Electrolyte balance

Aldosterone makes principal cells build more sodium channels, potassium channels and sodium–potassium pumps, so they reabsorb more sodium and secrete more potassium. Aquaporins are ADH's job; calcium and phosphate are PTH's.

• Correct: Increases sodium reabsorption and potassium secretion:
• Inserts aquaporins so water is reabsorbed:
• Increases calcium reabsorption and phosphate excretion:

A smaller concentration gradient means less potassium leaks out, so potassium's equilibrium potential and the resting potential move toward zero: less negative, closer to threshold.

• It becomes more negative:
• Correct: It becomes less negative:
• It does not change:

Parathyroid hormone releases calcium from bone, makes the kidneys keep calcium and excrete phosphate, and switches on calcitriol, which raises calcium absorption from food.

• Calcitonin:
• Aldosterone:
• Correct: Parathyroid hormone:

She gained about 2 L of water, and exercise and nausea kept her ADH high, so her kidneys could not pass it. The same sodium is diluted in more water. Hyponatremia is almost always too much water for the sodium present.

• She lost more sodium in sweat than she took in: She did lose some sodium in sweat, but sweat is more dilute than plasma. That loss alone would raise, not lower, her plasma sodium concentration.
• Her aldosterone release failed during the race: Nothing suggests aldosterone failed, and losing aldosterone takes days to lower sodium. Her problem developed over hours from drinking.
• Correct: She kept more water than she lost: Correct. A net gain of water dilutes her plasma sodium.
• Her kidneys excreted too much sodium: Her kidneys were keeping, not wasting, what they could; with ADH high they could not excrete the extra water.

Over about two days, brain cells in dilute plasma shed potassium and small organic solutes, so they stop swelling. If plasma sodium then rises quickly, water rushes out of these adapted cells and they are damaged, especially the myelin in the pons: osmotic demyelination. Slow correction gives them time to regain solute.

• Her kidneys cannot excrete water any faster than that: Healthy kidneys can excrete water much faster than this. The limit protects the brain, not the kidneys.
• Faster correction would cause hypernatremia: Raising sodium from 112 by 8 a day would take days to reach even the low normal range. Overshooting to above 145 is not the concern.
• Correct: Her brain cells have adapted, and fast correction can damage them: Correct. Adapted brain cells shrink too fast when the plasma is concentrated quickly.
• Salt given by vein takes several days to reach the plasma: IV salt enters the plasma at once. The limit on speed is chosen by the doctors, not set by delivery.

2% of 3,500 mmol = 70 mmol. Spread through 14 L: 70 ÷ 14 = 5.0 mmol/L added. 4.0 + 5.0 = 9.0 mmol/L, which is life-threatening.

• About 4.1 mmol/L: This treats the release as a tiny change. Because the ECF holds so little potassium, 70 mmol is more than it held to begin with.
• About 6.5 mmol/L: 6.5 is the result for a 1% release (35 mmol ÷ 14 L = 2.5 added). The question says 2%.
• Correct: About 9.0 mmol/L: Correct. 70 mmol ÷ 14 L = 5.0 mmol/L added to 4.0.
• About 74 mmol/L: 74 adds the 70 mmol to the concentration without dividing by the volume.

Hyperkalemia moves the resting potential of heart cells toward threshold. Raising extracellular calcium raises the threshold, restoring the gap between rest and threshold. It buys 30 to 60 minutes while insulin and glucose shift potassium into cells and dialysis removes it. It does not lower potassium.

• Correct: It raises heart cells' threshold, protecting the heart for a while: Correct. Calcium stabilizes the heart's membranes without changing potassium.
• It binds potassium in the plasma so it cannot act on cells: Calcium and potassium are both cations and do not bind each other; his potassium is unchanged.
• It stimulates the kidneys to secrete potassium: Calcium does not drive potassium secretion, and his kidneys have failed anyway.
• It moves potassium into cells, like insulin does: Calcium does not shift potassium into cells. That is what insulin, glucose and beta-2 agonists do.

Hyperventilation blows off CO₂, so the blood becomes alkaline. In alkalosis, albumin gives up hydrogen ions and binds more calcium, so ionized calcium falls although the total is unchanged. Low ionized calcium makes nerves and muscles overexcitable: tingling and cramps.

• Fast breathing removes calcium from the blood in exhaled air: Calcium is not a gas and does not leave in exhaled air. Breathing changes CO₂, which changes pH.
• Her parathyroid glands stop releasing PTH during panic: PTH takes minutes to hours to act, and panic does not switch it off. Her total calcium is normal.
• Epinephrine drives calcium into bone: Nothing like this happens in minutes, and her total calcium has not fallen.
• Correct: Alkalosis makes albumin bind more calcium: Correct. Only the free fraction falls, so the total calcium looks normal.

Feeding releases insulin, which drives glucose and phosphate into cells, where phosphate is used to make ATP, glycogen and phosphorylated sugars. His stores were already low, so plasma phosphate crashes. Without it, cells cannot make enough ATP, and the diaphragm weakens.

• The food contained too little phosphate: Food contains plenty of phosphate. The fall comes from phosphate moving into cells faster than it can be absorbed.
• Correct: Insulin drove phosphate into cells to make ATP and glycogen: Correct. This is refeeding: an internal shift into cells.
• His kidneys began excreting extra phosphate in response to PTH: PTH does increase phosphate excretion, but nothing here raised PTH. The fall came within two days of feeding.
• His bones took up phosphate to mineralize new matrix: Bone mineralization is slow and would not drop phosphate this fast. The fast shift is into cells.

Heavy alcohol use often causes low magnesium. Magnesium normally limits potassium flow out of principal cells; without it, potassium keeps leaking into the urine. Low magnesium also blocks PTH release and action, lowering calcium. Both correct only when magnesium is replaced.

• Plasma sodium concentration: Low sodium does not explain potassium that will not correct, or low calcium.
• Plasma chloride: Chloride changes with vomiting and diuretics, but low chloride does not block potassium replacement or PTH.
• Correct: Plasma magnesium: Correct. Low magnesium keeps potassium and calcium low until it is corrected.
• Blood glucose: Glucose affects potassium shifts through insulin, but it does not explain both the resistant potassium and the low calcium.