGY210- Lecture 37- Dr. Takano.docx

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Department
Physiology
Course
PHGY 210
Professor
Ann Wechsler
Semester
Winter

Description
Lecture 36- Friday, April 9 , 2010 Potassium Regulation 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%. nd 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 well. 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 physiology.] Hyperaldosteronism • The conditions in which the adrenal hormone aldosterone is released in excess. • The most common cause: adenoma of the adrenal gland that produces aldosterone autonomously. • 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 environment. -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
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