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Reversible injury commonly begins with impaired ATP production and failure of energy-dependent ion pumps. Sodium and calcium accumulate, water enters the cell, and swelling develops. Membrane blebs, loss of microvilli, dilated endoplasmic reticulum, and chromatin clumping may also occur; these changes can resolve if the stress is removed. Irreversible injury involves severe damage to plasma, mitochondrial, and lysosomal membranes, preventing ATP recovery and promoting enzymatic breakdown. Nuclear changes progress through pyknosis, karyorrhexis, and karyolysis. The lesson contrasts these features, explains how enzyme leakage can support clinical diagnosis, and introduces necrosis and apoptosis as possible outcomes. It also considers why reperfusion after irreversible ischemia may worsen injury.