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Renal system




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USMLE® Step 1 questions

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USMLE® Step 1 style questions USMLE

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A 67-year-old man with a history of hypertension and congestive heart failure presents to the primary care physician with muscle cramps, weakness, and fatigue. The patient has had multiple admissions to the hospital over the past several months and has been on escalating doses of bumetanide and hydrochlorothiazide. He was recently ill with food poisoning. Temperature is 37.0°C (98.6°F), pulse is 92/min, respirations are 16/min, blood pressure is 143/74 mmHg, and O2 saturation is 96% on room air. On physical exam the patient is not in acute distress, has 1+ peripheral edema in the lower extremities bilaterally, and has faint bibasilar rales. ECG is demonstrated below:  

Image reproduced from Wikimedia Commons

Which of the following is the most likely etiology of this patient’s symptoms?  

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Alkalosis p. 614, 616

hypokalemia with p. 614


hypokalemia and p. 615

Hypokalemia p. 615

antacid use p. 408

causes of p. 614

cystic fibrosis p. 58

on EKG p. 312

loop diuretics p. 632

nephrogenic DI p. 351

VIPomas and p. 380

Insulin p. 335

hypokalemia from p. 614


With hypokalemia, hypo- means under and -kal- refers to potassium, and -emia refers to the blood, so hypokalemia means lower than normal potassium levels in the blood, generally under 3.5 mEq/L.

Now, total body potassium can essentially be split into two components—intracellular and extracellular potassium, or potassium inside and outside cells, respectively.

The extracellular component includes both the intravascular space, which is the space within the blood and lymphatic vessels and the interstitial space—the space between cells where you typically find fibrous proteins and long chains of carbohydrates which are called glycosaminoglycans.

Now, the vast majority, around 98%, of all of the body’s potassium is intracellular, or inside of the cells.

In fact, the concentration of potassium inside the cells is about 150 mEq/L whereas outside the cells it’s only about 4.5 mEq/L.

Keep in mind that these potassium ions carry a charge, so the difference in concentration also leads to a difference in charge, which establishes an overall electrochemical gradient across the cell membrane.

And this is called the internal potassium balance. This balance is maintained by the sodium-potassium pump, which pumps 2 potassium ions in for every 3 sodium ions out, as well as potassium leak channels and inward rectifier channels that are scattered throughout the membrane.

This concentration gradient is extremely important for setting the resting membrane potential of excitable cell membranes, which is needed for normal contraction of smooth, cardiac, and skeletal muscle.

Also, though, in addition to this internal potassium balance, there’s also an external potassium balance, which refers to the potassium you get externally through the diet every day.

On a daily basis the amount of potassium that typically gets taken in, usually ranges between 50 mEq/L to 150 mEq/L, which is way higher than the extracellular potassium concentration of 4.5 mEq/L, so your body has to figure out a way to excrete most of what it takes in.

This external balancing act is largely taken care of by the kidneys, where excess potassium is secreted into a renal tubule and excreted in the urine.

Also, though, a small amount dietary potassium is also lost via the gastrointestinal tract and the sweat.


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