Pulmonary Thromboembolism:

Substances other than thrombi may traverse the venous system and lodge in the pulmonary circulation.

Fat embolism is the result of abrupt pressure changes in the long bones, which rupture thin walled venous sinuses and force marrow fat into them. It embolizes to the lung. In addition, levels of plasma triglycerides, free fatty acids, and lipase rise as part of the stress response. Endothelial damage is caused by fatty acids released from embolized fat and by mediators released during associated blood coagulation. Fat emboli can be recognized by ordinary histopathologic sections as sharply delimited, empty-appearing capillary loops or arterioles, but frozen sections stained for fat are required for confirmation.

Bone marrow emboli commonly follow vigorous cardiopulmonary resuscitation. Like thrombotic emboli they become adherent, endothelialized, and eventually organized.

Amniotic fluid emboli are a rare complication of pregnancy. Infusion of amniotic fluid occurs during tumultuous uterine contractions when the head is in the birth canal. The amniotic fluid is forced through a rupture in the chorion into the maternal veins, precipitating severe dyspnea, tachypnea, and hypotension. Disseminated intravascular coagulation is a common consequence. At autopsy the lungs are hemorrhagic. Squamous cells are lodged in the arterioles. Amniotic debris also contains lipid and mucin, which can be identified with appropriate stains. Reportedly, the clinical diagnosis can be confirmed by the demonstration of squamous cells in blood withdrawn by pulmonary artery catheter.

Air embolism can be produced during inspiration if negative intrathoracic pressure draws air into an open vein, an event most likely to happen during a neurosurgical or ear, nose, and throat procedure in which the patient sits upright and the operative wound is above the level of the heart. Air bubbles become trapped in pulmonary arteries and right ventricle where they mechanically impede blood flow. Reactions at the gas-fluid interface trigger blood clotting and the accumulation and activation of neutrophils. Small fibrin and platelet thrombi are found in pulmonary arteries. The physiologic consequences include transient airway constriction and vasoconstriction with great increases in pulmonary vascular resistance and pulmonary artery pressure. With large emboli  pulmonary edema, hypoxemia, systemic hypotension and myocardial ischemia are seen. Fatalities have been reported with embolism of 100 ml of air.

Foreign-body embolism can result from introduction of foreign material into the veins during medical procedures but is also common among intravenous narcotic users. Particles of insoluble material added as “fillers” to drugs intended for oral use embolizes to the lung and impact in arterioles and small muscular arteries where they cause thrombosis and proliferation of intimal cells. Often they migrate into the perivascular space or interstitium where they give rise to foreign-body granulomas composed of macrophages, multinucleated giant cells, and a few lymphocytes. The process of migration appears to involve the production of granulomatous response in the vascular wall with disintegration of muscle and elastic tissue. In cases where lesions are not numerous, their detection is aided by the use of polarizing filters, since cornstarch and talc, two of the materials commonly used as fillers, are strongly birefringent. When the vascular thrombosis is widespread, pulmonary hypertension results. Lesions may resemble those of primary pulmonary hypertension, particularly in view of the cellular proliferation induced by the foreign material. Extensive interstitial granulomas can produce roentgenographic nodularity  and a restrictive ventilatory defect.

Tumour embolism: Tumour emboli are occasionally seen in the lung and are thought to be the source of the lymphangitic form of carcinoma.

Schistosomiasis:click here : Schistosomiasis may be associated with the embolization of ova to the lungs from the bladder or gut. The ova occlude small vessels, excite a foreign body reaction, and elicit a granulomatous response. The ova also penetrate through the vessel into the surrounding lung.