BIOL 1104 Lecture Notes - Tubular Fluid, Ultrafiltration, Cardiac Arrhythmia

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11 Feb 2013
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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 flow into bloodstream
2. energy supplied by Na-K pump in basolateral mem.
ii. On high K+ diet see secretion of K & reabsorption of Na
1. increased activity of Na-K pump & increased # K channels in
apical membrane => secretion
2. K secretion is dependent upon Na reabsorption (Na-K pump)
3. Need at least 15 mM Na in tubular fluid for normal K secretion
e. Chemical Factors affecting K Transport
i. [K] in lumen of collecting duct
1. ↑ [Klumen] => ↑ secretion (pretty obvious)
ii. pH of ECF & ICF
1. Alkalosis stimulates K secretion; Acidosis inhibits K secretion
2. H+ competes w/ K+ for secretion => when high [H+] low K+
secretion (occurs in Principal cells)
3. Abnormal K Balance:
a. Vomiting → alkalosis → ↑ renal excretion of K+ BUT
alkalosis also → ↑ K+ into cells => even lower [K+plasma]
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