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Control of ECF and composition

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BIOL 1840U
Peter Cheung

Control of ECF & Composition IV Potassium Balance I. Potassium homeostasis a. Most K+ in body is contained w/in cells; very little in ECF!! This is ESSENTIAL for normal cell fxn b. Controlling Factors: i. Insulin & epinephrine—cause cellular uptake of K+ ii. Arterial pH—acidic = K+ shifts from cells to ECF; alkalinic = opposite iii. Kidney plays critical role—excretes ~5-15% K+ daily c. Effects of Abnormal [K+] i. Hyperkalemia—ventricular fibrillation ii. Hypokalemia—severe arrhythmia & decreased heart rate II. Sites & Mechanisms involved in Renal K+ Transport a. Low K+ Diet—amount of K+ in tubular fluid drops to very low level in final urine i. Ratio of concentration in tubular fluid vs ultrafiltrate remains ~1 ii. Ratio of amount of K+ in tubular fluid vs ultrafiltrate drops dramatically b. High K+ Diet—secretion of K+ in DCT/CCD => effective removal of K+ from ECF by increasing excreted amount i. Ratio of concentration remains ~ same as low K+ diet until CCD when it ↑ ii. Ratio of amounts remains ~ same as low K+ diet until CCD when it ↑ c. Principles of K+ Transport i. ~70% filtered K+ is reabsorbed by PCT, w/ 25% more reabsorbed in TAL ii. normal reabsorption is ~95% iii. During severe K+ depletion, distal Nephron can reabsorb K+ iv. K+ secretion is controlled by hormonal, metabolic, & physical factors!! d. Mechanism of K+ Secretion in CCD i. On Low K+ diet, see absorption of K & Na 1. K channel in basolateral mem. allows K to fl
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