The mechanism is believed to be the release into the bloodstream of muscle breakdown products—notably myoglobin, potassium and phosphorus—that are the products of rhabdomyolysis (the breakdown of skeletal muscle damaged by ischemic conditions). The specific action on the kidneys is not understood completely, but may be due partly to nephrotoxic metabolites of myoglobin. The most devastating systemic effects can occur when the crushing pressure is suddenly released, without proper preparation of the patient, causing reperfusion syndrome. In addition to tissue directly suffering the crush mechanism, tissue is then subjected to sudden reoxygenation in the limbs and extremities. Without proper preparation, the patient, with pain control, may be cheerful before recovery, but die shortly thereafter. This sudden failure is called the "smiling death".These systemic effects are caused by a traumatic rhabdomyolysis. As muscle cells die, they absorb sodium, water and calcium; the rhabdomyolysis releases potassium, myoglobin, phosphate, thromboplastin, creatine and creatine kinase.Crush syndrome can directly come from compartment syndrome, if the injury is left untreated. Symptoms include the 5 Ps: pain, pallor, paresthesias (pins and needles), paralysis, and pulselessness. Treatment
There is no distinct treatment option that can undo the effects and damage from rhabdomyolysis because it is a necrosis. However, the rate of the pathology that can lead to more complications can be decreased by acting early and consistently. Overall treatment depends on preventing kidney failure (renal failure) which is done by rehydrating the patient. It also depends on making urine have a more basic pH (alkalinization of urine).
External links

Life or Limb: What happens when your leg gets trapped under a building?