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Difficulty: HardCellular Injury and Adaptation

A 74-year-old woman with unmedicated persistent atrial fibrillation is admitted with sudden-onset excruciating pain in her right lower extremity. Clinical evaluation reveals a cold, pale, and pulseless leg, highly suspicious for an acute thromboembolic occlusion of the superficial femoral artery. Place the following subcellular events in the correct chronological sequence as they occur within the ischemic skeletal myocytes during the resulting hypoxic injury.

  1. 1Cessation of mitochondrial oxidative phosphorylation
  2. 2Failure of the Na+/K+Na^+/K^+-ATPase pump
  3. 3Isosmotic influx of water and cellular swelling
  4. 4Massive cytosolic calcium accumulation
  5. 5Activation of phospholipases and irreversible membrane damage

Answer

The correct sequence begins with the immediate loss of oxidative phosphorylation, followed by the failure of ATP-dependent pumps, cellular swelling, massive calcium influx, and finally, irreversible enzymatic membrane degradation.
Ischemic cell injury follows a highly predictable chronological cascade. First, the lack of oxygen halts mitochondrial oxidative phosphorylation, rapidly depleting ATP stores. This lack of ATP causes failure of the plasma membrane Na+/K+Na^+/K^+-ATPase, leading to intracellular sodium accumulation. The resulting osmotic gradient draws water into the cell, causing reversible cellular and organelle swelling. As ischemia persists, severe energy failure and early membrane stress result in a massive influx of cytosolic calcium. This high calcium concentration activates intracellular degradative enzymes (phospholipases, proteases, endonucleases) that destroy the cell membrane and cytoskeleton, marking the point of irreversible cellular necrosis.

Step-by-Step Solution

1
Identify the primary insult.
The acute arterial occlusion causes severe hypoxia (ischemia), which directly impairs oxygen-dependent mitochondrial processes.
Oxidative phosphorylation requires oxygen as the final electron acceptor.
2
Determine the immediate consequence of halted oxidative phosphorylation.
Failure of the Na+/K+Na^+/K^+-ATPase pump.
Ion pumps are highly ATP-dependent and are the first cellular mechanisms to fail when ATP stores are depleted.
3
Trace the osmotic consequence of ion pump failure.
Isosmotic influx of water and cellular swelling occur.
Trapped intracellular sodium creates an osmotic gradient that pulls water into the cell, representing the hallmark of early, reversible cell injury.
4
Identify the transition mechanism toward irreversible injury.
Massive cytosolic calcium accumulation takes place.
As ATP depletion becomes critical and prolonged, Ca2+Ca^{2+}-ATPase fails, and structural damage to the endoplasmic/sarcoplasmic reticulum releases sequestered calcium.
5
Establish the final irreversible mechanism.
Activation of phospholipases and irreversible membrane damage.
High intracellular calcium acts as a potent enzyme activator, turning on phospholipases, endonucleases, and proteases that destroy structural cellular components, finalizing the transition to necrosis.

Key Concept

Chronological sequence of subcellular events in ischemic cellular injury, progressing from reversible ATP depletion to irreversible membrane damage.
Estimated Time:1m 30s
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Topics

Cellular Injury and Adaptation

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